An astrometric examination of the well-photographed planets and moons of STAR WARS, in order to survey and interpret their physical and orbital characteristics.



This page is not intended to be an introduction to astronomical concepts, and some of the terminology used here is technical. Later versions of the page may be kinder to the general reader and/or contain more useful explanatory diagrams, but the present draft is simply meant to consolidate key ideas in a concise structure.

The purpose of this document is to examine the major filmed celestial bodies of the STAR WARS universe. In real life, astronomical objects are investigated by measurement of their motions and features as shown in photographs, spectrographs and other exotic instruments. Consideration of the geometry and physics underlying and giving rise to the photographic image allows the properties of the objects to be measured or constrained indirectly.

Although the STAR WARS movies do not allow us the freedom to change or even to know the details of the virtual observational instruments, we can reasonably assume that the screen images correspond to the effect and capabilities of a movie camera or the naked human eye. Certain features in the images of STAR WARS worlds provide mutually consistent points of reference (eg. the planet Yavin must be the same size in closeup and in diagrams of its moons' orbits). Sometimes the observations contain objects or circumstances that serve as absolute, universal points of reference (eg. the stellar classes of Tatoo I and Tatoo II are known).

Some commentators would naively dismiss this entire exercise on the basis that “it's only art”. Presumably most artists do not bother to structure their paintings or their movies with physical plausibility in mind. However there are plenty of documents about the special effects of the first generation of STAR WARS movies indicating that the original artists adhered to standards of precision and professionalism that took their work beyond mere impressionism. For instance the remarks of Richard Edlund about the size of the second Death Star are totally consistent with independent, astrophysically-grounded estimates.

Finally, works of art contain a great deal of inherent and implied physics. If an artistic creation looks “realistic” to the human eye and mind then it is usually possible to devise something “real” corresponding to the visual effect. Tacit human intuitions have evolved to deal with the real world. If an effect is pathologically unable to correspond to something real or potentially real, then the scene will be unconvincing and will seem surreal to movie viewers. Those kinds of images and animations, if they are ever made, usually die on the cutting-room floor.

This does not mean that there is no danger of inconsistency. It is entirely possible for a new film to be made showing astronomical objects with features so extreme that they cannot be made to fit our first assumptions. If, for instance, we were faced with a new planet with a bizarre internal structure that could not arise through natural processes then we may be forced to conclude that it actually is not a planet in the usual sense: perhaps it is some kind of artificial planet-sized body with its true origins lost in prehistory. Or, if we see a moon that looms implausibly large in the sky, we may be forced to grasp exotic assumptions about its density and composition, and perhaps its orbit, in order to save the planet from outrageously strong tides.

The greatest threat of trouble in this field of investigation comes from the spin-off literature. The written word is not open to interpretation and measurement in the same way as the visual experience of a movie. If the author of the book makes arbitrary quantitative statements about the distances, masses and orbital periods of celestial objects then there is a very real risk that they will conflict with physical law, whereas a visual presentation of the same phenomena would allow measurements to be made on the basis of physical law, yielding consistent conclusions which can then be explored for their implications. Fortunately, Lucasfilm's published terms for evaluating STAR WARS continuity give primacy to the movie canon. In the commentaries of this site, official non-canon sources will be considered only insofar as they are physically self-consistent and consistent with the canon.


In the study of astronomical objects photographed in STAR WARS, the two most general and basic questions to be answered are: “where is it?” and “how big is it?”

Determining absolute positions of objects within the galaxy is generally impossible and is not particularly useful. However the physical relationships between suns, planets and moons within their local system is of very great importance. The planet-sun distance relates to planetary climate directly, in conjunction with planetary rotation, the luminosity and the age of the star. Observed distances between sun and planet, or between planet and moon, constrain the properties of their orbits. If the gravitating objects' orbital velocities could also be estimated, they would yield information about the masses.

Knowing the dimensions of a celestial body, combined with reasonable assumptions about its bulk composition, indicates its mass and other properties like the surface gravity and escape velocity. Knowing the absolute sizes of two objects, when they are at very different distances from camera, allows their true positions to be calculated, based on their perspective-affected projected sizes.

Positions in three-dimensional space can be expressed as triplet coordinates that represent the location as displaced from a reference “origin” point in three perpendicular directions. In computerised images, such as those scanned from movies, the X and Y coordinates are conventionally taken to be the displacements from left to right and from top to bottom in relation to the upper-left corner of the picture. (This is how pixel-count coordinates are reckoned in PhotoShop and similar graphics programs.) For the purpose of this document, the third dimension is described by the Z coordinate which will be taken to be displacement out of the image plane, in the direction of the viewer. This is a left-handed coordinate system. (A right-handed system would either have the Y direction be up rather than down, or the Z direction be away from the viewer and into the screen.) The scales of the three coordinates are assumed to be the same, and perspective corrections will be performed where appropriate.

Most astronomical images in STAR WARS feature one dominant celestial body. For ease of reference, the coordinates and measurements pertaining to each image will be scaled to units in which the radius of the dominant object equals 1. Thus pictures of the Earth-Moon system would be described with the Earth radius as 1, the Moon's radius about 0.272 and the Moon's orbital radius being 60.3. Using these normalised figures allows discussion of the relationships between the objects without necessarily knowing the absolute size of any of them, (which is often unknowable using only the initial evidence). If the sizes are eventually determined (eg. the Earth's radius is 6378km), then it is a simple matter to scale the surrounding objects and orbits.


The painstakingly rendered planetary matte paintings of STAR WARS are rich in detail and are wide open for interpretation and analysis as if they were photographs of real worlds made by space probes. Using images of a spherical celestial object as seen from far away in space, the positions of features on the surface can be investigated as follows.

  1. Measure the screen coordinates of at least three points on the edge of the visible disk.
  2. Fit them to the equation for a circle r²=(x-xc)² +(y-yc to obtain the radius r, and the coordinates of the planet's centre in screen terms (xc,yc).
  3. Transform all coordinates to a frame of reference with its axis at the centre of the planet, and in which the global radius is scaled to 1: x' = (xxc)/r, y' = (yyc)/r,
  4. Then the z coordinate of any given point on the planetary disk is given by x'² + y'² +z'² = 1.

From these results it is easy to determine the distances between particular points on the surface of the globe, if you scale all your coordinates up by the true value of the celestial object's radius. Additionally, the three-dimensional coordinate data for surface features could potentially be used in more complicated mathematical expressions in order to plot maps of the world under study.


The illumination of planetary bodies seen from space provides important information about the orientation of the objects in relation to the distant sun. This is often the only way we can learn anything about the suns of STAR WARS, because they're shown directly on film so rarely. Studying the illumination and shadows on the globes of celestial bodies also has the potential to provide us with constrained estimates of the orbital and rotational parameters of the planets and moons, if comparable views are available at different times, or if surface markings move visibly due to global rotation.

The direction of the sun is calculated from the positions of representative points along the terminator between the night and day sides of the globe. Since the globe is a sphere to a good approximation, the third dimension is calculable for any of these terminator points, as described in the subsection above. The direction of the sunlight is at right angles to the plane than passes through every point on the terminator. Since the rays of sunlight are parallel at large astronomical distances, and since a planet does not move significantly in its orbit in the duration of one of the movies, the terminator information could provide an absolute spatial frame of reference to compare different events, eg. those taking place at Endor and Hoth.

A shadow cast by a moon or planet upon other objects in a scene provides additional information. Knowing the direction of the sun fixes the location of three-dimensional regions within which objects would experience night or eclipses. If a moon does cast a shadow on the surface of its primary, we could equate the size of the affected region with the umbra of eclipse and thus determine the angular size of the moon as seen from the planetary surface. We may also gain information about the angular size of the sun. All this deduced data may combine to inform us about the moon's absolute size and position, especially if the moon itself is visible in the same image as its shadow.


Perspective is the major blight of astrometric interpretations of STAR WARS objects. The size of an object as projected on a computer or movie screen is proportional to its true absolute size, and inversely proportional to its distance from “camera.”

The sizes of two objects in an image can only be compared validly if their distances are taken into account. In cases involving artificial objects like Imperial warships, the existence of a standard yardstick like the Kuat Drive Yards bridge tower, allows independent scaling to be done. In some other cases perspective effects can be neglected safely, if the camera range to each object is very much larger than the separation between the objects themselves. In this case different objects are affected by perspective almost identically. However this neutral-perspective assumption is exceptional and is not always easy to justify.

In the absence of any other evidence, the only way to determine which of two celestial objects is closer to camera is to see which one visually overlaps the other. If there is no overlap and no special yardsticks, then there is no way to prove spatial or size relationships, regardless of what eyeball intuition suggests.

The existence of overlap tells us which object is closest, but does not directly reveal how much closer it is. It also enables us to determine either an upper or lower limit on the ratio of the sizes of the two objects. For instance, if a moon is in front of its planet, then the true relative radius of the moon is less than the relative radius of the moon's image in the picture. (Because the closer object may appear larger due to perspective.)


This is not a treatise. Some familiarity with mathematics and basic physics is assumed. Readers who would like more instruction or explanation of unfamiliar concepts are advised to consult appropriate texts or popular science books. The material in this section is presented for the sake of giving some background for the practical results appearing in the commentaries about particular planets. If it proves desirable and if I can borrow enough time, I'll consider adding a few more clarifying subsections at a later date.


Inert objects moving in space in the absence of external forces follow straight lines, continuing to move at constant velocity indefinitely. Massive celestial bodies are caused to depart from free linear motion and follow curved paths because of their mutual gravitational attraction. If the bodies are not moving too quickly, do not collide and are not seriously disturbed by other influences (such as the gravity of a third object), their curved motions proscribe closed, repeating paths called orbits.

The orbit of a bound celestial body is an ellipse, and the centre of mass of the two bound objects is located at one focus of the ellipse. An ellipse is a round shape like a circle, but narrower in one direction than in the perpendicular direction. The foci are points on either side of the centre of the ellipse, on the major axis. The major axis is the longest possible straight line bisecting the ellipse, and its length is double the statistic known as the semimajor axis, which is customarily labelled a. Eccentricity is a number describing the degree of departure from a perfect circle; a circle has e=0, whereas the most extreme ellipses have eccentricity approaching 1. The distance between a focus and the centre of the ellipse equals ae.

When one of two objects in an orbital system is very much more massive than than the other, we call it the primary, and the lighter object is called a satellite. In this case the centre of mass is very close to the physical centre of the primary, and the primary can nearly be regarded as sitting fixed at the focus of the satellite's orbit. This is the case for a sun and its planets, and for a planet and its moons. In other cases, where the two objects are comparable in mass, such as the stars of a binary system, the centre of mass sits well between the objects, and both bodies revolve around this centre in separate orbits with the same orbital time period.

In a normal planetary system, the planes of the orbits of different planets are almost exactly aligned. The average plane of the orbital system is called the ecliptic. This is because planetary systems accrete and condense from a broad flat primordial circumstellar disk. It is thought that planetary ecliptics in most systems are usually aligned with the axis of the sun's spin. Likewise the orbits of a planet's moons tend to be arranged around a special plane, which often is aligned with the planetary spin, which does not have to be parallel to the ecliptic of the planets. The orbits of moons can show more variety than planetary orbits, however, because some moons are captured rather than born with the planet, and because satellite orbits evolve under the moons' mutual gravitational perturbations.

Below are a number of handy equations describing the orbits of bound planets and moons.


Physical and orbital data for several canonical and filmed planets and moons are tabulated below. Many of the blanks spaces will remain blank forever because of a lack of sufficiently detailed observations. Others may eventually be filled due to further studies of existing material, and fresh images from the prequel films. Tatooine and Coruscant are likely to benefit in the wake of The Phantom Menace. There also seems to be a good chance of seeing Alderaan again in a later prequel movie.

Where possible, the spatial measurements are given in terms of metric metres. However in some cases, ie. Hoth 6 and its moons, the basic scale of the system is less well known than the ratios of the various lengths. In these circumstances, spatial dimensions are measured in terms of the planet's radius. Another useful unit for describing planetary orbits is au which is literally an “astronomical unit”, the mean Earth-Sol distance, and is approximately equal to 1.496x1011m.

These abbreviations express different units of time:

yrterrestrial years
S.yrGalactic Standard (Coruscant) years = 368 days
days    terrestrial or Coruscant days = 24 hr
l.d.local days = rotation period of planet/moon
hrhour = 60 min
minminute = 60 s


Physical Data

object mass [kg] radius [m] gravity [m/s2] escape velocity [km/s]
Earth 5.976 × 1024 6.378 × 106 9.80 11.2
Alderaan ... ... ... ...
Bespin *8.6 × 1026 5.9 × 107 *16.5 44

[bright moon] ... < (4.7±1.2) × 106 ... ...
Coruscant 1.637 × 1025 8.854 × 106 13.93 15.71
Dagobah ... ... ... ...
Endor [sanctuary] (est.) 2.7 × 1024 (4.66 – 5.55) × 106
(est.) 5.2 x 106
(est.) 6.5
Hoth 6 ... < 8.3 × 106 ... ...

[red moon] ... [0.029±0.001 RH6] ... ...

[grey moon] ... [0.031±0.001 RH6] ... ...

[small moon] ... [0.011±0.001 RH6] ... ...
Tatooine ... ... ... ...

moon I ... ... ... ...

moon II ... ... ... ...
Yavin 5.299 × 1027 9.6239 × 107 38.17 85.71

Yavin 4 ... 6.55 × 106 ... ...

Yavin 13 6.310 × 1024 5.700 × 106 12.95 12.15

Orbital Data

object rotation period obliquity orbital period semimajor axis [m] eccentricity
Earth 23.9345hr 23.45° 365.25 days 1.496 × 1011 0.01675
Alderaan ... ... ... ... ...
Bespin 12hr >21±3° 14s.yr ... ...

[bright moon] ... ... *> 16.7hr > 1.7 x 108 ...

Drudonna ... ... 52.5 days *3.1 × 109 ...

H'gaard ... ... 105 days *4.9 × 109 ...
Coruscant 24hr ... 368 days 2.29 × 1011 0.0961
Dagobah 23hr ... 327 days
=341 l.d.
... ...
Endor [sanctuary] 18hr ... 0.823yr
=402 l.d.
... ...
Hoth 6 23hr *>20.2±0.1° 526 days
=549 l.d.
... ...

[red moon] ... ... *2hr21min – 2hr38min [1.41 – 1.52 RH6] *0

[grey moon] ... ... *5hr25min [2.46 RH6] *0

[small moon] ... ... ... ... ...
Naboo 26hr ... 312 day ... ...
Tatooine 23hr ... 0.80yr
=304 l.d.
=291.1 days
1.62 × 1011 ...

moon I ... ... ... ... ...

moon II ... ... ... ... ...
Yavin ... ... 13.2S.yr ... ...

“a” ... ... 134hr 1.28 x 109 0.726

“b” ... ... 114hr 1.14 × 109 0.111

“c” ... ... 286hr 2.12 × 109 0.154

Yavin 4 24hr ... 174hr 1.52 × 109 0.89

Yavin 13 ... ... ... ... ...


Coruscant, the glorious galactic capital, was originally devised as a major setting for Return of the Jedi and appears in Ralph McQuarrie's concept art from the early 1980s. The city-planet reappeared in the late 1980s in the early reference volumes of STAR WARS The Roleplaying Game. The environs around the Imperial Palace were eventually described in novels starting with Heir to the Empire and drawn as war-torn ruins in Dark Empire comics. The only point where the native rock of the planet is exposed to daylight, Monument Plaza at the peak of the city-encrusted Manarai Mountains, was portrayed in The Illustrated STAR WARS Universe, which recycled most of the original concept art. Coruscant was finally brought into the absolute canon with the radio play of Return of the Jedi and then the special edition of the movie in 1997. The first filmed views of the planet seen from space appear in the 1998 teaser trailer for The Phantom Menace. This image is close to McQuarrie's original vision, although it is a subdued dawn/dusk view with much more cloud cover than McQuarrie's planetscape of the glittering sunward side.

The coruscation of the surface of Coruscant, as seen from space, provides interesting information about the surface texture. The scintillation depends on the movement of the observer with respect to the planet, and the rotation of the planet relative to the Sun. If an observer rode in geosynchronous orbit, the daily solar rotation would be the only influence on the reflection: as different areas of the city come under sunlight at different angles in the sky. The timescale of each tiny scintillation flash, compared to the duration of the Coruscant day, yields information about the angular size of the detail on the reflecting surfaces. Combined with the radius of the planet, this yields information about the absolute physical scale of the reflecting lattice patterns of the building blocks. For instance a flash lasting a second may suggest surface structures with pattern detail on the scale of a hundred metres or so.

The actual surface of the planet is thoroughly buried. Those parts of the planet not covered by urban terrain include only small artificial lakes and the polar ice caps. The only place where the soil or rock of the planet sees daylight is Monument Plaza at the pinacle of the Manarai mountain range. The planet is not inert; the site of Imperial City was a volcanic plain and at least one massive eruption is recorded in legendary pre-Republic history. However Coruscant is a tame world today, and its inhabitants probably employ geological engineering technologies to ameliorate natural tectonic and volcanic processes.

Coruscant is considered the basis for timekeeping and planetary units in the Galactic Empire. Galaxy Guide 2 describes some of the planets and moons of the Yavin system in terms of both these plantary standards and absolute known units like the density of water and metric kilometres. These statistics combine to imply the true dimensions of Coruscant, as stated in the Standard Units page. Coruscant is a terrestrial planet somewhat larger than Earth. It has 2.74 times Earth's mass, 1.39 times its diameter, and 1.42 times its surface gravity. This is well within the range of habitability for humans, although newcomers from Earth or other mild-gravity worlds would go through a period of uncomfortable physical adjustment. (Indoor environments could use artificial gravitic devices to lighten the weight.)

Coruscant has four moons [Inside the Worlds of STAR WARS Episode I]. Two of them were visible in the night sky at the end of Return of the Jedi, but unfortunately they are unrecognisable due to the wide field of view and glare. The angular sizes of the moons [in ROTJ] were in the approximate ratio of 13:12, with the larger disk being higher in the sky. The absolute sizes of the moons are unknown because there is presently no way to determine their distances. Their angular separation in the sky is 5.6 times the angular diameter of the lesser moon disk. The true physical sizes of the moons cannot be as similar as their angular sizes; one moon is large and distant, and the other is smaller and nearer. Otherwise the moons would have perturbed each other gravitationally. The moons' separation in 3D space must be comparable to their distances from the planet.

If a line is drawn through the moons and extended to the horizon, it is about 30° from vertical. This would tell us something about the location of the scene if we knew more about the orbital distances of the moons. For instance, if the moons orbit in the same plane and that plane happens to coincide with the rotation of the planet, and if the moons are fairly distant compared to the planet's diameter, then the latitude of this part of Imperial City would be thirty degrees.

At least three of Coruscant's moons are seen in The Phantom Menace. These scenes and future movies may reveal three-dimensional information implicitly. Further studies of the projected relationships between the moons and the planet, at different orbital phases, may distinguish the system's orbital statistics and perhaps the planet's rotation.

One moon in the Coruscant system, named Hesperidium, was the site of a resort that was Han Solo's starting point in Planet of Twilight. However the novel does not indicate whether Hesperidium is a moon of Coruscant itself, or some other planet orbiting the same star. The moon appears to be large enough to hold an artificial or natural atmosphere, and the sky usually has a “rosy lavender” colour. This would have implications for the density of the atmosphere; it may be more diffuse than Earth's, or the colour may be affected by airborne dust.

The second trailer for The Phantom Menace shows an extremely valuable sunset scene with both Coruscant's sun and one of its moons in the sky. The angular diameter of this moon is about 0.185 times that of the sun, so this moon can never obscure enough of the sun to provide a total eclipse. The separation between the centres of the moon and sun on the sky is 2.39 times the angular radius of the sun. The phase of the sunlight on the moon is about 136°, which fixes the absolute angular size of the sun and the moon. The sun subtends about 1.8° on the sky, and the moon subtends 0.34°. (For comparison, Earth's sun and moon both subtend about 0.5° on the sky.)

This means that relative to the size of its sun, Coruscant orbits somewhat closer than Earth. However in absolute terms the mean orbital distance is 1.53 times Earth's [according to Inside the Worlds of STAR WARS Episode I], implying that Coruscant's sun is 5.5 times the diameter of Sol. In order to provide a comfortable climate, the age, mass and luminosity of this star must be substantially different from solar values. The orbital distance and the period of 368 days indicate a stellar mass of 3.53 times solar.

Unresolved Questions:

Composite image of Coruscant seen from orbit in the teaser trailer for The Phantom Menace.

Global view of Coruscant. [Inside the Worlds of Episode I]

The grandiose vistas of the upper levels of the Coruscant cityscape are one of the great wonders of the galaxy. Immense buildings two miles tall and hundreds of metres wide may each contain countless thousands of inhabitants.

The cavernous mid-levels of Imperial City descend to depths of about two miles, densely populated at all but the lowest and dankest levels.

Coruscant as seen from space glitters and shineslike a brilliant jewel, due to the refraction and reflection of sunshine off the countless angular surfaces of the city-covered globe.

* * * * * * * * * * * * *
The city-clad globe of Coruscant and halo of space traffic, as seen during Queen Amidala's arrival and departure [TPM]. Since this view of the planet is near the night/day line, either local dawn or dusk, the sunshine refracts and reflects off the city surface at very oblique angles. The interplay of nightlights, the city's texture and the curvature of the planet under sunshine causes the sparkling patterns.

* * * * *
Coruscant, a moon and space traffic as they appeared during Darth Maul's last conference with his master. [TPM DVD]

One of Coruscant's moons in the night sky, as seen from the Sith viewpoint. [TPM DVD]

* *
Coruscant as represented in the later part of the Palpatine era, during a hypothetical rebel invasion in the game Rebellion. In these views it looks more grey than in The Phantom Menace. Is this a genuine change of the planet's representation, or is because the Rebellion viewpoint looks down from a more sunward direction?

* * * *
Two of Coruscant's moons were seen together in the sky during this late-night celebration. The chiming of a huge clock singals three o'clock in the morning, in this time-zone. Coruscant moons are also visible in this concept painting, but now one of them seems larger. If these are the same moons then one requires an eccentric orbit to take it far and near at different times. However TPM shows more than two moons. 3: Sunset near the Jedi Temple. 4: Thin crescent of one moon visible over the day side of Coruscant.

The remarkable streetscapes of Coruscant, seen from the air near the Jedi Temple.


Tatooine is a harsh but habitable terrestrial planet with at least two moons, in a close binary star system. The suns, Tatoo I and Tatoo II, are of stellar spectral types G1 and G2, and the planet orbits their centre of mass at a distance much greater than the stars' separation. In The Phantom Menace one sun appeared bluer than the other, and it's not clear whether this could have been consistent with G1 or G2 classification. Perhaps the stars are extreme variables with some kind of pulsation period less than three decades? If they are actually G1 and G2 main-sequence stars, their total mass is about two solar masses, and roleplaying game references indicate that Tatooine's year is about 0.80yr (in Earth terms). Therefore the planet orbits at about 1.08au from the centre of mass of its suns.

It is possible to determine a lower limit on the absolute distance between the twin suns by measuring and comparing the suns' angular sizes and separation in the sky. Because there is no way to tell the alignment of the suns (ie. which one is closer to Tatooine than the other) this method only provides a lower limit. In the sunset in Return of the Jedi the upper and lower suns had angular diameters in the ratio of 15.1:10.5. Their projected separation was 10.2±0.5 times the radius of the smaller star, which means a separation of greater than (7.1±0.3) × 109m, assuming that the lesser star is the same size as Sol. Similarly, the ANH sunset scene yields radii in the ratio 109:105 and a projected separation of 5.40 times the smaller star's radius, at least 3.6 × 109m. In this moment the suns are aligned at an angle closer to the line of sight from Tatooine.

The wider lower-limit estimate, which is probably a large fraction of the actual value, is equivalent to 0.047±0.002 au. Tatooine orbits at about 1.08au, which is almost too hot for life because it gives it a solar energy influx 1.7 times that of Earth. In a binary system, planetary orbits are only stable if they are much larger than the stellar separation (moving around both stars as in the case of Tatooine), or else much smaller (orbiting one star closely). The former is definetly the case for Tatooine, and the likely separation of the suns and the likely orbit of the planet are physically consistent.

The oscillating gravitational influence of the two suns circling their common centre of mass is likely to have interesting tidal effects on the planets of the system. If Tatooine had seas, sailors might notice a subtle solar tide. However since the orbital period of the suns (lower limit >64±3 hours) is much shorter than the duration of Tatooine's year (0.80yr in Earth terms), there cannot be any effective resonance. Therefore the binary orbit shouldn't exert any gross effects on the planet's orbit or rotation within the span of human history.

If sufficient observations were made, it would be possible to constrain the phase of Tatooine in its orbit in A New Hope and Return of the Jedi, at least in relation to the shorter-period mutual orbit of the suns. Temporary climatic variations should be expected during the times when one star eclipses the other partially or wholly. Unlike eclipses due to the moons, the eclipse of one sun by the other would be seen over the entire globe of Tatooine, and sunlight would be half the usual level. This may well be the marker of the "seasons" mentioned by the moisture farmer Owen Lars in A New Hope.

Another harmless but interesting side effect of having two suns is that each object would cast two shadows during daylight. However since the suns are always close together in the sky, the shadows mainly overlap, so that they practially appear to be a single shadow with fuzzy edges, even when the sky is clear and the sunshine sharp. If, on the other hand, noticeably separate double shadows are ever observed (perhaps in The Phantom Menace), then they will set an interesting lower limit on the separation of the suns.

On Tatooine itself there are at least two basic types of terrain visible from space: rocky uplands, and dune seas filled with light-coloured wind-deposited sands. The lowland regions are not totally uniform and featureless. The largest known lowland area shows gradual variations in brightness, with lighter tones prevailing near the "shore" of the uplands. Perhaps this is due to lack of chemical degradation in fresh sands, or some kind of variation in the sand particle sizes. Perhaps the nearby rocky uplands affect the strength or nature of the winds that raise and deposit the sands.

The edges of the uplands appear to bear erosional channels of some kind. It is unknown whether these are due to wind erosion, prehistoric water erosion or an ongoing process involving nocturnal moisture. The A New Hope novel describes the existence of mysterious mists that arise from the ground near the edges of rocky outcrops. However the planetological experts of the galaxy are said to be unaware of the source of these mists (which probably indicates the inattention given to Tatooine, or the moribund nature of science in an ancient, static galactic culture).

Water must occur near ground level under some circumstances, otherwise plant and animal life would be impossible. Tatooine boasts some very large native animals, most of them dwelling in the rocky, sheltered highlands. In the canyon floors of the Jundland wastes there are numerous shrub-sized plants (less than a metre wide or high), but they have not so far been photographed with much resolution. The distribution of these shrubs probably follows the nighttime mists or the mysterious subterranean water source.

The scarce clouds in many space and ground views may attest to the presence of some lasting daytime water, and their visible shadows in principle allow the clouds' approximate altitude to be determined. The sky was cloudy near the Tantive 4 escape pod landing site and the nearby mesas and canyons. Clouds were seen near the horizon in each observed Tatooine sunset. The sky over Mos Eisley was clear. The sky was clear near the Lars' homestead when jawas brought droids for sale, but was cloudy the next afternoon after the farm had been wrecked. Could it be that a functioning moisture farm draws moisture from the atmosphere, and a wrecked farm allowed the clouds to pass unaffected?

What is the reason for the aridity of Tatooine? The development of macroscopic life probably required the presence of oceans at one stage. Perhaps there are processes at the top of the atmosphere that remove water to space or else consume it in chemical reactions of a kind that are unimportant on Earth. Perhaps the water has been subducted into the planetary interior at the descending edges of its tectonic plates, at a rate faster than the rerelease of water vapour from volcanic regions and the injection of new water from cometary impacts.

What is the basis of the Tatooine food chain? How are such large animals supported with so little vegetation? Is most of the biomass generated at the polar caps, nocturally, or underground? With so little above-ground flora, how is the oxygen atmosphere sustained? Is the atmosphere actually in decline? What is the oxygen cycle?

Tatooine has at least three moons [TPM, AOTC, ANH]. They appear to be dull grey orbs, and the moon with a larger projected size in the existing photograph has a dichotomous surface: dark, smooth basaltic maria, and rough, cratered, light highlands like our terrestrial Moon. The moons' astrometry and photometry deserves investigation. Unfortunately there are no definite cues to their sizes, because in the one available image, they fail to cast visible shadows on the observered areas of the planet, and they do not visually overlap each other, the planet or any nearby objects.

* *
Global views of Tatooine, with a mirror-flip correction to match the version seen in the movie. [Inside the Worlds of Episode I; The Art of Star Wars Episode IV] The colour balance of the second image is slightly suspect.

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Global views of Tatooine. The first is composed from several frames of the The Phantom Menace trailer, with foreground obstructions removed. The latter unedited images depict the arrival of the Naboo fugitives and Darth Maul. The planet is less clouded than in A New Hope, which may indicate a seasonal change. The illumination of the night side is also remarkable; it may mean that at least one of the moons is large, highly reflective and positioned off to the left of this field of view.

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Global view of Tatooine created by removing foreground spaceships from Return of the Jedi footage (except for some remaining parts at the bottom of the image).

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Tatooine and one of its moons as seen when Queen Amidala's ship departed for Coruscant. [TPM; TV advertisement One Love]

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Closer orbital views of Tatooine as seen from the scene of the capture of Tantive IV and later from the vantage of one of its escape pods. In the former image, two of Tatooine's moons are visible. The moons' features don't clearly match those seen in A New Hope, but this may be because a different face is illuminated. Of the two moons seen before, the three bands on the new image are more consistent with the smaller or more distant moon.

* * *
Moons of Tatooine [movie screenshots].

Sunset on the planet Tatooine shortly before the collapse of Jabba's criminal organisation. [ROTJ Topps widevision]

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Binary sunset on Tatooine [Topps Widevision; movie stills]. In the first scan, the yellow (upper) sun has it centre at (575.9 83.9) and radius 10.9; the orange (lower) sun has centre (612.4, 127.6) and radius 10.5. The projected separation of the sun centres is 56.9 pixels.

Tatoo I and Tatoo II as seen from space near Tatooine. One seems noticeably bluer than the other, although they are a similar brightness. [TPM DVD]

* * * *
The twin suns of Tatooine as seen in the sky of the Rogue Squadron computer game. The clouds are scarce, thin, high and distant. This particular mission is described as an "early-morning run over Mos Eisley." Although their edges seem indistinct in these images, it should be possible to place limits on the implies separation of the stars. According to the background material accompanying the game, these dogfights took place about six months after the events of A New Hope.

* *
Permanent habitations on Tatooine tend to be built on firm ground, almost always in the rocky highlands, for instance Ben Kenobi's hovel, the city of Mos Eisley and Jabba's Palace.

The canyon leading to Jabba's palace looks like a riverbed, or at least the path of a flood. It is definite evidence of water erosion occurring at some time in the planet's past.

Dark green forms at the bottom of this canyon in the Jundland Wastes may be a kind of hardy desert vegetation. They seem too uniform in size to be simple rocks. [ANH, Topps widevision]

This image taken from a pre-release version of the Rogue Squadron game shows numerous cumulous clouds in the sky. The final version of the game eliminates these clouds and leaves only higher wispier clouds near the horizon. The presence of cumulous clouds would have been inconsistent with the desert environment shown in the movies.

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ANH #49,50 widevision card from Topps. Two star destroyers and another Imperial ship of unestablished type pursue the Millennium Falcon from Tatooine. Compare the sunlight direction on the ships of the two scenes: portside for the destroyers, starboard for the ship that was closer to the Falcon. Therefore the destroyers are headed in a direction almost exactly opposite that of the closer ship. Either the rebels changed course or the Imperials came as two groups from very different directions. Unfortunately the shadowed side of the planet is off-camera in this scene, and thus cannot be used as a reference point.


The sixth planet in the Hoth system was a habitable world used by the Rebel Alliance for a major base to replace the one exposed at Yavin. The fact that it is the sixth world implies that either the Hoth sun is more luminous than ours, or the inner planets have narrowly spaced orbits.

An upper limit can be placed on the size of Hoth 6. When General Veers' walker came close enough to the rebel power generator for it to appear over the horizon it was at a distance of “one-seven-decimal-two-eight”. Considering the visual evidence and the tendency to use metric units in STAR WARS, this must mean 17.28km. Since the AT-AT head height is about 18m, if we assume that the walker was on flat ground then the radius of the planet is R < 8.3x106m. Of course, the walker really had the advantage of being on a ridge, so this is an upper limit rather than a direct calculation of the planet's radius.

A benefit of knowing the radius of the planet and the altitude of the rebel base (hence determining the distance to the local horizon) would be the ability to calculate a lower limit estimate on the area protected by the rebels' energy shield. Unless it penetrates the ground, the shield must extend at least to the horizon, otherwise Imperial ships could simply fire under the edges. According to Essential Guide to Weapons & Technology, the Hoth shield covered a region 50km in diameter meaning that it either wraps past the horizon, had a single generator placed at an exceptionally high altitude, sat on a planet somewhat larger than Earth, or else there were several nodes of generation up to and beyond the base horizon.

The surface of Hoth 6 is largely shrouded with cloud. About half of the surface area is visible from space. Half of the regions seen to date are oceans; the remainder appears to be covered with ice. Essential Guide to Moons & Planets suggests that the oceans are equatorial, and STAR WARS The Visual Dictionary hints that there may be some tundra in the equatorial regions as well, but it remains unseen in The Empire Strikes Back. The absence or scarcity of land plants is not cause for concern about the composition of the atmosphere; oceanic plankton is more than sufficient to maintain global oxygen levels and serve as the base of the food chain. It seems likely that much of Hoth's life may be concentrated in the seas and near the shores.

According to General Rieekan, the system had a great amount of “meteor activity”. This probably means that there are a lot of asteroids crossing the planet's orbit, and it may have something to do with the presence of the Hoth asteroid field described in The Illustrated STAR WARS Universe. (This may have something to do with the dense and vigorous asteroid field seen in The Empire Strikes Back, but the location of that field is controversial.) Minor impact events are commonplace, like the one Luke Skywalker thought he would investigate. If small impacts are common, large impacts must be frequent on geological timescales. The planet would suffer the “nuclear winter” effect often, although not necessarily within recent history. The transient nuclear winter effects from modest and major impacts may cool the planet to the point where ice formation brightens the surface to the point where enough sunlight is reflected to sustain a perpetual ice age.

Several of the moons of Hoth have been photographed in front of the planet's disk, which places upper limits on their sizes and lower limits on the dimensions of their orbits. Three available images are usful for this kind of analysis. The first one is a distant portrait of the planet and its moons without any foreground objects. The second useful image is the view of Hoth during the arrival of Lord Vader's taskforce. The third image is a closer view of the planet and moons, near conjunction, during Luke Skywalker's departure.

Distant Portrait

In the longer-range portrait matte painting the projected radii of the moons are 0.031±0.001, 0.029±0.001, and 0.011±0.001 times the planetary radius. By direct projection from this picture, the orbital radii (divided by the planetary radius) are not less than: 2.093, 1.406 (for the grey and red-patch moons respectively) and indeterminate for the smallest moon. Because the planet has oceans, the moons must raise small tides. In general, the tidal interaction of a satellite with the oceans or atmopshere of its primary causes exchange of angular momentum and energy and tends to reduce the eccentricity and inclination of the satellite's orbit. Since Hoth's known moons are close to the planet, their orbits ought to be essentially circular, and possibly coplanar with the planet's rotation.

The large grey moon casts a shadow on Hoth 6, which indicates that it is closer to camera than the planet. Furthermore the planet's illumination determines the direction of sunlight, (0.904,0.167,0.395), which in turn allows use of the shadow to determine the exact position of the moon in relation to the planet. In the same coordinate system, the moon is at (1.785,1.086,1.292), which implies an exact orbital radius of 2.456 times the planet's radius. If the bulk density of Hoth 6 is similar to that of other habitable terrestrial worlds, the orbital period of this grey moon is approximately 5 hours 25 minutes.

At this instant, the orbital vector of the grey moon makes an angle of 20.3° to the direction of the sunlight. This is worth noting, because future information may combine with this observation to determine the exact orientation of the moon's orbit in relation to the planet's solar orbit.

For reference, the normalised coordinates of the large patchy red moon and the small red moon are (-0.996,-1.025,_) and (-0.694,-0.656,_) where the Z coordiante remains unknown.

Vader's Arrival

The Imperial naval squadron attacking the rebel base arrived from a heading perpendicular to the direction of the sun. This implies that their trajectory took them into the system without ever going past or directly towards the star. It is not clear whether the Imperial approach was perpendicular or oblique to the system's ecliptic, or tangential to the planet's orbit and in the orbital plane. An approach perpendicular to the system's ecliptic plane is desirable for the sake of avoiding asteroids and other small obstacles. However the fact that the two visible moons appear roughly aligned with each other and the planet's centre suggests that the trajectory was almost parallel with the average plane of the satellite system, but it is not yet certain how the satellite orbits relate to the orientation of the planet's orbit. (The orbital vector of the grey moon at this moment makes an angle of 42.4° to the direction of the sun.)

The inbound trajectory may indicate the direction of the last system visited by Lord Vader's taskforce. It also may have important implications for Ozzel's bungled naval tactics in the Battle of Hoth; the direction of approach may have affected the rebels' ability to detect the attackers. If the approach was in a direction far from the ecliptic then it would have been easier to distinguish the warships from natural comets or asteroids in highly inclined and eccentric orbits. (This is in addition to the improved chances of detection caused by emerging from hyperspace too close to the system.)

Skywalker's Exit

In the third decisive image, the larger red-patchy moon and the small red moon are in front of the planet, which gives independent upper limits to their radii. These limits are 0.0324±0.0005 and 0.0091±0.0005 times the planetary radius respectively.

The grey moon is does not overlap the image of the planet, so there is no independent limit on its size. However since its absolute size is already calculated from the long-range portrait, the projected size of the grey moon in this frame (0.0408±0.0005 planetary radii) implies that the distance to the moon is 76% of the distance to the planet. Using the orbital size determined above, and accounting for perspective, the absolute position of the grey moon is therefore (-0.177,0.821,2.308) in the normalised coordinate system of this image.

Similarly, if the red moon is scaled according to its apparent size in the long-range portrait, the moon is at 89.5±4.5% of the planetary distance, implying that its orbit is interior to the grey moon's orbit. Its perspective-corrected normalised coordinates are (-0.134,0.508,z) where the z coordinate is only known to be within the range 1.30 to 2.40. By calculating the viewer's distance from the planet (about 4.17 planetary radii) and then relating it to the range of the red moon, a stricter upper limit on the moon's orbital radius is found: 1.52, implying that z=1.43 in this image.

The projected size of the smallest moon doesn't make much sense however, since the figures indicate that it may actually have a smaller projection in this image. The discrepency must be due to uncertainties in the barely measurable size of the moon in the long-range portrait.

It is interesting to compare the orbital periods of the largest two moons. The orbital radius of the grey moon is known exactly, and the orbital radius of the patchy red moon is known to be within narrow limits. These results lead to limits on the ratio of the moons' orbital periods, via Kepler's third law: somewhere between 1.00:0.49 and 1.00:0.43. The former limit is almost a perfect 2:1 ratio. In real life, when the periods of two moons have a simple integer ratio, the small gravitational perturbations they experience during close encounters tend to have a cumulative effect. Moons in such a relationship experience either a constructive or destructive orbital resonance: either it has a stabilising effect on the orbits, or else the moons will evolve away to different orbits that are not resonant. Within the bounds of uncertainty, there is a fair possiblity that the two largest moons of Hoth 6 are in a 2:1 orbital resonance, but more precise measurements are required. The investigation of possible orbital resonances of the moons of Hoth 6 could be a fruitful area for future study.

Using the new upper limit on the orbital radius of the red patchy moon, is is possible to place limits on its position in the long-range portrait picture. Then it is possible to determine how the red moon and the grey moon have moved in relation to one another. The plane of the red patchy moon, the grey moon and the planet's centre has a perpendicular vector somewhere between (-0.665,0.646,0.375) and (-0.629,0.736,0.250) in the portrait picture; but it is not well constrained in the scene of Skywalker's departure. This direction is important if the two moons truly orbit in the same plane. If the moons orbit exactly around the planet's equator, as is likely considering the strong tides for satellites that are this close to the planet, then this vector is the direction of the planet's spin axis. Comparing it with the direction of the sunlight in the portrait picture determines that the axial tilt of Hoth 6 is equal to or greater than 20.2±0.1°, under the present assumptions. If the planet's orbital plane could be determined, the exact obliquity would be calculable.

Unresolved Questions:

* *
Distant orbital view of Hoth 6 and three moons. The two larger moons show interesting surface features: the one on the top left has strikingly different terrains with red and white colours. The grey moon on the right seems monochromatic, but also shows strong variations of surface brightness. The smaller moon near the planet's limb is remarkably red also, but its features are indistinct at this scale.

The Executor and attending Imperial destroyers approach the planet. One of the moons is visible towards the upper right; another redder moon is to the left of the planet's limb.

Hoth and three moons seen during the departure of the last of the rebel defenders.

* * * * * *
An Imperial probe droid's hyperspace pod speeds towards a landing on the sixth planet of the Hoth system.

* * *
The first rebel transport makes its escape from Hoth 6. Note that the ion cannon fire comes from a position on the terminator. This means that the cannon is experiencing either dawn or dusk. Presumably it is somewhere near the main base. If that is the case, the fact that the ground battle took place in daylight suggests that the cannon is in dawn.

General Veers sights the rebel power generator Hoth 6, at a range of "one seven decimal two eight."

The last rebel soldiers retreat across the glaciers of Hoth 6. The single sun is low in the sky, indicating that it's either dawn or dusk. Another possibility is that the base is at a very high lattitude or that the axial tilt is extreme enough so that the sun is continually near the horizon. If the battle occurs near dawn then the rebels are headed approximately west at this moment.


“The planet is geologically unique, and its bizarre structure remains unexplained by the few planetologists who have visited it. Rather than consisting of a magma core and a rocky crust, Naboo is instead a very ancient planetary body with no molten core. A tremendous honeycomb structure surrounds the largest solid bodies of rock, which are thousands of miles in diameter. This cave-filled rock formation structure pervades much of the planet's interior, reaching the surface to create myriads of swampy lakes between the interior land masses and the open seas.”
— Naboo, Behind the Magic multimedia CD-ROM

This is an intriguing description of the astrophysical problem of Naboo. How can it be rationalised? The planetologists' studies must have been cursory. Analyses of the ratios of radioactive isotopes in Naboo's minerals ought to reveal something about the planet's age and the circumstances of its formation. Seismic studies and gravitational measurements would help characterise the interior.

Naboo's most severe mystery is its alleged lack of a molten core. This observation constrains its possible age, size, composition and particularly its internal heat sources. In nature, three principal sources of planetary heating are known.

  1. A planet is initially endowed with heat from its accretion process: the energy of infalling material is thermalised on impact, raising the temperature to hundreds or thousands of degrees. More heat is released by the sinking of denser materials to form a core. The surface cools radiatively, but heat from the deep interior takes longer to conduct or convect outwards.
  2. Radiogenic heating from the nuclear decay of natural unstable isotopes in the minerals provides continuous power. A very young planet experiences strong radiogenic heating due to short-lived radioisotopes. An aged planet is more weakly heated by residual radioisotopes with half-lives of billions of years.
  3. Tidal effects by a sufficiently massive satellite may also heat a planet's interior. Unless the moon orbits synchronously (remaining above one location on the spinning planet) its moving gravitational influence causes the rock to flex periodically. Frictional heating from the flexing warms the planetary interior. This effect is generally much weaker than radiogenic heating, unless the moon is of exceptional size and orbit.

If radiogenic heating is unimportant in Naboo then it may mean one of several things. The planet may be anomalously poor in long-lived isotopes due to either great age or a freakish initial composition. It may help if Naboo formed with a poverty of silicates and heavy elements generally, but such explanations can't be driven far before the planet would have to be a midget gas-giant (which it isn't). If Naboo formed during an early stellar generation then its metallicity would be lower than younger planets, and age would have diminished its radiogenic heating by the present day. Naboo's reputation as an “ancient” planet is consistent. However it can't be many times older than Earth (4.5Gyr) otherwise its sunlike star would have died long ago. (Earth today exists roughly halfway through Sol's lifetime.) If Naboo had a less luminous, low-mass, sun then it could have existed longer, but a low-mass star would cast conspicuously ruddy daylight (which isn't observed). The star is reported to be yellow [SoN p.19].

An exceptional, ad hoc theory seems necessary. Some possibilities include: (a) that Naboo's solar system formed from a molecular cloud which was especially old and escaped seeding of fresh radioisotopes by any nearby supernovae; (b) that Naboo didn't form in its present solar system but is much older and was captured from interstellar space; (c) that Naboo was built or restructured artificially.

The size of a planet is critical to its internal temperature balance. A small planet has more radiative surface area compared to its volume, and so it cools efficiently to space. Conversely, a larger planet has proportionately less surface per unit volume, and it cools poorly. As smaller orbs, Luna and Mars lack plate tectonics and are less internally active than Earth. Supposing that Naboo is a small world may help alleviate the mystery of its cold interior, but it wouldn't be a complete explanation. A small-Naboo theory is difficult to reconcile with the signs of vigorous geological activity (creating mountainous relief). In any case there are limits to how small Naboo could be: the surface gravity appears similar to Coruscant or Earth standards, and the escape velocity needs to be high enough to retain an atmosphere.

The honeycomb mentioned as permeating the bulk of the planet is a much stranger constraint than a merely solid core. The weight of many miles of overlying rock should crush deep caves. Any air or water within the caves would displace upwards as the rock collapses downwards. Perhaps these caverns are artifacts of powerful intelligent or unintelligent life forms? More evidence is needed to test such theories. Wahtever the cause, Naboo's cold, honeycombed structure has unusual consequences and implications:

The city of Theed sits on a precipice running with waterfalls [TPM]. Ths is apparently the boundary where highland crust erodes into the lower terrain. This relief implies rapid geological activity: without ongoing uplift, water erosion would erase the highlands within mere millions of years. Unless there are artificial conservation measures, Like a creeping shoreline, Theed's eroding cliff face must creep into the city on humanly observable timescales unless the topography is artificially conserved. The area around the Palace may have been protected in this way: at least one tower now stands on an isolated rock buttress. The lowland plains and swamps include areas strewn with statues in human likeness, implying that they once stood in upland cities belong to a human civilisation.

Despite their planet's lushness, the inhabitants of Naboo must have a particular lack of self-sufficiency, otherwise they would not have been a vulnerable, credible target for the Trade Federation's demonstration of power. There are different estimates of the population of Naboo: some say 1.2 billion humans [SoN, p.21] but others [ITW:E1, p.5] count only 600 million in total. The latter could be rationalised as the effect of starvation during the Trade Federation blockade, invasion and concentration camps. The former figure could refer to the pre-conflict population. As Sio Bibble said, “the death toll is catastrophic!”

Naboo has at least one satellite. During the Trade Federation occupation, a silvery moon hung in the night sky above the Theed Palace. Queen Amidala's starship escaped past a moon with splotchy areas of grey and brown, and rough shadowed impact craters. The largest craters extend to diameters approximately 1/20 of the global diameter. A moon with apparently larger craters was visible from the N-1 starfighters battling the Federation control ship. This may be another moon or a different side of the same moon. Sources claim that Naboo has one moon [ITW:E1, p.5] or three moons [SoN, p.21].

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Global views of Naboo. [Inside the Worlds of Episode I; PAL widescreen DVD of TPM]

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The planet Naboo seen from the bridge of Qui-Gon Jinn's diplomatic ship. [PAL widescreen DVD of TPM; STAR WARS Insider #42]. This face of the planet is almost uniformly green (presumably vegetation), with smaller areas of blue (open water).

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A face of Naboo seen during Queen Amidala's flight into exile. The blue tinge suggests extensive water, but we can't be sure that this is an oceanic surface because no landform outlines are visible.

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Water erosion near Theed shifts the cliff boundary between the dichotomous terrain types. Over centuries, some great monuments of Naboo's humans have apparently fallen into the lowlands. [Topps Widevision]

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Moonlit skies above Theed during and shortly after the Trade Federation occupation. [TPM DVD]

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Naboo's splotchy moon: during Queen Amidala's escape; above Theed during the occupation. [TPM DVD; One Will]

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Heavily cratered moon visible from the N-1 fighters flying to attack the Trade Federation control ship. [TPM DVD]


As seen in A New Hope, the illuminated side of Alderaan was blue with vast oceans and generous white cloud cover. Clearly it is an Earthlike world with a dichotomous crust divided into distinct continental (granitic, light, thick) and oceanic (basaltic, dense, thin) slabs. The low elevation of the oceanic crust causes most of the world's water to settle into large, continuous oceans, rather than sitting as isolated lake chains as on Endor or Yavin 4, with their singular crustal structures.

This image conflicts with some second-generation published descriptions indicating that the planet has very few seas and is mostly covered by undulating grasslands. The notion of oceanless Alderaan is a mistake that arose because a picture of Yavin 4 was misfiled as Alderaan in The Art of STAR WARS Episode IV: A New Hope, which slipped into STAR WARS Chronicles, and then was expanded upon in the text of The Illustrated Star Wars Universe. The two production reference books are simply showing a bookkeeping error; the coffee-table book can be salvaged if it is reinterpreted so that the descriptions apply only to Alderaan's land areas rather than the entire global surface.

According to Obi-Wan Kenobi in the novel of A New Hope, Alderaan never had a moon. This has curious implications for some of the paintings in The Illustrated STAR WARS Universe, which seem to show an Alderaanian satellite in the sky. Obviously this object must be some kind of lightweight orbital reflector created and placed in orbit as an Alderaanian artistic whimsy, as a clean power source, for an atmospheric/environmental experiment, or as a skyhook space station.

The violent destruction of Alderaan is one of the most fascinating physical processes in the entire movie saga. Assuming that PAL video preserves every frame of the movie and does not repeat any frames, the characteristic speed of the outermost portions of the debris cloud is about 1.8 x 107m/s. This is 6% of lightspeed, or about 1600 times escape velocity. It follows that the energy of the explosion is up to 2.6 million times the gravitational binding energy threshold. The exact number depends on how the energy is distributed throughout the former planet's mass. This is only an upper limit because it's based on the most rapidly expanding (highest velocity) particles.

The explosion also gave rise to two flaming planar rings expanding outwards from the centre of the planet in what seems to be the equatorial plane. The rings moved at different speeds, but both were highly relativistic. A direct Newtonian interpretation of the motion of the rings is invalid; relativistic corrections are needed. The first ring moves at about 0.29c, and the second ring that overtakes and consumes it has a velocity of at least 0.91c. There is even an irritating possibility that it might be mildly supralight.

It is difficult to explain why this phenomenon should be planar, rather than expanding in a spherical front. The destruction of a planet is not the sort of thing that should have a preferred direction or plane. The rings are not aligned with the shot from the Death Star. Neither can the planet's spin be the determining factor, because the rotational energy of the planet is very much less than the magnitude of the kinetic energy of the blast.

Perhaps the effect really is spherical, but its light emission is not isotropic. In a particular observer's point of view, the beamed, directional emission might only be visible from a certain annular part of a generally transparent spherical structure. Put another way, from a given position and angle, light is received only from a ring because of the narrow range of angles at which the light is beamed from particular parts of the shell. However this model might not be consistent with the symmetry of the explosion; it may imply that different sides of the hypothetical shell concentrate their light in different directions relative to the centre of the explosion. Perhaps the beaming is concial rather than unidirectional? Perhaps a more subtle physical effect is at work, like a rainbow. This deserves further thought.

Alternatively, there may really be a special plane in the explosion that governs the ring aspect, but does not affect the rest of the flying debris. In the case of the similar explosions of the Death Stars, the special plane may be determined by the path travelled by the chain reactions leading to the final detonation, which probably is determined by internal regions of weakness in the station's structure. However a planet like Alderaan is more homogenous and solid than a Death Star. The spin axis of the object would be an attractive culprit, providing a preferred plane if only there were a way for small rotational effects to spontaneously grow into large effects within the explosion. Alternatively, the preferred plane may be determined by the nett angular momentum of the planetary or Death Star deflector shield, or some other shield characteristic.

Alderaan did have an operational planetary shield, as demonstrated by the fact that the initial part of the superlaser shot fragmented in the telltale fashion of a blaster ray striking an energy shield. Even before the bolt splintered, it caused a diffuse green glow over the entire globe (including locations without a line of sight to the Death Star) just above the atmosphere, which is a sure sign of a functional, albeit overwhelmed, shield. Layers of the shield may have circulated with angular momentum and/or rotational energy that was vast relative to the planetary spin.

See also: ⇒ Alderaan

* * * * * * * * * * * *
The planet Alderaan, seen in the instants immediately before its demolition. Note the greenish nibus surrounding the planet as the Death Star's weapon hits; this may be due to an interaction between the beam and the planetary shield.

* * * * *
Alderaan as it appeared on the tactical screen of the Death Star's firing control room, the “overbridge” [so named in Death Star Technical Companion]. There are two different views. One of them must have been transmitted to the Death Star from another location in nearby space.

Alderaan as pictured in the revised second edition STAR WARS: The Roleplaying Game rulebook. This image is consistent with the planet as seen in A New Hope, except that it was mirror-inverted. This scan has been deliberately flipped back to the correct handedness.

A heavily vegetated moon pictured in Anakin Skywalker: The Story of Darth Vader, and misidentified as Alderaan. It really is the Endor sanctuary moon.

Thrantas of Alderaan flying in the light of some sort of space object. Since Kenobi explicitly stated that Alderaan has no moon, the caption to this picture must be confused. Either the object in the sky is merely an artificial satellite, or else these are examples of thrantas transported to a different world.

* *
Incandescent rings emerging from the explosions of Alderaan and the first Death Star.


The surface of Dagobah, seen from high in the atmosphere or from orbit, appears to be nearly evenly covered in dark green jungle foilage and brown muddy lakes. The fact that the lakes are muddy is important. It means that the water is either too shallow to appear blue (perhaps less than a few feet deep), or else there are processes preventing the mud particles from settling to the bottom. Perhaps this is due to bottom-dwelling biota stirring the silt, or the mud may actually be living plankton-like life (or its organic remains).

Mud is usually made when water flows (especially rivers) erode fresh sediments off rocks, washing from elevated regions down into seas or other basins. The persistence of silt and mud in Dagobah's water implies that there are highland regions somewhere. Erosion destroys mountains within mere millions of years, unless mountain-building processes compete continually. This implies tectonic activity. However there are no obvious highlands on the observed parts of Dagobah's globe. There is at least one swamp stream near Yoda's hut, descending several centimetres into the lagoon. Was this creek part of a broad alluvial system originating in highlands (which force precipitation locally) or was this stream just a random gathering of condensation running off the nearby trees?

Surface of Dagobah as seen from near the top of the atmosphere.

The globe of Dagobah as seen from nearby space. [The Art of Star Wars: Episode V]

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Dagobah seen from space during Luke Skywalker's arrival during his first (adult) visit to the planet.

* * *
The horizon of Dagobah, as seen from Luke Skywalker's X-wing fighter cockpit.

* *
Dagobah as seen during Luke Skywalker's departure for Bespin. Yoda's home was experiencing night time when Luke launched; it may be close to the day/night terminator off the top or left sides of this image.


Sometime after the Battle of Yavin, Princess Leia, Luke Skywalker and their droids were stationed in a rebel base under the command of General Bob Hudsol. The base was buried in a large rocky fragment in the ring of a planet, which has not yet been named. The planet was probably close to Bothan territory, since Hudsol's SWCCG game card indicates that this was his region of authority.

Luke Skywalker took one of the base's Y-wings, designated Y4, to pursue Chewbacca and the Millennium Falcon, which were overdue from a vital mission. He tracked the freighter to “a moon in the Panna system”. Panna appeared to be a gas-giant planet with a spectacular set of moons, and the one chosen by Chewbacca has a hospitable climate and at least one significant city.

This “water moon” was almost entirely covered in oceans of a red gellatinous substance of unknown, though possibly biological, composition. Occassional green circular mats of up to several metres diameter on the surface of the mucus resemble the growth of some kind of simple algae.

The only rocky areas seen to date served as the foundations of a large city. It looks like an outcrop of vertical basalt columns standing several hundred metres above the mudline. Refuse and sewerage are laid indiscriminately at the base of this island, where populations of various large and small animals (including introduced dianoga pests) feast on the detritus.

The cosmopolitain city itself is enclosed within a canopy made of translucent bubble domes which are kilometres in diameter. The reason for this shell is not clear; the weather during Skywalker's visit was sunny and placid. Perhaps the moon is subject to episodes of violent wind and storms. Perhaps the moon's citizens suffer predation from flying animals, or sapient raiders in the air or space?

Neighbouring moons and the planet Panna itself loom in the green sky. An analysis of their dimensions and orbits promises to be an interesting future investigation.

Unresolved Questions:

* * *
During Luke Skywalker's first encounter with Boba Fett, at least four significant celestial objects were in the sky. One may be the primary planet Panna itself, but that isn't certain. The moon behind Fett, with an irregular terminator suggesting a rough surface, has a phase noticeably different from the rest, with important implications for its spatial location.

Two globes in the sky near the horizon behind the Millennium Falcon just prior to the arrival of Fett and Skywalker.

* * *
Exotic shelled city on the moon of Panna.

The rebel companions head for the stars, we gain a glorious view of several of the Panna worlds together. This image is a composite of three frames from the cartoon.


The forest moon of Endor, which apparently was considered a nature sanctuary, was originally the largest of nine satellites in orbit around a pale silvery gas-giant. The gas giant was present at the time of early Imperial surveys [The Illustrated Star Wars Universe], but was out of sight by the time of the Battle of Endor [Return of the Jedi novel]. The most likely explanation, if a Death Star test-firing is ruled out, is that the forest moon was originally in a weakly-bound, long-range orbit at the margins of the planet's gravitational influence. Outer moons of gas-giant satellite systems can be lost into independent planetary orbits and then recaptured again many times over. In a system with two suns [as shown in the Ewoks cartoon television series] the varying solar tidal influence may make this occurrence more likely, perhaps to the point where it happens often on human historical timescales.

The size of the Endor moon is neatly determined by an abundance of photorealistic and schematic images showing the moon in relation to the second Death Star. The ratio of the radii of the moon to battle station is consistently 11.5±0.1. Writing in CINEFEX in 1983, Richard Edlund reported that this Death Star was over five hundred miles wide. Therefore the absolute lower limit on the radius of the moon is (4.63±0.04) × 106m. Similarly, an upper limit of (5.55±0.05) × 106m can be imposed if we assume that the Death Star is less than six hundred miles wide, (because otherwise Edlund would have said “over six hundred miles”). The true size preferably should be in the larger end of the range, in order to ensure that the surface gravity is strong enough to hold a habitable atmosphere for an appreciable fraction of the moon's age. At the lower limit, the escape of a given mass of Endorian atmosphere takes much less than half as much energy for the escape of the same mass from Earth. Near the upper size estimate, the escape energy may reach something like two-thirds of the terrestrial value, allowing for a realistic range of density for the moon.

At either extreme of possible size, if the moon has a physically and chemically plausible composition, the surface gravity will be noticeably weaker than Earth gravity. The Endorian gravity is unlikely be greater than 8m/s² (versus Earth's 9.8m/s²) even if the moon is wildly enriched with heavy chemical elements. Light gravity may also be necessary to justify other evidence in Return of the Jedi, particularly the ability of rebel heroes to fall many metres from an ewok trap without breaking any bones, and the ability of an ewok combat glider to carry not only an adult ewok but also a load of rocks heavy enough to annoy an AT-ST. The enormous size of the Endorian trees and dangerous mega-fauna is directly attributed to the low gravity [The Illustrated Star Wars Universe].

Endor's topography is much flatter than that of Earth or Alderaan, but possibly more uneven than Yavin 4. There appears to be no distinction of continental and oceanic crust; the elevation of the surface is remarkably uniform and therefore should lack tectonic plate motions, as explained below. However the topography is not totally trivial. There do exist modest mountains or hills in the regions surrounding the Imperial shield generator station. Without active tectonic plates, the moon's mountains are probably built by regional upwellings and subsidence due to hot-spot volcanism. This in turn means that the moon must have molten interior, although the crust may be thick compared with the crusts of Earth or Alderaan.

The fraction of the globe covered with water is greater for Endor than for Yavin 4. The cloud cover also seems to be more extensive. Perhaps the Endor moon is less effective at keeping its moisture at ground level, or as uncondensed vapour. Endor is in the inner part of its solar system, is relatively well illuminated and therefore does not require a radically strong greenhouse effect or other unusual sources of warming. Therefore it is unsurprising that the role of water in the Endorian climate is very different from that of Yavin 4. Endor's meteorology must be more like that of Earth or Alderaan, except that Coriolis forces are stronger (due to the moon's faster rotation) and whatever differences arise from the near-uniformity of the Endorian terrain. (For instance the absorption and reflection of light, and the heat capacity and emissivity of the surface, are relatively constant over Endor's entire globe, rather than varying radically over different terrains as on Earth.)

Unresolved Questions:

Whole-disk view of the forest moon.

* * * *
Size and spatial relationship of the forest moon and the second Death Star is consistent between the rebel schematics and the actual long-range space view. The nature of the markings on the tactical hologram are uncertain; they may be contours of topography and boundaries of lakes.

* *
The shield generator base on the Endor moon was set in a valley surrounded by high hills, demonstrating that the moon does undergo at least some modest mountain-building.

Phases of the second Death Star and the santuary moon below it may allow the determination of the moon's rotation period, and the timing of events during the Battle of Endor.

Lord Vader arrives at Endor. Another brownish object is in the sky; it may be a minor sibling moon captured by the forest moon when it escaped the orbit of the gas giant.

* *
The illumination of the second Death Star and the sanctuary moon in these two frames indicates the time of day on near the Imperial bunker on the surface: probably sometime in the early morning.


The planet Yavin is the dominant gas giant in the Outer Rim system of the same name. Its exact basic statistics are given explicitly or can be derived from figures given in Galaxy Guide 2. Compared to Jupiter, it is about 2.79 times as massive and 1.35 times as wide. Its surface gravity is about 3.89 times Earth's, vs 2.54 for Jupiter. Large gas-giants that are more massive than Jupiter tend to be not much greater in diameter because the greater mass typically causes greater gravitational self-compression. Yavin conforms with this general rule.

Yavin's orbital period is 13.2 standard years, yet it is close enough to its sun to permit the existence of life and comfortable climates on several of its moons. The know orbital period and clement insolation limits the range of possibilities for the mass and luminosity of Yavin's sun. Greater mass implies greater orbital radius for a given orbital period, whereas the solar heating received by a body decreases as the inverse square of distance. This system is said to be about 7.5 billion years old [Galaxy Guide 2], which sets an additional upper limit on the sun's mass. More massive, luminous stars consume their nuclear fuel relatively rapidly, causing them to have shorter lifetimes than less massive stars. The upper limits on the mass an luminosity of Yavin's sun are 1.1 and 1.5 times solar values, respectively.

Visually, the planet Yavin is a baleful orange globe partly streaked with clouds that are slightly lighter than the general tone. The streaks do not seem to form well-defined cloud bands like those of Jupiter, although they do seem roughly aligned and each streak seems to stretch across a large fraction of the planet's circumference. The causes of gas-giants' cloud colours are not yet well understood, but it must have something to do with trace chemicals present at different altitudes. The surprising blandness of Yavin's face suggests that either the atmosphere is more homogeneous than Jupiter's, or else the uppermost visible layer obscures lower cloud levels completely. Perhaps there is a haze of photochemicals high above the windy, banded regions of the atmosphere, or perhaps the colouration has something to do with the presence of airborne life.

The orbits of four of Yavin's moons were shown on a tactical schematic in the Death Star's overbridge. If the central circle in this display marks the planet Yavin, the orbital elements of the four displayed moons can be determined or constrained. Orbital dimensions are calculated assuming that the central circle represents the planet's circumference. Determining the eccentricity depends on the fact that the planet is at one focus of each ellipse [Kepler's First Law of Plantary Motion]. Because of three-dimensional projection effects, it is somewhat to determine the actual orientation of the orbits. (The mathematical procedure involves measuring the shapes of the on-screen orbital projections and fitting them with ellipses with consistent dimensions and 3D orientation.)

moonea [m] a [RY]a(1-e) [RY]a(1+e) [RY] T [hr]
“a”0.7261.28 × 109 13.33.6322.9 134
“b”0.1111.14 × 109 11.910.613.2 114
“c”0.1542.12 × 109 22.018.625.4 286
Yavin 40.891.52 × 109 15.81.7429.9 174

e = eccentricity; a = semimajor axis; a(1-e) = periyavin distance; a(1+e) = apyavin distance; T = orbital period; m = metres; RY = radius of Yavin; hr = hours

Yavin 4 is unambiguously identified with the green orbit on the screen. For the sake of discussion, the other three displayed moons are given provisional labels according to the diagram above. In terms of mean distance to the planet, the four measured moons are “b”, “a”, Yavin 4 and “c” from innermost to outermost.

The moons of Yavin comprise a highly disturbed system. The orbital inclinations cover a great range of angles, whereas conventional gas-giant satellite systems [eg. those of our own solar system] are settled to within a few degrees of the giant planet's equatorial plane. The high orbital eccentricities are also remarkable. It is a strange, but not unrealistic configuration. Perhaps it is the result of complicated orbital resonances and severe mutual perturbations between the moons. Or perhaps it is a result of recent violent astronomical events in the system, maybe even due to Naga Sadow's experiments or the aftermath of the Sith War [Dark Lords of the Sith, The Sith War]. The cause is either strong and ongoing (stronger than the tidal forces that would regularise the orbits relative to the planetary spin) or so recent that corrective forces have not yet been effective.

* * *
The gas-giant planet Yavin, as seen in full-disk views and in a closeup of the cloudtops near the limb.

* * * * * * * *
Millennium Falcon passes Yavin, on course for the moon Yavin 4.

The Falcon's passage around Yavin, in the pre-Special Edition version of A New Hope. In this version the planet's and moon's crescents of illumination are difficult to interpret. Judging by the moon's crescent, we should see at least part of the dark side of the planet. This is not the case. Is the planet self-luminous at this moment? It may be a blooper. It was replaced in the Special Edition. Onscreen projected radii of planet and moon: 26.2 and 1859 pixels respectively. [Topps Widevision]

* * * *
The Death Star arrives in the Yavin system and heads towards the planet. In these frames the station is moving at a speed of a few tens of km/s relative to the camera. The camera's velocity relative to the planet is not measurable.

An impression of the Yavin satellite orbits accoring to Star Wars Technical Journal. It is worth noting, but the upper drawing doesn't exactly match the movie/canonical schematic, and the lower plan is not consistent with the orbital values (above) calculated according to Kepler's Laws. Furthermore, the Death Star's trajectory was highly eccentric.

Yavin 4

Several of the basic properties of Yavin 4 are well determined by tactical schematics provided to Grand Moff Tarkin in A New Hope. Some of the orbital elements are listed above in the commentary about Yavin. The semimajor axis (mean moon-planet distance) of the orbit is about 1.52 million kilometres, which implies an orbital period of 174 hours. Yavin 4's orbital eccentricity is very large (approximately 0.89, where the maximum is 1.0 for any bound orbit) and its closest approach is less than two times the gas-giant's radius. Within about 2.46 planetary radii (known as the Roche limit), tidal forces deprive a moon's surface of equilibrium. It is thought that major moons cannot form in such proximity to the planet, but fortunately Yavin 4 only spends part of its orbit in the extreme zone. However surface waters are likely to migrate significantly during the hours of closest approach.

In the astronomical future, the moon may be vulnerable to tidal disruption or collision with the planet. Forecasting the outcome would be difficult without more information about the interior structures of the moon and planet.

A later computer schematic shows the position of the moon behind the limb of Yavin as seen from the battle station's viewpoint. This image necessarily indicates the size of the moon in relation to the gas giant. Assuming the planetary radius stated in Galaxy Guide 2, the moon's radius must be just under 6.55x106m, which is slightly greater than the radius of the Earth.

Despite being of a size similar to Earth, Yavin 4 appears to have a very different surface and interior. The topography is very flat and simple. There are no large or distinct oceanic basins; the water is instead spread across the whole globe with the largest bodies being lakes and isolated inland seas.

For terrestrial worlds in general, surface elevation is mainly determined by underlying crust composition. The local density of rock determines the local thickness of the crust, as it floats on top of the mantle. In order to cause inter-regional variations of dozens of kilometres in elevation, for true ocean basins, the gross composition of the crust must be divided into continental and oceanic components with grossly different composition. Sections of crust with a light, granite-like composition ("sial") tend to be thicker, giving continental masses. Dense basalt-like crust ("sima", as in Earth's ocean floors), must be thinner to achieve the same buoyant equilibrium. The lack of relief on Yavin 4 implies that the moon's crust has a fairly uniform composition.

While water erosion reduces relief and fills basins with sediment, plate tectonic motions have the effect of collecting sial crust into discrete continental blocks. The absence of continental regions on Yavin 4 is a sign that tectonic plate activity is absent. On the other hand, the driving of plate tectonic motions may require the different thermal and buoyant properties of the disparate sections of a dichotomous crust. Since Yavin 4's crust is apparently not dichotomous, it probably couldn't sustain Earthlike tectonic activity regardless of internal heat sources. Like that of Venus or Mars, Yavin 4's crust must remain a single motionless piece.

Any uplifting (mountain-building) is probably restricted to hot spots where upwelling convection in the mantle enhances volcanic activity. Since a few hills were visible near the Rebel Alliance outpost, there must be at least some of this activity. The existence of convective motions in the mantle would require that the moon's interior be warmed by residual heat from the moon's formation, tidal forces, or radiogenic heating processes (decay of naturally radioactive substances inside the world). A terrestrial planet of this size has sufficiently small surface area compared to its volume to slow the escape of heat and maintain a large temperature difference between the interior and surface. Yavin 4's internal heat must be comparable to Earth's, because it is a similar size and probably a similar composition.

Tidal heating must be an additional significant contributor. Since the orbit of Yavin 4 is highly elliptical, the distance to the planet varies greatly in a matter of days. Tidal flexion of the moon's bulk is probably a major heat source in the moon's interior, and may be sufficient to drive volcanic activity regardless of other heat sources. Furthermore the moon's rotation is not synchronous with its orbit, which ought to cause minor daily tidal heating as well.

The most awkward fact of this system is that the constraints on Yavin's type of sun don't allow a planet with a thirteen-year orbit to receive more than about a twentieth of the flux of sunlight that falls on Earth. Although the human eye would readily adjust to this dimness, it poses a serious climatic problem. If all else was equal, Yavin 4 would have a mean effective temperature about a third of Earth's on the absolute scale, ie. far below freezing.

In order to keep the climate hospitable so far from the sun's warmth, Yavin 4 requires a remarkably powerful greenhouse effect. If this is the case then the atmospheric constituents are unlike those of Earth. Rather than existing only in traces, the greenhouse gases must be a major fraction of the atmosphere. The total air pressure may be much higher than standard; it may be useful to visualise this moon as a milder version of Venus moved to the outer solar system. Of course, oxygen must still be a key component of the air, otherwise the human rebels couldn't breathe.

If the chief greenhouse gas is carbon-dioxide, then a special explanation is needed to account for how it remains abundant in the atmosphere. In the presence of water, carbon-dioxide tends to be captured and laid down as carbonate minerals in sedimentary rocks. On Earth these rocks are eventually subducted as they ride the descending edges of tectonic plates, and this heating bakes out carbon-dioxide which is vented back to the atmosphere in volcanic regions. With no recycling of its crust, how does Yavin 4 avoid depleting carbon-dioxide from its atmosphere? Are the surface soils and rocks saturated with carbonates, so that no more is deposited? Does outgassing of carbon-dioxide from the deep planetary interior contribute enough to the atmosphere? Is the role of life in maintaining atmospheric equilibrium different on Yavin 4 than on Earth?

* * *
Death Star computer schematics displaying the moon Yavin 4 in relation to its primary, and the orbits of four of the moons. The faint orbit extending to the lower left is the one belonging to Yavin 4. The second image has had its proportions corrected; but the first image has not.

Final Yavin 4 firing schematic from the overbridge of the Death Star. The surface features are intriguingly regular. The background grid scale is unknown (and there is no certainty whether it is spatial or angular), but the points outside the moon's outline are probably background reference stars on the sky. [STAR WARS Chronicles]

The Yavin 4 image first published in The Art of STAR WARS Episode IV: A New Hope and misidentified as Alderaan in STAR WARS Chronicles. This painting was seen when the Millennium Falcon descended to the rebel base for the first time.

* * * * *
Millennium Falcon descends onto Yavin 4.

ILM artists working on A New Hope: Special Edition used the correct matte painting for Yavin 4.

ANH #113 widevision card from Topps. The surviving rebel pilots return to Yavin 4. In coordinates of this scan, the moon's centre is at (440.6, 73.3) and the projected radius is 71.4. The subsolar point is at (505.0, 51.0, 21.4) in scan pixel units, or (0.902, −0.312, 0.299) in normalised units.


According to Galaxy Guide 2, the radius of Bespin is 5.9 × 107m. In the tabulated data elsewhere in this page, the mass and surface gravity figures for Bespin are based on the assumption that the planet's bulk density is the same as water. This is a good estimate for a gas-giant intermediate between the masses of Jupiter and Saturn, but the existence of the Life Zone raises seerious questions about Bespin's composition. However if the planet is deemed to have an overall composition dominated by elements heavier than hydrogen and helium then the surface gravity would be much higher.

The orbital period of Bespin about its sun is 14yr. In order to receive a flux of sunlight consistent with what is seen in The Empire Strikes Back, and compatible with human comfort, the star must be significantly more luminous than Sol. Assuming that it is a main-sequence star, it must be significantly more massive than Sol, and must therefore have a shorter lifetime. Thus the system is almost certainly younger than Earth's, and the biological history of Bespin shorter than our own. In order to enjoy an Earthlike amount of sunshine, Bespin's sun would have to be 2.9 solar masses, almost seventy times as luminous, and would have a total lifetime of about four hundred million years.

However a large gas-giant doesn't only rely on sunshine for warmth. Giant planets have a formidable amount of interior heat leftover from the planet's formation, and the rate of heat coming up from the interior can be comparable to the heating from the sunlight above. Therefore the existence of a warm, habitable zone in the atmosphere is not a problem, unlike a wholly solid world like Yavin 4. Matching the illumination seen on Cloud City is the only argument for a massive, short-lived, luminous star; climate is not a constraint.

In the scene showing Luke Skywalker's arrival at Bespin, the left-handed vector for the sunlight direction was was (0.057,0.906,0.420) in terms of the movie image. Since Bespin's brownish clouds show parallel bands aligned with the planetary rotation, the planet's rotation axis is also observable. Taking the dot product of these two vectors yields a lower limit on the sine of the planet's obliquity: ie. the angle between the rotation axis and the orbital plane. Using this method, the obliquity of Bespin's rotation is not less than 21±3°. Thus the polar regions of Bespin experience very long periods or perpetual daylight and perpetual night during summer and winter seasons respectively. Cloud City, on the other hand, is near the equator and its daylight has nearly constant duration throughout the year.

According to Galaxy Guide 2, Bespin's two major moons are H'gaard and Drudonna. They orbit in the same plane, which is usual for major natural satellites of a gas-giant, in close orbits. From the surface of Bespin they appear to have roughly the same angular size. The orbital period of Drudonna is half that of its partner, which by Kepler's third law constrains the orbital radii to be in the ratio 1:1.59. They are said to appear together in the sky for one week every three standard months: five days per 105 days. This means that the orbital period of H'gaard is about 105 days. Using the above assumption about Bespin's density, the semimajor axis of H'gaard's orbit is 4.9 × 109m, and Drudonna's is 3.1 × 109m. These distances are 83.5 and 52.5 planetary radii, which is a distant but reasonable range for the formation of natural satellites of a gas-giant (cf. the moons of Jupiter and Saturn).

H'gaard and Drudonna are also said to be only 2.5km and 5km wide, which is almost a thousandfold too small for major satellites. Something is wrong with these statistics. Moons of this size would not be visible to any unaided eye on Cloud City. Furthermore, if these moons are to appear the same angular size in the sky, the ratio of their true sizes must be close to 1:1.59, not 1:2.

Neither of these two moons was seen in The Empire Strikes Back, but another moon was visible closer to the planet. It's projected position puts it at an orbital distance greater than 2.95 planetary radii. Its apparent radius is 0.08±0.02 planetary radii, but this is an upper limit because the moon appears very bright and over-exposed and therefore lens flare may increase its perceived size.

Despite its Jupiter-like dimensions and rotation period (twelve hours), Bespin has quite a distinctive appearance. As viewed from a long distance, it is uniformly beige, rather than having bands of white, red and brown clouds. Yet it looks more stormy and turbulent than consistently coloured worlds like Saturn, Uranus, Neptune and Yavin. The colouration of gas-giant planets is not very well understood and it probably depends on subtle trace chemicals in the atmosphere. Since Bepsin is a relatively warm and and bright world which boasts indigenous life, the chemistry of its atmosphere must be significantly different from any conditions found in Jupiter. Indeed Galaxy Guide 2 states that the colour of the clouds is given by airborne algae known as "pinks". That suggests that the life zone has the highest visible cloud layer, and the higher levels of the atmosphere are clear.

The life zone is 30km thick and is 150km from space, although it is not clear how "space" is defined in this context. It is also written that Cloud City uses no life support or environmental control. Since Luke Skywalker did not suffocate or die of gas poisoning when he hung from the city's underside, it can be assumed that this level of the atmosphere contains a breatheable concentration of oxygen, and a remarkable absence of the noxious gases and vapours commonly found on gas-giant planets. It is unknown whether the whole atmosphere contains appreciable amounts of free oxygen, or whether the life zone a special layer of air that is far from chemical equilibrium with the layers above and below, maintained by the activity of living organisms.

Aside: Taloraan

Taloraan is another colonised gas-giant, with Tibanna mining and a city-platform similiar to but smaller than Cloud City. It is shown in the Rogue Squadron computer game as a world with coloured bands like Jupiter, and yet the streets of its city are open to the air, implying that there is a Bespin-like life zone. If the life zone of Bespin is responsible for that planet's colouration, then the life zone of Taloraan must be of a different nature. Perhaps it sites at altitudes below the main cloud belts, or else may be restricted to within a single hospitable atmospheric band.

Unresolved Questions:

* * * *
The globe of Bespin as seen from space during the Luke Skywalker's arrival, at around midday on Cloud City. The bright object in the lower lift of the first image must be a moon; it is too bright to be a distant planet, and it is in the wrong direction to be the primary sun. It might alternatively be a distant binary companion to the prime sun, but such a star is not mentioned in the official literature.

* * * *
The night side of Cloudy City during the departures of Lord Vader's shuttle and the rebels aboard the Millennium Falcon, during Cloud City's dusk. [TESB movie frames; Topps' Widevision card]

Bespin's interior as drawn in Galaxy Guide 2. It is written that the pressure at the base of the liquid "rethen" layer puts the thrusters of a star destroyer to shame.

[anonymous rimward system]

The probot that discovered the rebel base on the sixth planet of Hoth launched from the star destroyer Stalker [named in SWCCG card]. This launch took place with three celestial orbs hovering in the background. The surfaces of each of these bodies appeared desolate and lunar.

On the larger/closer world there is a distinction between light terrain like lunar highlands, and darker brown terrain which might be similar to maria (basaltic volcanic floodplains). There is a lighter region near the upper edge of this world's visible face; perhaps it is a polar cap like those of Mars?

The name of the system is not known. Since there are supposed to be star destroyers in most sectors of the galaxy, the system is probably within the same sector as Hoth, perhaps only a few hundred light-years away.

* * * * * * *
Apparently barren celestial bodies in a remote system somewhere near the Hoth system.


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