Neil F. Comins

As a science fiction film Avatar contains phenomena, such as floating mountains and chains of rocks rising upward, that defy the laws of nature. These we accept by suspending our disbelief and enjoying the “ride” on which the movie takes us. For the most part, however, it appears that writer and director James Cameron was trying to create a scientifically plausible world and by and large, he did. Nevertheless, the relationship between the moon Pandora and its mother world Polyphemus is portrayed incorrectly.

Visually, Polyphemus is a “gas giant” planet similar to Jupiter or Saturn. The gravitational attraction of such a world creates tides on the spherical moons orbiting it that force these moons into synchronous rotation. This is the condition in which a moon rotates (spins) at the same rate that it revolves around (orbits) its planet. Earth, by the way, did the same thing to our Moon, creating tides of molten lava on the Moon when it was first forming. Indeed, all the spherical moons in normal orbits in our solar system are in synchronous rotation around their respective planets. Pandora would be in synchronous rotation around Polyphemus.

The consequence of being in synchronous rotation is that, as seen from any place on the moon, the planet never moves in the sky. Imagine, for example, that you were on Pandora with Polyphemus directly overhead. Set out a lawn chair and get a glass of your favorite libation to sip through your facemask. Sit back and watch Polyphemus for, say, a month. During that time, the planet will not budge in the sky. Day and night, it will be in the same place. If you did this at any other place on Pandora where Polyphemus is visible, say where it is visible on the eastern horizon, you would see the same effect, namely that the planet would not move from there.

The action on Pandora takes place in a region of the surface spanned by a few hundred “klicks” (military slang for a kilometer, which is just under two thirds of a mile). Therefore, from all places in the storyline, Polyphemus is within a few degrees of being in the same place in the sky. This is the first astronomy error in the movie: sometime Polyphemus is shown well above the horizon, while at other times it is on or partially below the horizon. If the story line ranged over, say, the entire half the surface of Pandora facing its planet, then seeing Polyphemus at different places would be plausible. But that would obviate the justification for destroying the tree that was over the “closest” source of unobtanium.

The second astronomical error was that Polyphemus changed size significantly in Pandora’s sky. Well, one might argue, the moon could have a very elliptical (egg-shaped) orbit allowing the apparent size of the planet to vary as shown in the movie. This couldn’t happen if Pandora is to sustain life. As a moon changes distance from its planet, the tide-generating gravitational tug on the moon by the planet changes dramatically. The changing strength of the tides forces the moon’s entire shape to alter – the farther away the moon is, the more spherical it becomes; the closer it is, the more egg-shaped it is. The result of the internal rock rubbing against itself as the moon changes shape is the generation of enormous amounts of heat. Pandora’s elliptical orbit would lead to so much heat created inside the moon that its surface and most of its interior would be molten rock. No life-supporting Pandora for you.
The half error (in that verifying it would require a set of geometric calculations based on a screen shot I don’t have available) was presented in the first image of Pandora and Polyphemus together. We saw the planet span the screen while the moon was a relatively small body between us and Polyphemus. The geometry of that image suggested that Pandora is so close to its planet that the tidal force from Polyphemus would be great enough to tear the moon apart. You can read more details of all of these issues and much more in my upcoming book What if the Earth Had Two Moons. The science of Pandora and Polyphemus is presented in Chapter 2 of the book. After reading the book, you may also have questions about night on Pandora.

(c) 2010 by Neil F. Comins

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