In the wake of the total eclipse of the sun a few weeks ago, I remembered a day-dream of mine.
One of the things I do is to mentally elaborate on other people’s fiction. Occasionally the result is fan fiction (mostly short, and I haven’t published any of it yet), but more often I’m just thinking about how stuff works and what stuff means. It’s one of the ways I play, an enjoyable exercise for my brain.
Case in point–the movie, Avatar. For those not in the know, the basic idea of this film is that humans have a mining operation on another planet, called Pandora, which is populated by a very vaguely humanoid species of very tall blue people, called the Na’vi. The humans are treating the Na’vi very badly. A human man who came to Pandora as part of its private security force ends up joining the Na’vi and helping them fight back. The real high point of the movie is its CGI effects, which are gorgeous.
But let’s leave the plot and the characters alone, for the time being, and look at physics.
Introducing the Game
Pandora isn’t really a planet. This is never mentioned in the movie, but is made obvious by certain shots–it’s clearly a moon orbiting around a Saturnlike planet. In our solar system, our gas giants orbit very far out, but astronomers have found systems with gas giants near their stars. Apparently, Pandora is part of such a system.
The only other things we know about Pandora is that it is slightly smaller than Earth and has a thicker atmosphere that is dangerous to humans, that many of its animals and plants are much larger than those of Earth, that many of the plants glow in the dark, especially when you touch them, and that the biosphere is sentient and named Eywah.
The game I want to play involves attempting to explain these details without violating either the information provided by the movie or the known facts of how the universe works.
For example, how can the atmosphere be thicker than ours, even though the gravity is weaker? If Earth’s atmosphere were applied to a smaller planetary body, the lesser gravity would allow the atmosphere to expand and to thin, like a sponge released from compression. Yet Pandora’s atmosphere is thicker. How?
Obviously, it’s atmosphere isn’t like Earth’s. Either the proportion of gasses is different or there is an additional gas, but either way, the mix is heavier. If a different, heavier, non-reactive gas replaced some of the nitrogen, for example, the air wouldn’t be toxic, but breathing it in might conceivably trick the body into developing a poor balance of blood gases. That’s close to what we see in the movie. So far, so good.
Ok, Now, Orbital Mechanics
Now that you know how the game works, let’s look at Pandora’s astronomy.
Just to keep things clear, let’s name the giant Hephaestus, since in Greek myth, he is one of the deities who created the woman, Pandora. Also, let’s refer to Pandora’s orbital period–the time it takes to go around Hephaestus–as it’s month, and the time it takes Hephaestus to go around its sun as the year of both Pandora and Hephaestus. Pandora’s rotational period is its day, as with Earth.
There are a number of interesting aspects of Pandora’s astronomical movements. Playing around with these ideas led me to look up things online such as the diameter of the orbit of Jupiter’s moon, Callisto, the relative weights of Earth and Jupiter, and what happens if you don’t breathe enough nitrogen. Weird questions are the best thing for finding out new stuff.
Two things seem quite clear, though: Pandora experiences very complex tides; and Pandora has very long eclipses.
Let’s ignore the tide, since the characters of the movie live nowhere near the sea. But the eclipses are interesting.
Jupiter’s major moons have orbital periods from almost two to almost 17 Earth days. Pandora is likely somewhere in that range, because Hephaestus in its sky looks similar in apparent size to pictures taken of Jupiter from among its moons. Pandora’s day is also probably similar in length to that of Earth, for the simple reason that none of the characters mentions day length being odd–and Earthlike days are only possible if Pandora’s orbit is close to its system’s plane of the ecliptic (where eclipses are possible).
So, if Pandora has a month about two weeks long, it’s going to spend a few of those days more or less behind Hephaestus. How much of that time would actually be in eclipse would depend on the distance at which Pandora orbits, since planets cast cone-shaped shadows (that’s why on Earth totality is only about seventy miles wide–we catch only the small end of the cone). But it does seem plausible that Pandora has regular eclipses that cover the entire world and last least a day.
The most dramatic scenario would be a solar eclipse that began before dawn and ended after sunset, such that once a month you’d get a 36-hour night. But if the eclipse began during the day, what would that look like?
What an Eclipse on Pandora Looks Like
On Earth, our total solar eclipses owe their special, weird character to a cosmic coincidence; from our vantage point, our moon and our sun appear to be exactly the same size. As a result, during a total eclipse, the moon blots out the entire sun, but not the corona, so we get a black hole in the sky ringed by ghostly white fire. And since we do only get the tail end of the cone of shadow, the area of totality is not very big. You can see a rim of sunset colors from inside the shadow, and the sky does not fully darken. Totality doesn’t look like night. It doesn’t look like anything but itself.
Pandora wouldn’t get those same coincidences. Hephaestus looks much larger than the sun, so the corona would be covered as well. I suspect that if the entire planet were in shadow, the sky would be as dark as night as well.
One thing would not be different, though–on Pandora, as on Earth, you don’t see the eclipse coming. You can’t see a planetary body that doesn’t have sunlight on it, so the approaching moon–or Hephaestus–is invisible. The day looks ordinary. And unless you have specialized glasses, you can’t even tell anything is happening until most of the sun is blotted out. Even a little sunlight is enough to light up the world. The air just gets a little cooler. You’re not sure if you’re imagining it.
Then the day starts to darken, like a storm is coming. Or maybe there is something the matter with your eyes. The air is undeniably cooler, now, and there is something definitely and dreadfully wrong with the air–it’s going dark, even though the directness of the sun, the colors, everything, still shows you it is mid-day. There are no storm-clouds. Power bleeds out of the light, somehow. The day has anemia. Even if you know intellectually exactly what is happening, the metaphors are undeniable because the brain will accept every explanation for what it’s seeing except the right one.
You see the greater darkness coming–perhaps a cloud goes suddenly grey. Perhaps the land in this distance goes dim, if you have a good view in the right direction. Seconds later, it’s upon you.
You look up. It’s safe to do that now, the sun is gone, it can’t hurt your eyes anymore. You see the blackness, the ghostly white fire.
But because this is Pandora, not Earth, the black circle is not the size of the sun–it’s blotted out almost a quarter of the sky’s width. The far edge of it is indistinct, but the near side appears as a giant, black bow. The ghost fire breathes out from the side of the bow, silent, but it looks as though it should be howling, and then, after a few more minutes, it, too, is blotted out. The bow keeps moving. The sky darkens. You can’t see any hint of daylight on the clouds anywhere anymore. It’s still getting colder.
In a a few hours, the entire planet is under totality. The sky is as black as night, and all the stars are out, except that there is a hollow place, a black circle, which has no stars. If one of the other moons is in the sky, you might see it go behind the black circle, in which case you get multiple kinds of eclipses at once. Hours pass, and the circle with no stars gradually sets. You sleep. You wake. It’s still dark. The circle with no stars rises again. Then, gradually, the sky begins to brighten. All but the brightest stars are washed away. Sunrise, of a sort, blossoms on distant clouds, and the ghost fire leaps forth like a jet, like a flower, like the arc of a crown. Look away now, because the sunlight is about to leap out, and could blind you. A minute more, maybe two, and it is broad daylight. Give it ten or twenty minutes, and everything will look normal again, unless you have glasses. An hour after that, and even glasses couldn’t show you the monster that ate the sun.
You can expect this every month.
The Inconvenient Truth of Pandora
If you are a plant on Pandora, you have a problem; insects can eat you just fine in the dark, but insect predators, such as birds, have trouble finding them. Echolocation is no good for insects sitting flat against leaves. So every month, you get munched on for up to 36 hours straight, while you sit in darkness, unable to photosynthesize. What do you do?
You glow–especially is something (that might be an insectivorous animal) touches you. By lighting up, you help insectivores to help you.
If you are an animal on Pandora, things are easier. Presumably, you’ve either evolved the ability to function in the dark, or you can take 36 hours off from feeding. You will have to alter your behavior patterns, though.
If you are of the Na’vi, you also have a problem. How you plan hunting, warfare, and travel depends on when these eclipses are going to happen and whether any of the other moons are providing light at the time. It’s not enough to know an eclipse is due every 14 days, you have to calculate when it will begin and end to the hour, and where the other moons will be. These kinds of calculations aren’t simple. You’re certainly smart enough, but learning complex math takes time and a certain type of mental training, which also takes time, and most of your people are too busy coping with an extremely complex environment to be available for any of that. What do you do?
Your tribe can nominate a single individual to learn the material, which is passed, priest to acolyte, down through the generations. Since the knowledge is critical to both hunting, war, and travel, this person can also be your chief. You also have another tribal specialist, the wisewoman whose job is to understand and commune with Eywah. What makes more sense than for these two, the secular and spiritual heads of your community, to be a mated pair? And since training must begin very young, a child of the couple is the natural successor of one, that child’s betrothed the natural successor of the other.
In fact, in the movie, plants do glow when you touch them, the war chief and the wisewoman are a couple, and their daughter is the future wisewoman and is promised to the future war chief.
The movie never tells us that the tribal war leader is also its mathematician, but it certainly seems plausible.
What’s the Point?
Do I think that all this was actually in the minds of the people who made the movie? No, I don’t think that, nor do I care–I’m not trying to be “right.” I’m just playing a game, and it’s not the kind of game you can win or lose. It’s just fun.
But I can’t play this game with all movies. Avatar is unusual in that thinking about the movie in this way yields implications that actually remain compatible with the movie. The Matrix, in contrast, has a premise that violates the Second Law of Thermodynamics, so if you think about the movie too deeply you inevitably lose the plot. Most movies, even most “science fiction” movies, have the same limitation that The Matrix does. They are like those tempting confections made of plastic for display. No matter good they look, they are ultimately unsatisfying. This is the distinction between ordinary science fiction, which violates reality for the sake of the story, and science-based fiction, which creatively augments reality, but never violates it.
Why do I write (and prefer to read) science-based fiction? Because I want to make you a cake you can go head and eat.