A while back, I wrote a blog post about Pandora, the gas giant moon that is the setting for Avatar, the visual masterpiece by James Cameron.
To celebrate the opening of Avatar 2: the way of water this week, I made a slideshow to give a little more astrophysical context to Pandora. And, like the movies, it’s all about the visuals — just click through the slides, like you’re sliding down a waterfall, with a gas giant hanging in the sky…
Additional Resources
- Real-life Sci-Fi Worlds: a series of blog posts at the intersection of science and science fiction
- Pandora (from the movie Avatar), the habitable moon of a gas giant planet (older post)
- The Dune planet Arrakis
- Kalgash, a planet in permanent daytime (from Isaac Asimov’s Nightfall).
- The hot Eyeball planet
- Black Holes, Stars, Earth and Mars: my astronomy poem book (just in case you need to buy a present for someone who loves both astronomy and poems!)
PLANETPLANET 
Neat summary! However, I think there is one small error. If the planet is around the size of Jupiter, and presumably the mass of Jupiter, then a 15-day orbit corresponds to an orbital radius of 1.75 x 10^9 m, or 1,750,000 km. At that distance from Polyphemus, and with a 29 degree orbital tilt to the plane of the orbit of Polyphemus, the configurations shown in the “Seasons” slide (i.e. the solstices) would not create eclipses for Pandora. It would experience the eclipses during some of the year around the equinoxes, but not every day.
Depending on the mass-radius relationship the Polyphemus follows, it seems like it would have to be well into the Neptunian regime in order for a 15-day orbit with a 29 degree obliquity to always experience eclipses.
Hi Josh — You are right, eclipses shouldn’t happen every time if Pandora’s orbit is aligned with Polyphemus’ equator. They should be seasonal, like you suggest. Thanks for catching that!
I’m not a fan of Avatar, but i find this topic super entertaining. That was a nice read!
How does one calculate the eclipse period of a moon? I’d like to play with numbers for a double-planet system I invented.
The orbital period calculation is pretty straightforward – you just use Kepler’s third Law that relates the distance between the planet and the moon and the mass of the planet to calculate the orbital period. In the above comment, I just did that in reverse to calculate the distance between the planet and the moon. You can likely assume that the moon’s mass is much less than the planet’s mass to make the calculation simpler.
The question of how often eclipses happen is tricky – it depends on the orientation between the orbital plane of the moon around the planet and the orbital plane of the planet around the star. So if the angle between the two were zero degrees (i.e., the moon was orbiting the planet in the same plane as the planet orbited the star), then the planet and the moon would eclipse every orbit of the moon. If the angle were 90 degrees (i.e. the moon was orbiting the planet perpendicular to the orbital plane of the planet around the star) then there would only be very few eclipses right when the orbital plane of the moon passed through the star (twice a year). For angles between zero and 90, you have to consider the geometry of the moon, planet, and star and see when you get the proper alignment over what fraction of the year. And it gets even more complicated if either the planet’s orbit or the moon’s orbit is eccentric (non-circular).
Great exercises for high school or college physics classes!