How to find an eclipse around another planet

Last week, we talked about how moons in the outer solar system can see total eclipses as good as the ones we see on Earth. To recap, if you’re standing on one big round moon of a planet like Jupiter or Saturn, you can watch another big round moon just barely block out the Sun, the same way you can see the Moon just barely block out the Sun on Earth. From Earth, this is visible as one moon suddenly dimming as it enters the shadow of the other – one type of what’s called a “mutual event”.

Which combinations of moons can see perfect eclipses? Geometrically, it’s a bit of a complicated question. You need the right combination of size and distance.

Let’s take two of Jupiter’s big moons, Ganymede and Callisto, as an example. If they’re lined up next to each other on the same side of Jupiter, the closest possible distance between them is Callisto’s minimum distance from Jupiter (it’s “perijove”) minus Ganymede’s maximum distance from Jupiter (it’s “apojove”). This is 1.869 million km minus 1.072 million km, or 0.797 million km.

Their maximum distance is when they’re lined up on opposite sides of Jupiter. In that case, it’s Callisto’s apojove plus Ganymede’s apojove. This is 1.897 million km plus 1.072 million km, or 2.969 million km total.

Are these distances right for eclipses? Well, the Sun is 264 times bigger than Ganymede, so to see a perfect eclipse it also needs to be 264 times further away. In orbit around Jupiter, the Sun is anywhere from 816 million km to 740 million km away. That means that for Ganymede to perfectly block the Sun you’d need to be anywhere from 3.09 to 2.80 million km away from Ganymede. This just barely overlaps with the range of possible distances. So in principle, if Ganymede and Callisto are lined up with the Sun, while on opposite sides of Jupiter, while Jupiter is close to its farthest from the Sun, you could stand on Callisto and watch the Sun get eclipsed by Ganymede. Assuming you have a good spacesuit, of course.

There’s a few other combos that are good candidates. Here’s a chart for Jupiter’s four round moons:

And here’s one for Saturn’s seven round moons:

Uranus also has 5 round moons, but none of them are the right size to have perfect eclipses as viewed from the others. Neptune only has one (Triton). So that leaves us with just the candidates above.

So when is the next total solar eclipse in the outer solar system? All the big moons of Jupiter and Saturn orbit pretty much exactly around their parent planet’s equator, so mutual events only happen close to each planet’s equinoxes, when the Sun appears right over that equator. We’ve just missed Saturn’s equinox (in September 2025). But Jupiter’s next equinox is coming on December 16, 2026, and the mutual events have already started.

To find which ones produce good total eclipses, we can look at a table of upcoming mutual events for Jupiter’s moons, and cross check against WebGeoCalc from NASA’s Jet Propulsion Laboratory. Specifically, here’s the procedure I came up with, using as an example an eclipse by Io viewed from Callisto:

1. Use the “occultation finder” to find the exact times when Io is covering the Sun as viewed from Callisto. You can just search a few hours around each eclipse listed in the table, to account for the most up to date orbital info and corrections for the speed of light. (Note that the calculator uses an imaginary telescope at the center of Callisto.)
2. Use the “angular size” calculator to find the angular sizes of the Sun and Io viewed from Callisto during each eclipse. If Io appears bigger than the Sun, there’s a chance of a total eclipse. And if Io is less than 1.12 times the size of the Sun, the magnitude of that eclipse would be as good as eclipses of the Moon on Earth.
3. Use the “angular separation” tool in “sphere” mode. This gives you a negative number representing the angular overlap between the Sun and Io as viewed from the center of Callisto. Find this value for the remaining candidate events.

At this point, we know what the eclipse looks like from the center of Callisto. It probably won’t be perfectly lined up, but that’s OK: just like a total eclipse on Earth, totality might only be visible from a small part of Callisto’s surface.

Suppose a solar eclipse by Io looks like this from the center of Callisto:

We want to know if this eclipse is total somewhere on Callisto’s surface. Since Callisto is roughly a sphere, we get one Callisto radius worth of leeway in any direction. That needs to change our perspective enough so that Io appears to cover up that last little bit of the Sun.

Some geometry tells us that the amount that the angular change in an object’s position in the sky from two different observers is equal to the angle between those observers as viewed from the object. So:

1. We use the “angular size” calculator again to find the angular size of Callisto from Io and divide by 2, to see how much leeway we have.
2. If the overlap from step 3 plus the leeway from step 4 is greater than the angular size of the Sun from step 2, congrats! There is a total eclipse visible from somewhere on Callisto.

Using this methodology, we can see that the next good total solar eclipse in the Jupiter system is indeed by Io from Callisto: on November 16th, 2026 at 18:30:43 UTC, with a magnitude of 1.07 – a bit lower than next year’s eclipse in the Mediterranean here on Earth. It’ll be a little underwhelming compared to our eclipses: the eclipse will last only about 30 seconds, and the Sun will appear only about 20% as big as on Earth. But hey, at least there won’t be any clouds.