Do Total Solar Eclipses Happen at Other Planets?

With a total eclipse of the Sun coming up tomorrow (as I'm assembling this webpage), I have seen some discussion on social media about whether an observer on another planet in our solar system could ever see a total solar eclipse. I ran the numbers, and made this page to describe the results. To calculate this you need the distance of each planet from the Sun and of each moon from its planet, and you need the diameters of the moons, the planets, and the Sun. Most of these numbers I had previously assembled for my webpage on the relative pull of the Sun's gravity and each planet's gravity on its moons; I found diameters for the planets from NASA's planetary science webpage.

(Click on each figure below to view it twice as large in each dimension as is shown here.)

Above is a diagram showing the circumstances needed for an eclipse to occur. Planetary and lunar orbits are not circles but rather ellipses; this diagram is not to scale and the non-circularity of the orbits is exaggerated to make clear what can in reality be some very subtle effects. Any moon can pass between its planet and the Sun; what it looks like from the planet (ignoring the fact that the gas-giant planets from Jupiter to Neptune don't have solid surfaces on which an observer could stand) depends on how big the shadowed disk of the moon appears compared to the bright disk of the Sun (the photosphere). If the moon appears much smaller than the Sun in the sky, it just appears as a silhouette over part of the Sun's disk; this is called a transit rather than an eclipse. If the moon appears just a bit smaller than the Sun but passes directly in front of it, the silhouette of the moon blocks all of the photosphere except a narrow ring; this is an annular eclipse, sometimes called a "ring of fire" eclipse. This is the situation at the left of the diagram; note the dotted lines showing the relative sizes of the disks of the Sun (yellow) and moon (gray) as seen from the surface of the planet (blue). If the moon's disk is just a little larger than that of the Sun, the photosphere is completely blocked for a little while and we get a total eclipse, like that occurring on April 8, 2024. The obscuration of the brilliant photosphere allows the viewer to see the red/pink chromosphere, possibly with extensions called prominences poking above it, and also the ethereal pearly corona of the Sun's faint outer "atmosphere." This is the situation at the right of the diagram; again note relative apparent sizes of the Sun and moon indicated by dotted lines. Finally, if the moon appears substantially larger than the Sun in the planet's sky, the Sun will merely appear to "set" behind the curve of the moon rather than behind the planet's horizon. This is also called an eclipse, but (assuming there is no atmosphere to scatter light and blot them out) the chromosphere and corona would only be visible in part after the entire photosphere had "set."

This plot shows the average ratio of the diameter of the disk of each of a planet's moons to the diameter of the Sun's disk in the sky of each planet (taking the old-skool view and counting Pluto as a planet, because of its decent-sized moon Charon). The "dot" for each moon is to scale with its physical diameter for all moons over 200 km, with all other moons (down to 10 km diameter) plotted as if they were 200 km. In order to get an impressive annular or total eclipse, this ratio should be just about equal to 1; too low and you get a mere transit, too large and the Sun simply "sets" behind the moon. You can see that, of all the moons for which I plotted data, only Earth's own Moon sits right on the solid line with a value of almost exactly 1. Because the orbits of the Earth about the Sun and of the Moon about the Earth are not perfectly circular, you can get both annular and total eclipses at different times; I sketched these two sets of circumstances for the same planet/moon system in the diagram at top to emphasize that what you get each time depends on where the Earth and Moon are in their orbits, how near or how far from each other and the Sun.

The four Galilean moons of Jupiter (Io, Europa, Ganymede, and Callisto) are the large circles in that planet's stack of points in the figure above; note that the ratio is plotted at the center of the circle, so the value for the outermost of these, Callisto, is 1.48 despite the circle overlapping the horizontal line indicating exactly 1. Thus an observer at Jupiter's cloud tops would find most of the corona blocked by the moon rather than the full display that weather-lucky observers will marvel at tomorrow inside the path of totality at Earth. There are a few small moons of Jupiter, Uranus, and Pluto whose points lie near the value of exactly 1; however, smaller moons tend to be irregularly shaped, so even if the eccentricity of their orbits may cause them to appear the same size as the disk of the Sun in their planets' skies sometimes (I didn't calculate this, just averages), an observer wouldn't see a clean circular blockage of the photosphere like that produced by Earth's larger, nearly spherical Moon. So, in fact, Earth is the only planet in our solar system that can view the full beauty of the chromosphere and corona during a total solar eclipse.

However... in a planetary system like that of Jupiter with multiple moons, some of which are large enough to be nearly spherical and thus present nearly circular silhouettes when passing in front of the Sun, the distance between the two moons will change as they move in their orbits, and very rarely (since each "observer" moon is a small and rapidly moving target, compared to the planet itself) the distance may be just right so that one moon just blocks the Sun's photosphere as seen from the other, like our Moon does during a total eclipse. This is sketched in the diagram above (again, not to scale), with two moons orbiting a planet (orange). At one position in their orbits, shown in green, the moons are too close together and, as shown by the dotted sightlines, one moon blocks out much more than the disk of the Sun; at other positions in their orbits, shown in blue, the distance is just right relative to the diameter of the eclipsing moon and the distance and diameter of the Sun, so that the observer would see an Earth-like total eclipse. Again, this would be a very rare circumstance, but it could happen. My calculations (not shown) indicate that Callisto could see any of the other three Galilean moons with a disk the same size as that of the Sun, Ganymede could see Europa or Callisto similarly, and depending on orbital eccentricities Io or Europa might see Callisto thus; there are probably many other possibilities among the numerous moons of the outer planets.

This brings up another point altogether, which has nothing to do with orbits and diameters. A spacefaring civilization could arrange a "perfect" total eclipse any time they wanted to, by simply putting a spacecraft at the right distance from a nearly spherical moon; in fact, humans do this inside special telescopes called coronagraphs, where an opaque disk is mounted at just the right position to block the Sun's photosphere so the corona can be monitored continuously. I have seen total solar eclipses in 1976, 1991, and 2017 (I deeply regret that I will be unable to share this one with my family in 2024), and the utterly alien apparition that takes the place of the brilliant solar disk in the sky for a precious few minutes is unforgettable. However, what is perhaps even more marvelous is the way that the living biosphere responds to the eclipse, and no spacefaring civilization could synthesize that just by putting a ship at the right place in space. The light just before and after totality is the color of full sunlight, not merely gray like a cloudy day, but its dimness gives a silvery, otherworldly sheen to trees and grass. The air cools; birds and insects, confused, start to settle into their evening routines with sunset chirps and buzzes. I've seen at least a couple of science fiction stories in which this unique circumstance, of a total solar eclipse on the surface of a living planet, is an interstellar tourist attraction! Humans are certainly fortunate to be able to experience this. I wish safe travels and clear skies to everyone who is going to be trying to catch this opportunity!

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New 7 April 2024