It is amazing when you think about the coincidence of the Sun being 400 times bigger than our Moon, yet 400 times farther away, producing an almost perfect fit for a total solar eclipse. Is there another planet whose moon(s) would fit perfectly over the Sun, or are we just the lucky ones?

By my count, 35 other moons can hide the Sun’s disk completely as seen from their planet’s surface. Topping the list is Pluto’s Charon, which looms 200 times larger than the Sun in Pluto’s sky. Next is Neptune’s Triton, 27 times larger than the Sun in Neptune’s sky. Other whopping-big moons include Neptune’s Despina (17×) and Uranus’s Ariel (15×). Jupiter’s Galilean moon Callisto can appear just 1.3× larger than the Sun when low in the Jovian sky.

Weird moon Prometheus
The small Saturnian moon Prometheus, is 74 miles (119 km) long. NASA's Cassini orbiter snapped this image on December 26, 2009, from 36,000 miles (59,000 km) away.

The only moons that seem to be capable of a near-perfect fit are Saturn’s Epimetheus, Prometheus, and Pandora. Each has an irregular shape, so a total eclipse by one of them is not likely to show the Sun’s corona in all its glory, nor the beautiful array of prominences and Baily’s Beads that we often behold from Earth.

Many other moons in the solar system would always appear smaller than the Sun’s disk. Although the size of Neptune’s Nereid is still poorly known, when near the low point of its highly elongated orbit it might cover up to 80 percent of the Sun’s diameter. And Mars’s Phobos is wide enough to span 69 percent of the Sun’s diameter when Mars is at aphelion — but for less than 10 seconds as this tiny moon glides by!

Yes, we are very lucky.

— Roger W. Sinnott


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September 3, 2020 at 5:04 pm

Actually, the Sun can be covered much closer to perfection by Galilean moons as seen from fellow moons seasons of mutual events.

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Roger W. Sinnott

September 3, 2020 at 7:46 pm

Good point! The Sun's angular size, seen from the vicinity of Jupiter, is about 0.10°, and the outermost Galilean moon, Callisto, when viewed from Jupiter itself, appears almost half again larger than the Sun. But if, instead, you could view Callisto from Ganymede at a time when they were nearly on opposite sides of Jupiter, you'd see Callisto subtending just 0.09°, a tad smaller than the Sun. So there can be rare occasions (rarer than total solar eclipses seen from Earth) when Callisto and the Sun are very nearly matched in size as seen from another Jovian moon. -- Roger

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