Is Earth's moon the only one where a total eclipse of the sun covers the entire sun?
Are any other moons the same size as the sun as viewed from their planet like Earth's moon?
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2$\begingroup$ Welcome to Astronomy. I assume you mean a terrestrial planet? I don't see how you could define where the sun would be viewed from either a gas- or ice-giant planet (Jupiter, Saturn, Uranus, Neptune). $\endgroup$– Bob516Feb 4, 2020 at 15:38
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2$\begingroup$ The radius, and hence the location from which to view an eclipse, of a gas planet is somewhat arbitrarily, but quite well, defined as where the pressure equals 1 bar. $\endgroup$– pelaFeb 4, 2020 at 16:21
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6$\begingroup$ @pela Pretty sure teh OP just wants to know if any other planets' moons subtend a solid angle greater than the sun does. Part Two might be whether, or how often, those moons pass into an occulting position. $\endgroup$– Carl WitthoftFeb 4, 2020 at 18:32
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3$\begingroup$ Fun fact: suppose you hold up an object -- a quarter, say -- at such a distance from your eye that it exactly covers the Moon. Say, arm's length. At that moment, the ratio between the length of your arm and the size of the quarter is the same as the ratio between the distance to the Moon and the size of the Moon. That is, if you made a model of the Earth and Moon where the Moon was a quarter, the distance between the model Earth and model Moon would be arm's length. $\endgroup$– Eric LippertFeb 5, 2020 at 1:15
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3$\begingroup$ It's worth mentioning that, as the earth moon distance increases over time, even if the apparent sizes of the sun and the moon are similar now it has not always and will not forever be the case. $\endgroup$– armandFeb 6, 2020 at 0:24
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1 Answer
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As planets get farther from the Sun, the Sun takes up a smaller part of the sky. The Sun is about 31 arc-minutes when viewed from Earth, but just 6-7 from Jupiter and 3-4 from Saturn. Less than 2 from Uranus and about 1 arc-minute from Neptune, not much bigger than Venus appears from Earth when Venus is visibly large in the sky and when Venus transits the Sun, viewed from Earth, its size is about 1 arc-minute, about the same size that the Sun appears from Neptune. To get a sense of what size the Sun is from Neptune, look at a photo of a Venus transit.
Jupiter has 5 satellites capable of creating a total eclipse, the 4 Galilean satellites and the tiny but close Almathea, which is irregularly shaped, so it wouldn't be the same size as the Sun.
Saturn has 6. All are larger than the Sun when viewed from Saturn.
Uranus, because the Sun is quite small that far away, has 12 moons that can create a total eclipse. All 12 are considerably larger than the Sun in the Uranus' sky but because of Uranus's nearly sideways axial tilt, solar eclipses are rare and only happen at the midpoints in its orbit every 42 years.
Neptune has 7, but due to its axial tilt and Triton's off-equator orbit, eclipses are rare.
Io probably casts the largest shadow. Though Pluto and Charon cast a shadow over the largest percentage of the other object, about once every 120 years. Two of Pluto's smaller moons, Nix and Hydra are large enough to block the Sun completely though eclipses from them are probably rare.
As far as I can tell, having checked, our Moon appears unique in our solar-system in being a nearly perfect fit over the sun.
Further reading if interested, though there's some variation in the answers, they only mention 4 of Jupiter's moons.
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2$\begingroup$ I wonder whether Jupiter's moon Callisto (the farthest one that can cause an eclipse) is about as big as the Sun from Jupiter? $\endgroup$ Feb 5, 2020 at 13:38
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3$\begingroup$ "not much bigger than Venus appears when Venus is large in the sky and when Venus transits the Sun" I assume here, you are referring to Venus as seen from Earth? $\endgroup$– IzzyFeb 5, 2020 at 14:25
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2$\begingroup$ @user30007 Callisto appears larger somewhat larger from Jupiter than the Sun. If Earth's Moon was where Callisto is, then it would be about the right size. Callisto is larger than our Moon. $\endgroup$– userLTKFeb 5, 2020 at 16:54
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2$\begingroup$ @Izzy I edited the answer to make it more clear. $\endgroup$– userLTKFeb 5, 2020 at 16:54
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$\begingroup$ Religious apologists often use the fact that the sun and moon appear to be exactly the same size from Earth as some sort of proof of divine intelligent design. $\endgroup$ Feb 6, 2020 at 20:41