Is our moon "special" for creating a total solar eclipse? Is it common or rather rare for a moon to create a total solar eclipse the way our moon does, with corona and such?

  • $\begingroup$ Related: physics.stackexchange.com/q/24588/2451 $\endgroup$
    – Qmechanic
    Sep 9, 2013 at 19:08
  • $\begingroup$ Are you asking inside our solar system our outside? Inside nothing will look exactly like our eclipse because the sun appears much smaller as does the corona. Outside our system we haven't found any moons yet...they will be very hard to detect. $\endgroup$
    – user6972
    Sep 10, 2013 at 19:05
  • $\begingroup$ @user6972 no i meant outside, or moons in general. $\endgroup$
    – kutschkem
    Sep 11, 2013 at 7:28
  • $\begingroup$ In that case we don't know. But we've found over 3,000 extra-solar planets and if we compare that to the 9 we know there are about 166 moons, meaning 30,00 planets with possible 55,000+ moons. Odds are good some can create a total eclipse. $\endgroup$
    – user6972
    Sep 11, 2013 at 17:23
  • $\begingroup$ typo..."meaning 3,000 planets with possible 55,000+ moons" $\endgroup$
    – user6972
    Sep 11, 2013 at 17:38

2 Answers 2


While the chosen answer isn't incorrect it doesn't really answer the question -- that it isn't that rare on other planets.

For a total eclipse you have to fall into the Umbra portion of the shadow. From this image you can see that the size of the moon and the distance from the sun as well all play an important role.


If we look at the planets and moons in our solar system (we don't know much about others outside our system) it breaks down like this:

Mercury/Venus: 0% -- no moons.

Mars - 0%, the moons are too small for a total eclipse you only get partial eclipses. The moons themselves experience total eclipses often when Mars blocks the sun. During some seasons this can happen everyday for Martian moons. Here's partial eclipse caused by Phobos passing in front of the sun taken by the mars rover Opportunity:

Mars partial

Jupiter, Saturn, Uranus and Neptune can experience total eclipses, since they all have substantial moons and the Sun appears small from them. Eclipses are most common on Jupiter, because its moons orbit in the same plane with the Sun. In fact Jupiter just had a total eclipse June 19, 2013 but it was not visible from Earth. There are many nice spacecraft photos that show dark shadows of the moons on Jupiter's disk. Unfortunately, since none of the giant planets have solid surfaces (all of their outer parts are composed of gas), one cannot stand on them and watch the eclipses. But their moons have solid surfaces and offer spectacular view. Moons of Jupiter experience eclipses once per each orbit, and those around Saturn can regularly see the Sun being eclipsed by Saturn's rings before and after the regular eclipse.

Here's IO causing an area of a total eclipse on Jupiter taken by Hubble telescope: jupiter

Pluto has large and close moon Charon that can eclipse its tiny Sun for more than an hour. Due to the large tilt of Charon's orbit and long year of Pluto, eclipses happen only for a couple of years per each century.

  • $\begingroup$ that's very interesting (also nice pictures). Does that mean moons tend to orbit the planets in a way that the shadow will fall on the planet sometimes? Does this have to do with (i dont know how to express this differently) how the planets got their moons? $\endgroup$
    – kutschkem
    Sep 11, 2013 at 7:52
  • $\begingroup$ Odds are higher if the moon's orbital plane is in the same orbital plane as the sun. There is no real rule about how moons tend to orbit that we have discovered. For more reading try en.wikipedia.org/wiki/Solar_eclipse and the "See Also" links. $\endgroup$
    – user6972
    Sep 11, 2013 at 17:35

The total solar eclipse is only possible because the Sun and Moon both subtend the same angle when viewed from Earth (about half a degree). Amongst planetary systems this is likely to be rare because it requires the moon to be at just the right distance, or conversely to have just the right radius.

Even now not all eclipses are total. Because the distances from the Earth to the Moon and Sun change continuously (because the orbits involved are elliptical), some eclipses are annular. Because the Earth-Moon distance is increasing this will become more common, and indeed if you wait half a billion years or so all eclipses will be annular.

Response to comments:

My answer assumes the apparent sizes of the moon and star need to be the same. Obviously the apparent size of the moon can be greater than the star, but I note that kutschken's question includes the requirement with corona and such. If the apparent size of the moon is much greater than the star we wouldn't be able to see the inner portions of the corona. It's because the apparent sizes of the Moon and Sun are similar we can see the corona right down to the Sun's surface, and this has been an important contribution to our understanding of the Sun's atmosphere. It is this that makes the eclipses seen Earth special, and such eclipses are indeed rare.

  • 2
    $\begingroup$ it's not about angular size of the moon but rather the position of the moon's shadow--whether it lands on the planet's surface or not. the angular size only comes into the question of how large the area is where a total eclipse is visible from the planet's surface. $\endgroup$
    – gregsan
    Sep 9, 2013 at 17:31
  • $\begingroup$ @gregsan: it's still necessary for the moon to appear at least as large as the sun is in the sky--there aren't any visible eclipses on the earth due to the ISS (though there obviously would have to be transits, that may or may not be detectible) $\endgroup$ Sep 9, 2013 at 18:17
  • $\begingroup$ I disagree. Annular eclipses do require the rare coincidence of an almost-perfect match in angular sizes between the satellite and the Sun. Total eclipses simply require that the satellite appear larger than the Sun, and that is relatively easy (just put it closer if it's small). Five jovian moons, for example, can cause total eclipses (known technically as occultations). $\endgroup$ Sep 9, 2013 at 18:28
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    $\begingroup$ @JerrySchirmer: re the ISS "eclipses", see universetoday.com/64776/… for an absolutely wonderful example! $\endgroup$ Sep 10, 2013 at 6:05
  • $\begingroup$ I took the "with corona and such" to mean that when you are in the area of the planet with a total eclipse shadow you can view the faint corona which extends millions of kilometers out from the sun. It doesn't necessarily require an annular eclipse. But you're right each eclipse will look different on different planets for a variety of reasons. $\endgroup$
    – user6972
    Sep 10, 2013 at 18:53

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