Say a terrestrial planet is orbiting two stars as if they're a single gravitational well.(I haven't worked out the distance yet as I still on the phone...) I don't need the planet to be habitable but it does come in full packages; moons and lots of it. So how would the spectacular event differs between ours and this? While you're at it could you do lunar eclipse too? The terrestrial planet is as five times as big as Earth and as for the pair of identical Sun-like stars I think they are cute!

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    $\begingroup$ Keep in mind that the spectacular solar eclipses we get on Earth are the result of an exceptionally unlikely set of circumstances, resulting in the Earth's Moon subtending almost exactly the same angle as seen in the sky from Earth as does Earth's Sun. That's a consequence of the relative sizes of the Moon and the Sun, and the relative distances from the Earth to the Moon and the Sun respectively. Insofar as I know, at least in our solar system, even with the abundance of moons around other planets, Earth is unique in this regard. $\endgroup$
    – user
    Dec 22, 2016 at 8:52
  • $\begingroup$ I started to answer this, and realized there are two many possibilities. We need the mass and luminosity of the two stars (so we can calculate the barycenter around which the planet orbits), orbital characteristics of the planet (inclined orbit? eccentricity? distance from the barycenter?), and characteristics of the moons (orbital inclination, size, distance, eccentricity). Needs specifics or it is too broad. $\endgroup$
    – kingledion
    Dec 22, 2016 at 13:59

2 Answers 2


There are numerous possible scenarios.

  • An annular eclipse of one star if its angular diameter is larger than that of the moon, or total eclipse if it isn't, with the other star providing daylight. Partial eclipse is possible whether the angular diameter of the moon is larger or smaller.
  • An annular, total or partial eclipse of both stars while one is hidden behind the other, looking similar to when there is only one star.
  • A partial eclipse of both stars while they are close together in the sky (i.e. the moon covers a part of each).
  • Or one star can be partly hidden behind the other, so that there is a total or annular eclipse of one of them while the other is only partly eclipsed.
  • An annular, total or partial eclipse of one star shortly after the first dawn or before the last sunset, so that the other star is still hidden, also looking similar to when there is only one star.

Expect to see less of the corona than on Earth. The stars are close together so it will be outshone by the other star when only one is eclipsed. When there is a total eclipse of both stars it should be visible, though the angular size of the moon probably isn't so neatly matched (as on Earth) so the inner part (or more) of the corona may be also hidden.

Lunar eclipse is more similar to those on Earth. An eclipse will happen when the moon is in the umbra of the planet's shadow as seen in the light from either star.

  • This shadow will no longer be circular, depending on how the stars are arranged. In other words, if one star isn't behind the other, the shadow will look like the overlapping part of two mostly overlapping (since the stars are close together in the sky) circles. It may be possible to see this dark shape on the moon, with illuminated parts on either side.
  • If the angular size of the moon is small enough, there can be a total lunar eclipse. Unlike on Earth, if the orbit of the moon is highly inclined this may start and end as a dark wedge moving onto the moon, due to the previous point.
  • Eclipses with one star behind the other are similar to those on Earth, except that whether total eclipse or annular eclipse of the moon is possible depends on angular sizes.

There will still be some darkening in the penumbra illuminated at least partly by at least one of the stars, as in the Earth's shadow on the moon.


Your moon would be there for the effects only for a while.

The stars orbit around each other, so the gravitational force on the planet and moon would change. That means that the planet's orbit isn't a ellipse; the planet will "wobble". The moon, in the planet's gravitational field, will "wobble" too - but because the planet wobbles around it.

Soon (a few years to millions of years, but most likely years), our moon would get wobbled too far off it's orbit; One of the following scenarios would happen:

  • Our moon gets way too far towards our planet; It crashes at a very small angle and part of it gets into space

  • It gets so close to our planet, it "uses" it as a gravitational slingshot and gets propelled into the stars or to open space

  • It starts to orbit closer and closer to the planet until it gets torn to parts due to the Roche Limit (the minimum distance to which a satelite can approach its primary body without being torn apart by tidal forces). It will maybe steal some of the water in the oceans, if any.

  • It starts to orbit farther... and farther... until it loses Planet's gravity and gets disintegrated by one of the stars, or slingshoted into space

  • It gets too far from out Planet; it will orbit as a planet around the binary star system

According to this planet simulator and many, many tries, scenario #5 is the most probable (67/80).

But until that happens...

Assuming each sun and the moon covers the same angle distance as here on Earth:

You'll have eclipses a bit (almost 2x) more often; The main change is that partial eclipses, eg. when at least a part of one sun is visible almost 10x more often against full (both suns covered) eclipses; Eclipses covering one sun only would be 2x more often in this binary system then in a "normal" one.

If the moon gets torn apart by the gravity of the planet, a spectacular view would occur: The planet would have it's own rings! Imagine half the derbis blocking the sun... and a few years later, crashing into your spaceship! Astonishing!

Helpful links, some used to calculate values:


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