What would make an earth-like planet have an eclipse every day? does it need additional moons and what would happen if it had those? A Total Solar eclipse across most of the planet's surface

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    $\begingroup$ Solar eclipses, to my understanding, aren't likely to cover the whole daylight side of the planet at once. To do that, you'd need a moon that was a match for the size of your planet itself, and at that point you've got a double planet instead. $\endgroup$
    – Palarran
    May 1, 2017 at 16:15
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    – Secespitus
    May 1, 2017 at 16:17
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    $\begingroup$ ok, that's fine I don't really need it to cover the entire planet just most of it. my idea is that they have an eclipse cycle every day. $\endgroup$
    – Akil Ryan
    May 1, 2017 at 16:23
  • $\begingroup$ Same time every day? $\endgroup$ May 1, 2017 at 16:45
  • $\begingroup$ Month long day? $\endgroup$
    – House
    May 1, 2017 at 22:22

2 Answers 2


A solar eclipse is the shadow of the moon crossing the earth.

For this to happen at all the moon must be closer than the sun is big. Our moon is about the same angular area as the sun, so it can block it. If the sun was bigger or closer, or the moon was smaller or farther away it wouldn't work; there could be dimming but the sun would peek out around the sides. So you need either a small distant sun or a huge close moon.

With a moon and sun of proper sizes at proper distances the next hurdle is getting in line. This happens when the orbit of the moon around the earth takes it through the ecliptic while it is on the sun side. To make this easier we'll assume the moon's orbit is in the same plane as the ecliptic.

Now how often can there be an eclipse? Once per lunar orbit. That's bad, our moon takes a month to orbit us, so yours will need to be much much closer. In fact we have a name for the orbit that takes one day, geostationary, and it means a satellite stays over the same location on earth, never seeing the far side.

If my math holds a moon would only need to be 35 miles across at geostationary to look like our moon. It probably wouldn't stay there long and it getting there in the first place is a bit of a stretch. related on astronomy.se. And it would only cast a 35 mile diameter shadow, so not really most of the planet.

But now what if we change the problem a little? The earth is in lunar-stationary orbit, and is bigger than the moon so the eclipse would be over the whole (facing) surface. And if we handwave the moon into an orbit in the ecliptic it will have a lunar eclipses every lunar day. Unfortunately that's still only every about 1000 hours.

I'm going to guess 28 moons wouldn't work either. They would be a nightmare to actually math out, but they would weigh about a third as much as the earth, almost certainly would be unstable and would very certainly be doom once they stopped being perfectly balanced.

  • $\begingroup$ I don't think ecliptic means quite what you used it for in the second to last paragraph. $\endgroup$
    – user
    May 1, 2017 at 17:41
  • $\begingroup$ @MichaelKjörling No, it does. It's referring to the moon's orbital inclination with respect to the ecliptic. I think that your answer would be greatly improved if you were to add a few pictures to illustrate your point. There are lots of good simple ones out there. $\endgroup$
    – Phiteros
    May 2, 2017 at 1:42
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    $\begingroup$ @Phiteros notstoreboughtdirt has apparently clarified that point considerably between my comment and yours (which was the aim of my comment). See revision 1 for the wording I was commenting in relation to. $\endgroup$
    – user
    May 2, 2017 at 4:44

What you need is a gas giant. Rather than having an earthlike planet, you have an earth sized moon of a gas giant with an incredibly fast orbit (that takes twenty four hours. This would allow you to have the gas giant cover the sun for a large portion of each day, causing an eclipse.

If this doesn't work, use a large, close moon with a forty-eight hour orbital period. This will give you a total eclipse over much of the planet once every day.

  • $\begingroup$ An Earthlike planet in such an orbit around a gas giant may be within its Roche Limit. $\endgroup$ May 1, 2017 at 17:07
  • $\begingroup$ Good point, hadn't thought about that. Would have to put it just outside the Roche Limit. Don't know whether that would allow for a twenty-four hour day though, unless the earthlike moon isn't tidally locked, which seems unlikely. $\endgroup$
    – Gryphon
    May 1, 2017 at 17:09
  • $\begingroup$ Wouldn't a twenty-four hour orbit be a geosynchronous orbit? You'd need an orbit no more than two days long, but exactly one day would simply black out part of the planet permanently with probably disastrous consequences for life. Regardless, I think the point is irrelevant; that's definitely going to be within the Roche limit, and something is going to get torn to rubble before long. $\endgroup$
    – Palarran
    May 1, 2017 at 17:16
  • $\begingroup$ Thanks, edited question. The Roche Limit is the problem here. Maybe I would have to go to multiple moons, and figure something out with that. $\endgroup$
    – Gryphon
    May 1, 2017 at 17:18

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