I'm thinking of a planet with one side too hot, and one too cold due to always facing its star. With the planet having a small ring, a "twilight zone" which may be habitable, I was wondering if a moon with some kind of orbit can somehow give this zone a consistent day/night cycle.

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    – JDługosz
    Commented May 15, 2017 at 4:39

3 Answers 3


The problem with a tidally locked planet is it can't have a big moon orbiting it, and as the previous answers mention it, it would cast a tiny shadow.

But think of this : on your twilight zone, you have 2 sides : one side closer to the sun, when the sun is actually really low on the horizon (always kind-of day), and one side further, with the sun a bit below the horizon (always kind-of night).

Now, if you allow the planet not to be quite totally tidally locked, you can have it slightly oscillating (something like a span of a few degrees). Maybe in a few million years the planet will be effectively tidally-locked, but today is not that day, it's not rotating anymore but still oscillating. This way, the sun-close side of the ring of habitability have a long day / short night cycle, while the sun-away side has a short day / long night cycle.

Of course, it's not really day/night, but more of a dusk/dawn. Think of the course of the sun on the poles, but with day/night cycle of a few hours instead of 6 months.

  • $\begingroup$ You mean, oscillating like our Moon? That's totally feasible, we can see this happening. $\endgroup$
    – Mołot
    Commented May 15, 2017 at 9:15
  • $\begingroup$ Exactly, with a bit more amplitude though (our Moon libration is really tiny) $\endgroup$
    – Keelhaul
    Commented May 15, 2017 at 9:35
  • $\begingroup$ Dang! That's what I was going to say. You got to it first. $\endgroup$
    – ozone
    Commented May 16, 2017 at 13:12

A moon would provide a very short night.

Note that rings can be arcs not full rings! You'd have to look into that, but just off the top of my head I would suppose that an arc ring would have to be very narrow and thus would ot eclipse the sun anyway. But it’s a type of phenomenon you might not have heard of, and might give you more ideas.


That's a really interesting idea. I'm actually working on a tidally locked planet as well right now, and I had never thought of that. I can't see why it wouldn't be feasible. All I can see is that the moon would have to be really quite large, possibly even the size of the planet itself, for there to be a day/night cycle like we have on Earth. For reference, in the 2006 total solar eclipse, the moon only cast a shadow on a portion of Turkey and Cyprus: https://en.wikipedia.org/wiki/Solar_eclipse#/media/File:Eclipse_fromISS_2006-03-29.jpg
The implications of having a moon that large would most likely drastically change the path of orbit of the planet, the two would orbit around each other as well as their star.

  • $\begingroup$ A moon that large would'nt be also tidally locked to the planet? I suppose you can't have a tidally locked moon to a tidally locked planet. Wouldn't that make that moon stationary? $\endgroup$
    – Rekesoft
    Commented May 15, 2017 at 7:21
  • 1
    $\begingroup$ No, the moon would still revolve around the planet. The problem is : you can't have (for long) a large moon around a tidally locked planet : if the moon rotates faster than the planet (wich is the case), it will be slown down and will inevitably crash onto the planet. Plus, there's the problem of the unstable moon orbit due to the proximity of the planet to its star (this proximity is usually what causes the tidal-lock in the first place). $\endgroup$
    – Keelhaul
    Commented May 15, 2017 at 15:02

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