# Is it plausible for a planet to have a face constantly sunlit while the other is constantly moonlit, with the sun/moonlit territories never changing?

I'm writing a sci-fi & fantasy novel, I won't get into details, but in essence it's set on a fantasy planet that orbits around a sun, and has a moon. The basic premise (that I'm hoping for anyway) is that one side of the planet exclusively faces the sun and is in constant day-time, while the other face is submerged in darkness, with only the light of its moon to light it. What I'm wondering is, is it astronomically plausible for a planet to have one face be constantly sunlit, while the other is constantly moonlit?

At first, I considered the moon, which for some reason I had previously very incorrectly thought did not rotate around its own axis, and the same face was always in darkness/lit up. I'm now thankfully aware the moon does spin around its own axis, but the time it takes to orbit the Earth is practically equal to the time it takes to rotate around its own axis, keeping the same side of the moon facing the Earth throughout the month. If the moon didn't rotate around its axis/rotated at a different rate, different parts of the moon would face the Earth during the month.

In recap, I'm hoping for this planet to be plausible and definitely have the following criteria:

1. Have a moon and sun

2. Have the exact same face constantly facing its moon, and the other constantly facing its sun, so the exact same territory is always moonlit/sunlit, the boundaries don't change.

As long as these two criteria are generally met, I'm happy with any solutions/explanations for how this could be (if at all) astronomically/physically plausible.

edit: i have made a rather large change, but don't want to change the question -- in essence, to see my temporary solution (still have to do some work on it) in progress, top answer's comments :)

• No. You need to seriously re-think your geometry, as that's not what actually happens, or can happen. Anything in orbit has to go all the way around the planet, so it could only light the back side of the planet half the time. (And you can't keep anything at L1/L2 without active control.) The only way to come close to doing it is to have several moons, and even then, you'd get occasional dark periods when all the moons were on the sunlit side of the planet. Commented Apr 9, 2021 at 16:51

Having a planet tide-locked to its sun is easy and straightforward.

Having a moon that always faces the opposite side of the planet requires the moon to be at the Second Lagrange (L2) point. This is tricky for three reasons:

First, the L2 point is awfully far away. Roughly 5 times the distance Earth's moon is. This problem is solvable by having a heavier planet or a lighter sun. The formula is in wikipedia.

Second, eclipses. The moon will experience a permanent solar eclipse. If the moon is too close (e.g. as close as Earth's), it will be a total eclipse accompanied by a total lunar eclipse as seen from the planet. If the moon is farther away, it will experience an annular solar eclipse with corresponding dimming. At a narrow band of intermediate distances, the moon will experience a total solar eclipse at noon at the equator, but partial everywhere else, corresponding to an annular lunar eclipse as seen from the planet.

Finally, the L2 point is unstable. If the moon gets nudged (e.g. by a meteor) it will wobble out of this position and into 3-body chaos, likely ending up in either a lower planetary or higher solar orbit. Things can stay at the L2 point for a while, but they don't tend to fall there by nature. Maybe hint that some other factor is at work.

• Thank you so much for the in depth answer! I've been thinking, and (I think) I've come up with a solution that's less tricky - the setting will be on an actual moon instead of a planet, at an L1 point. I cannot attach images, but here's a (clearly not to scale) diagram of what I've come up with: drive.google.com/file/d/1LKlZJVfU2Wvdt9hmGDTFguJ0ypB8WjWs/…. As such, the moon is lit up half by the sun, half by the light reflected off the gas giant - except assuming I understand correctly, that light from the gas giant will only be enough to make a plausible twilight :) Commented Apr 8, 2021 at 23:44
• Generally I'm not well rounded in astronomy -- will appreciate any comment on whether the previous comment's potential solution seems like it could work! Really appreciate the idea :) Commented Apr 8, 2021 at 23:45
• @Guest You should ask that as a separate question so that the question and answers are searchable and easily found, not buried in comments here. Commented Apr 9, 2021 at 14:55
• @MikeScott ah sure, thank you so much! Commented Apr 9, 2021 at 15:45

A planet can expose always the same face to the star, it's precisely the case of tidal locking. It happens with our Moon, which show us always the same face.

It is also possible for a moon to be facing always the same side of the planet, if the moon and the planet are mutually locked.

However for the two lockings to happen simultaneously, you would need to have a very precise and unlikely combination of circumstances: the orbital period of the moon around the planet should be equal to the orbital period of the planet around the sun.

That at most would happen with one moon, not with more.

As pointed out by Novotny in the comments, for any combination of Sun, planet, and Moon, to get an orbital period of the moon equal to the orbital period of the planet, you always wind up needing a radius of the moon's orbit larger than the Hill Sphere of the planet.

• "a very precise combination of events" - I have a feeling that this combination is nearly unrealistic. Commented Apr 8, 2021 at 19:11
• @Alexander, I agree with that
– L.Dutch
Commented Apr 8, 2021 at 19:15
• Ah I see! So theoretically if I just had this planet have one moon, even though incredibly unlikely, it is possible for the planet to have one face exclusively facing the sun and the other exclusively facing its moon? Commented Apr 8, 2021 at 19:22
• For any combination of Sun, planet, and Moon, to get an orbital period of the moon equal to the orbital period of the planet, you always wind up needing a radius of the moon's orbit larger than the Hill Sphere of the planet. Commented Apr 8, 2021 at 19:28
• @DWKraus hi! I've actually been thinking and have come up with something fairly similar, with a gas giant and a sun, and now it would be set on a moon at an L1 point. Explained it more in depth under dispeyer 's answer :) Commented Apr 8, 2021 at 23:47