# How does the day/night cycle work on a non-tidally locked moon?

So to keep this simple, imagine a planet much like ours, orbiting a star. For the sake of this example, let's give the planet a 24 hour rotation, and a 364 day orbit around the star.

Now, this planet has a moon, the moon is not-tidally locked to the planet, it also has a 24 hour rotation, but it orbits the planet on a 28 day cycle. This gives the moon 13 full orbits around the planet per year.

So, what I'm trying to wrap my head around is what the day/night cycle on the moon actually looks like. When the moon is on the "sun-side" of the planet, the rotation of the moon would give the moon a pretty typical day of 12-ish hours of light and 12-ish hours of dark. But what about when the moon is behind the planet? I feel like at this point the day/night cycle is pretty meaningless because the moon is always in the planet's shadow, so it would be night all the time. Over a 28 day moon cycle, half of the cycle (14 days) would have a regular day/night, then the other half of the cycle (another 14 days) would just be one long night.

I get that depending on the position of the planet and the moon with respect to the star will shift the cycle around a bit, and also that when the moon is near the "edges" of its orbit around the planet the days/nights will be longer/shorter.

Is my (very basic) understanding basically correct? When the moon is behind the planet it will just be night for half the moon's cycle?

• Read up on sidereal (sigh-dear-e-all) time. For instance, note that the Earth actually rotates around its axis just over 366 times per year, this results in 365 day cycles because one is effectively cancelled by the orbit. Also note that it's highly unlikely the moon will be in shadow for half its cycle. Own own moon is placed such that it's in shadow for hours at most. Overall, you're overcomplicating it. Apr 9, 2018 at 19:23

Not exactly. What you've specified is something pretty much like our own Earth-Moon-Sun system except that the Moon rotates in 24 hours rather than 28 days. I'm assuming that they're all about the same size as in the Earth-Moon-Sun system, also.

The Moon's day-night cycle would be pretty much the same as the Earth's is here and is in your new system: 24-hour days with on avers 12 hours of dark and 12 hours of light. The Earth is far enough away and small enough that eclipses of the Sun (as viewed from the Moon) would last a few hours and happen at most once a month (if you aligned all the orbital planes).

Besides the eclipses, there'd be a host of smaller effects: * The Sun's apparent path through the Moon's sky would have a monthly loop in it, the details of which would depend on the orbital inclinations of Earth and Moon. * Because of that, the Lunar analemma (the "equation of time") would be very complicated and solar noon would differ from mean noon with an extra jitter of some minutes depending on whether the Moon was ahead of the Earth or behind the Earth in its orbit around the Sun.

But most people wouldn't notice anything but the longer and more frequent eclipses.

• If you re-read my question, you'll see that I specified that the moon has a 28 day orbit around the planet, and a 24 hour rotation. Not a 24 hour orbit around the planet as that first paragraph of you answer seems to imply. Apr 9, 2018 at 19:31
• +1. Good answer, and beat me to it. The only other things I can think of worth consideration are the effects of partial versus total eclipses and how eclipse frequency varies depending on the orbital inclination of the moon. Apr 9, 2018 at 19:32
• @fiend He isn't suggesting a 24-hour orbit — in fact, the Moon does rotate as well as orbit once a month, and must do so to point the same side toward Earth at all times. Relative to Earth, of course, the moon doesn't appear to be rotating, but relative to a fixed direction — say, toward the center of the galaxy — Earth's moon rotates once every 28 or so days. Apr 9, 2018 at 19:35
• @fiend If I'm understanding correctly, that is what he's saying — a 24-hour rotation period but with an orbital period of 28 days just like Earth's moon. Apr 9, 2018 at 19:38
• @AidanF.Pierce oh I see what he's saying now, reading things at the end of the workday on Mondays isn't my strong suit apparently :P. Apr 9, 2018 at 19:41