# Does the moon have days?

As like Earth, does the moon have a day (24 hours). If it has, how many hours are in a moon day?.

• vsauce > How Earth Moves Sep 8, 2016 at 16:33
• Solar day or Terran day? Synodic or Sidereal? Sep 8, 2016 at 17:18
• Lunar day (Wiki) Sep 9, 2016 at 2:38
• @OrangeDog: Probably you could have helped the OP (& me) understand different definitions with answers. Why confuse someone knowing that they don't know something (I believe you guessed that correctly on reading the question) that you know. Just a thought came first to my mind on reading your comment, cheers. Sep 10, 2016 at 6:19

If you define a day as we usually define it in the Earth (time between the Sun is at noon or average time between sunrises, a 24 hours day in Earth), the length of a day in the Moon is the synodic period of the Moon and it takes 29.530589 days (29 d 12 h 44 min 2.9 s).

Anyway, if you define a day as rotation on own axis, that is, time between a far away star being at noon, you have what in Earth is called sidereal day (23 hours, 56 minutes in Earth). In the moon the day defined this way would be 27.321661 days (27 d 7 h 43.19 min 11.5 s).

Please notice that since the Moon is tidally locked to the Earth, rotation periods equal orbital periods.

• Mine's more of a fast and simple answer as I am on phone and agree that this should be the accepted answer. Although not sure how my answer warrants a down vote. Sep 8, 2016 at 12:11
• @CipherBot, Keep it Up.. Sep 8, 2016 at 18:12
• You should probably express your lengths in hours instead of days to avoid ambiguity, since the definition of a "day" is part of the topic of your post. For example, is "27.321661 days" in 24-hour days, or in Earth sidereal days? Sep 9, 2016 at 16:20
• @JasonC I think it is clear that if someone uses day as a unit of measurement next to hours and minutes, it clearly means the earth 24-hours day. Sep 10, 2016 at 10:52

Defining that a day is a rotation on the axis, 1 moon day is approximately equal to 27.3 Earth days. This link on moon rotation may interest you.

• Comments are not for extended discussion; this conversation has been moved to chat. Sep 12, 2016 at 13:24

Another valid definition is the time between earthrises instead of sunrises, since the moon is in orbit around earth. In that case, there is no direct concept of a day, because the moon is tidally locked to earth.

• Well... there's always the new moon phase where it is dark. But I sort of see your point. Sep 8, 2016 at 12:12
• @CipherBot If you're basing days on what's illuminated, then an eclipse would be night and a nearby supernova would be day, both of which kind of ruin any consistent length of a day. Sep 8, 2016 at 14:54
• You can get days under this definition, due to the lunar libration cycle. Sep 8, 2016 at 17:20
• I can't see how this works. Basically all of the moon's surface has no earthrises or earthsets. Sep 11, 2016 at 0:32
• I know. That's why I said "basically". My point is that "time between earthrises" doesn't give a workable definition of a "day on the moon". Sep 12, 2016 at 9:47