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

RANSARA009
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3 Answers3

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The answer depends on your definition of a day.

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.

Pere
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    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. – CipherBot Sep 08 '16 at 12:11
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    @CipherBot, Keep it Up.. – RANSARA009 Sep 08 '16 at 18:12
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    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? – Jason C Sep 09 '16 at 16:20
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    @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. – kajacx Sep 10 '16 at 10:52
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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.

NVZ
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CipherBot
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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.

MSalters
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    Well... there's always the new moon phase where it is dark. But I sort of see your point. – CipherBot Sep 08 '16 at 12:12
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    @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. – 8bittree Sep 08 '16 at 14:54
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    You can get days under this definition, due to the lunar libration cycle. – OrangeDog Sep 08 '16 at 17:20
  • I think it's pretty universal that "day" means the the time when the sun is above the horizon. Any other definition is pointless sophistry. From most [thin libration zone notwithstanding] of the Near Side of the Moon, the Earth is permanently in the sky so doesn't rise nor set. On the Far Side, the Earth is never seen. Everywhere [polar regions notwithstanding] on the Moon experiences a normal day that lasts 27.3 Earth-days – Oscar Bravo Sep 09 '16 at 09:48
  • @OrangeDog: The problem with libration is that it leads to significantly different "day" lengths depending on your position on the moon surface. Earth days have pretty consistent length across the earth (down to microseconds IIRC) – MSalters Sep 09 '16 at 09:53
  • @OwenBoyle, Google for "term of art". The meaning of a word can be "pretty universal" for most of the human population, but have a very different meaning in some special context. For starters, if you simply say "day" to an astronomer, they will ask whether you meant to say "sidereal day" or "solar day." I am not an astronomer, so I don't know whether there are other meanings of "day" when talking about a body that revolves around something other than the Sun. – Solomon Slow Sep 09 '16 at 20:53
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    I can't see how this works. Basically all of the moon's surface has no earthrises or earthsets. – Dawood ibn Kareem Sep 11 '16 at 00:32
  • @DavidWallace: Most but not all (I think it's approximately 94%). – MSalters Sep 12 '16 at 08:39
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    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". – Dawood ibn Kareem Sep 12 '16 at 09:47
  • That definition of "day" in the Moon would be equivalent in Earth to the time that the Moon is over horizon. That time could be interesting, but it's no way called a day. – Pere Jan 23 '17 at 14:29