<p>The answer depends on your definition of a day.</p>

<p>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 <a href="https://global.britannica.com/topic/synodic-period" rel="noreferrer">synodic period</a> of the Moon and it takes <a href="https://en.wikipedia.org/wiki/Moon" rel="noreferrer">29.530589 days (29 d 12 h 44 min 2.9 s)</a>.</p>

<p>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 <a href="https://en.wikipedia.org/wiki/Sidereal_time" rel="noreferrer">sidereal day</a> (23 hours, 56 minutes in Earth). In the moon the day defined this way would be <a href="https://en.wikipedia.org/wiki/Moon" rel="noreferrer">27.321661 days (27 d 7 h 43.19 min 11.5 s)</a>.</p>

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

CC BY-SA 3.0
2016-09-08T12:11:48.370Z
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
CC BY-SA 3.0
2016-09-08T18:12:27.187Z
@CipherBot, Keep it Up..
RANSARA009
CC BY-SA 3.0
2016-09-09T16:20:55.247Z
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
CC BY-SA 3.0
2016-09-10T10:52:59.320Z
@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
CC BY-SA 3.0
2016-09-08T08:12:03.503Z
2016-09-25T12:58:43.530Z
2016-09-25T12:58:43.530Z
Pere
<p>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. </p>

CC BY-SA 3.0
2016-09-08T12:12:51.463Z
Well... there's always the new moon phase where it is dark. But I sort of see your point.
CipherBot
CC BY-SA 3.0
2016-09-08T14:54:07.513Z
@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
CC BY-SA 3.0
2016-09-08T17:20:13.483Z
You can get days under this definition, due to the lunar libration cycle.
OrangeDog
CC BY-SA 3.0
2016-09-09T09:48:59.463Z
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
CC BY-SA 3.0
2016-09-09T09:53:03.140Z
@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
CC BY-SA 3.0
2016-09-09T20:53:35.147Z
@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
CC BY-SA 3.0
2016-09-11T00:32:13.027Z
I can't see how this works.  Basically all of the moon's surface has no earthrises or earthsets.
Dawood ibn Kareem
CC BY-SA 3.0
2016-09-12T08:39:30.470Z
@DavidWallace: Most but not all (I think it's approximately 94%).
MSalters
CC BY-SA 3.0
2016-09-12T09:47:25.370Z
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
CC BY-SA 3.0
2017-01-23T14:29:02.010Z
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
CC BY-SA 3.0
2016-09-08T11:48:15.073Z
2016-09-08T11:48:15.073Z
MSalters
<p>Defining that a day is a rotation on the axis, 1 moon day is approximately equal to 27.3 Earth days. This link on <a href="http://www.universetoday.com/48788/moon-rotation/" rel="noreferrer">moon rotation</a> may interest you.</p>

CC BY-SA 3.0
2016-09-12T13:24:24.997Z
Comments are not for extended discussion; this conversation has been [moved to chat](http://chat.stackexchange.com/rooms/45272/discussion-on-answer-by-cipherbot-does-the-moon-have-days).
called2voyage
CC BY-SA 3.0
2016-09-08T07:00:26.653Z
2016-09-10T10:44:45.503Z
2016-09-10T10:44:45.503Z
CipherBot
<p>As like Earth, does the moon have a day (24 hours). If it has, how many hours are in a moon day?.</p>

CC BY-SA 3.0
2016-09-08T16:33:56.587Z
[vsauce > How Earth Moves](https://www.youtube.com/watch?v=IJhgZBn-LHg)
Remi.b
CC BY-SA 3.0
2016-09-08T17:18:37.137Z
Solar day or Terran day? Synodic or Sidereal?
OrangeDog
CC BY-SA 3.0
2016-09-09T02:38:58.320Z
[Lunar day](https://en.wikipedia.org/wiki/Lunar_day) (Wiki)
Mazura
CC BY-SA 3.0
2016-09-10T06:19:29.110Z
@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.
0xc0de
<time><the-moon>
Does the moon have days?
RANSARA009
# Does the moon have days?

(Asked by: username_0 on 2016-09-08)

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

**Tags:** time, the-moon
> **Comments:**
> **username_1**: [vsauce > How Earth Moves](https://www.youtube.com/watch?v=IJhgZBn-LHg)  
> **username_2**: Solar day or Terran day? Synodic or Sidereal?  
> **username_3**: [Lunar day](https://en.wikipedia.org/wiki/Lunar_day) (Wiki)  
> **username_4**: @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.  

---

## 3 answers

### Answer by username_6 on 2016-09-08. Score: 19 | Accepted answer

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](http://www.universetoday.com/48788/moon-rotation/) may interest you.

> **Comments:**
> **username_14**: Comments are not for extended discussion; this conversation has been [moved to chat](http://chat.stackexchange.com/rooms/45272/discussion-on-answer-by-cipherbot-does-the-moon-have-days).  

---

### Answer by username_5 on 2016-09-08. Score: 49

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](https://global.britannica.com/topic/synodic-period) of the Moon and it takes [29.530589 days (29 d 12 h 44 min 2.9 s)](https://en.wikipedia.org/wiki/Moon).

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](https://en.wikipedia.org/wiki/Sidereal_time) (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)](https://en.wikipedia.org/wiki/Moon).

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

> **Comments:**
> **username_6**: 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.  
> **username_0**: @CipherBot, Keep it Up..  
> **username_7**: 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?  
> **username_8**: @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.  

---

### Answer by username_9 on 2016-09-08. Score: 7

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.

> **Comments:**
> **username_6**: Well... there's always the new moon phase where it is dark. But I sort of see your point.  
> **username_10**: @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.  
> **username_2**: You can get days under this definition, due to the lunar libration cycle.  
> **username_11**: 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  
> **username_9**: @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)  
> **username_12**: @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.  
> **username_13**: I can't see how this works.  Basically all of the moon's surface has no earthrises or earthsets.  
> **username_9**: @DavidWallace: Most but not all (I think it's approximately 94%).  
> **username_13**: 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".  
> **username_5**: 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.  

---

