<p>As noted in comments, the earth rotates on its axis in about 23hrs 56 minutes. This is known as the <a href="https://en.wikipedia.org/wiki/Sidereal_time" rel="nofollow">sidereal day</a>, that is, the day with respect to background stars. (that's not <em>quite</em> true, but it's good enough unless you're dealing with periods of thousands of years with high accuracy - see the linked wiki article if you care about the detail).</p>

<p>At the same time, the Earth is also orbiting around the Sun in the same direction, and the combination of the Earth's spin and its orbit around the Sun gives us the 24-hour solar day that we are familiar with, and that you have already modelled in your game.</p>

<p>Now consider how the Moon rises and sets, when seen from a position on Earth. The primary cause of this is, again, the Earth's rotation with it's just-less-than-24-hours period. In addition, this time, one must consider the motion of the Moon around the Earth, which is again in the same direction as the Earth's spin. This results in a period of about 24 hr 50 mins for the apparent motion of the Moon across the sky (<a href="http://oceanservice.noaa.gov/education/kits/tides/tides05_lunarday.html" rel="nofollow">Source</a>). For your game that may be sufficient info, although if you want to be accurate it is <a href="https://en.wikipedia.org/wiki/Orbit_of_the_Moon" rel="nofollow">complicated</a>.</p>

<p>Remember to also take account of the <a href="https://en.wikipedia.org/wiki/Lunar_phase" rel="nofollow">phases of the moon</a> - because the whole earth-moon system is also in orbit around the sun, and because the moon is only visible via reflected light, the mere fact that the moon is above the horizon when seen from a given point on earth does not mean that it is visible.</p>

CC BY-SA 3.0
2015-06-10T11:10:17.313Z
Nice VSauce episode about the moon terminator: https://www.youtube.com/watch?v=Y2gTSjoEExc
Jan Doggen
CC BY-SA 3.0
2015-06-12T22:10:45.447Z
Thank-you, that is what I was looking for was the 50 minute delay, I've got my system all working with moon & sun now. And moon phases are working as well, although I can't have lunar eclipses because my game world is not a globe.
Jayden Miller
CC BY-SA 3.0
2015-06-10T06:17:21.200Z
2015-06-10T06:17:21.200Z
Semidiurnal Simon
<p>I am working on a game, and I have a day/night cycle. Currently I have the sun. Currently every "sun day" is 24 hours, with that being said, how long is every "moon day"? In other words, how much does the moon lag behind the sun. I just need to figure out what the moons speed should be in relativity to the sun. I've done some searches, but wasn't quite sure how to properly word this question.</p>

CC BY-SA 3.0
2015-06-09T02:41:54.393Z
the sun and the moon are essentially static... it's the Earth that rotates in 24 hours.  The moon falls behind by about 50 minutes every day.
f.thorpe
CC BY-SA 3.0
2015-06-09T10:02:47.403Z
@farrenthorpe The Earth rotates in 23 h 56 m 4.1 s.
gerrit
CC BY-SA 3.0
2015-06-09T12:30:52.810Z
@gerrit yes with respect to the stars, not with respect to the sun.  Not sure  what your point is here other than to prove you know decimal places?
f.thorpe
CC BY-SA 3.0
2015-06-09T13:13:13.753Z
@farrenthorpe My point is that a solar day does not correspond to a single rotation of the Earth, but that it is a single rotation of the Earth /plus/ a fraction of the Earth rotation around the Sun.  This is quite relevant for astronomical questions such as this one, as it causes the length of a solar day to vary during the year, which makes the answer to Jaydens question more involved.  I'm not sure if a rotation on an axis is a relative or an absolute quantity.
gerrit
CC BY-SA 3.0
2015-06-10T05:04:32.577Z
Technically this is probably a question for astronomy.se, but we should be able to provide an answer here... The Google terms for the comment conversation above are "solar day" vs "sidereal day". The "moon day", as you put it is a little less straightforward. I'll try to put an answer together when near a PC later.
Semidiurnal Simon
CC BY-SA 3.0
2015-06-10T14:23:15.557Z
gerrit perhaps re-read the question... he is not asking about the spin rotation of the Earth... he is asking about the relative time between our day (which is 24 hours, not 23h 56 m) and the moon position.
f.thorpe
<moon>
How often does the moon "set" for us compared to the sun?
Jayden Miller
# How often does the moon "set" for us compared to the sun?

(Asked by: username_0 on 2015-06-09)

I am working on a game, and I have a day/night cycle. Currently I have the sun. Currently every "sun day" is 24 hours, with that being said, how long is every "moon day"? In other words, how much does the moon lag behind the sun. I just need to figure out what the moons speed should be in relativity to the sun. I've done some searches, but wasn't quite sure how to properly word this question.

**Tags:** moon

> **Comments:**
> **username_1**: the sun and the moon are essentially static... it's the Earth that rotates in 24 hours.  The moon falls behind by about 50 minutes every day.  
> **username_2**: @farrenthorpe The Earth rotates in 23 h 56 m 4.1 s.  
> **username_1**: @username_2 yes with respect to the stars, not with respect to the sun.  Not sure  what your point is here other than to prove you know decimal places?  
> **username_2**: @farrenthorpe My point is that a solar day does not correspond to a single rotation of the Earth, but that it is a single rotation of the Earth /plus/ a fraction of the Earth rotation around the Sun.  This is quite relevant for astronomical questions such as this one, as it causes the length of a solar day to vary during the year, which makes the answer to Jaydens question more involved.  I'm not sure if a rotation on an axis is a relative or an absolute quantity.  
> **username_3**: Technically this is probably a question for astronomy.se, but we should be able to provide an answer here... The Google terms for the comment conversation above are "solar day" vs "sidereal day". The "moon day", as you put it is a little less straightforward. I'll try to put an answer together when near a PC later.  
> **username_1**: username_2 perhaps re-read the question... he is not asking about the spin rotation of the Earth... he is asking about the relative time between our day (which is 24 hours, not 23h 56 m) and the moon position.  

---

## 1 answers

### Answer by username_3 on 2015-06-10. Score: 3

As noted in comments, the earth rotates on its axis in about 23hrs 56 minutes. This is known as the [sidereal day](https://en.wikipedia.org/wiki/Sidereal_time), that is, the day with respect to background stars. (that's not *quite* true, but it's good enough unless you're dealing with periods of thousands of years with high accuracy - see the linked wiki article if you care about the detail).

At the same time, the Earth is also orbiting around the Sun in the same direction, and the combination of the Earth's spin and its orbit around the Sun gives us the 24-hour solar day that we are familiar with, and that you have already modelled in your game.

Now consider how the Moon rises and sets, when seen from a position on Earth. The primary cause of this is, again, the Earth's rotation with it's just-less-than-24-hours period. In addition, this time, one must consider the motion of the Moon around the Earth, which is again in the same direction as the Earth's spin. This results in a period of about 24 hr 50 mins for the apparent motion of the Moon across the sky ([Source](http://oceanservice.noaa.gov/education/kits/tides/tides05_lunarday.html)). For your game that may be sufficient info, although if you want to be accurate it is [complicated](https://en.wikipedia.org/wiki/Orbit_of_the_Moon).

Remember to also take account of the [phases of the moon](https://en.wikipedia.org/wiki/Lunar_phase) - because the whole earth-moon system is also in orbit around the sun, and because the moon is only visible via reflected light, the mere fact that the moon is above the horizon when seen from a given point on earth does not mean that it is visible.

> **Comments:**
> **username_4**: Nice VSauce episode about the moon terminator: https://www.youtube.com/watch?v=Y2gTSjoEExc  
> **username_0**: Thank-you, that is what I was looking for was the 50 minute delay, I've got my system all working with moon & sun now. And moon phases are working as well, although I can't have lunar eclipses because my game world is not a globe.  

---

