<p>tl;dr: Perfectly full moon lasts, never more than, approximately 2 minutes.</p>

<p><strong>Estimating an answer from the point of view of Plain Geometry:</strong></p>

<p>If we approximate, that the sun is at an infinite distance, then exactly half of the moon is illuminated.</p>

<p>Next, how much of the Moon do I see when I look at it from Earth? (This has nothing to do with illumination of the Moon.) Estimate: Radius of moon (Rm), 1740km. Distance to the moon (Dm), 384,000km. Doodle this into a diagram that is VERY much not to scale...</p>

<p><a href="https://i.sstatic.net/HIPGI.jpg" rel="nofollow noreferrer"><img src="https://i.sstatic.net/HIPGI.jpg" alt="enter image description here"></a></p>

<p>arctan(Rm / Dm) gives us 0.260 degrees. So as long as the observer is within that angle from the Moon-Sun-centerline, the portion of the Moon which is <em>visible</em>, would be a circular disc fully illuminated.</p>

<p>Here, I suspect that the variation in the Moon's distance is the biggest thing I've estimated away.</p>

<p>The question now is: combining the Moon's motion of revolution (aka orbit about the Earth) with the Earth's rotation, how long does it take an observer to perceive the moon moving half a degree (1/4 degree either side of the Moon-Sun-centerline)? Rotation of the Earth gives 15 degrees-per-hour, minus the Moon's 1/2 degree per hour... 14.5 degrees per hour apparent motion.</p>

<p>I get approximately 2 minutes duration when the Moon's shifting of apparent position would only hide/show illuminated portions of it's disc.</p>

<p><strong>But the lunar orbit is inclined</strong></p>

<p>So it's only 2 minutes if the Moon is very close to the plane of the Equator. As the Moon's orbit carries it out of the plane, the time would be reduced. Beyond half a degree out of the plane, you would actually never (pure geometry) seen a fully illuminated disc.</p>

<p>So I think the best answer is probably, "Never more than 2 minutes."</p>

CC BY-SA 3.0
2015-08-03T17:38:55.407Z
2015-08-03T17:51:06.320Z
2015-08-03T17:51:06.320Z
Craig Constantine
<p>Is the moon the most full only for one moment </p>

<p>As stated  <a href="https://answers.yahoo.com/question/index?qid=20080721155747AAsraEM" rel="nofollow noreferrer">here</a> and <a href="http://m.space.com/24511-moon-myths-lunar-new-year.html" rel="nofollow noreferrer">here</a></p>

<p>Or does if stay the same full for a longer period of time (maybe because, the sun is grater then it, and light spreads out)</p>

<p><a href="https://astronomy.stackexchange.com/q/8380">Full Moon Position of Sun</a></p>

CC BY-SA 3.0
2015-08-02T22:32:35.640Z
It seems to be, because from the Moon, the earth takes up a fair bit more sky space than the sun does, we never get a truly entirely round full moon.  You can get it full in one axis but not both.    But I think your argument is largely correct.   The sun takes up 1/2 degree of the sky, so there should be a 1/2 degree of space where the Moon would appear completely illuminated as it crosses the far side of Earth.   It's the Moon's orbit around the earth that matters so 1/720 x 28 days = 56 minutes, (I would think), but that's only across it's equator.   it's still not full pole to pole.
userLTK
CC BY-SA 3.0
2015-08-06T14:25:54.367Z
@barrycarter I understand that, that is why I used the words "**most** full" and "**same** full"
hazoriz
<the-moon><moon-phases>
How long is the moon full
hazoriz
# How long is the moon full

(Asked by: username_0 on 2015-08-02)

Is the moon the most full only for one moment

As stated [here](https://answers.yahoo.com/question/index?qid=20080721155747AAsraEM) and [here](http://m.space.com/24511-moon-myths-lunar-new-year.html)

Or does if stay the same full for a longer period of time (maybe because, the sun is grater then it, and light spreads out)

[Full Moon Position of Sun](https://astronomy.stackexchange.com/q/8380)

**Tags:** the-moon, moon-phases
> **Comments:**
> **username_1**: It seems to be, because from the Moon, the earth takes up a fair bit more sky space than the sun does, we never get a truly entirely round full moon.  You can get it full in one axis but not both.    But I think your argument is largely correct.   The sun takes up 1/2 degree of the sky, so there should be a 1/2 degree of space where the Moon would appear completely illuminated as it crosses the far side of Earth.   It's the Moon's orbit around the earth that matters so 1/720 x 28 days = 56 minutes, (I would think), but that's only across it's equator.   it's still not full pole to pole.  
> **username_0**: @barrycarter I understand that, that is why I used the words "**most** full" and "**same** full"  

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## 1 answers

### Answer by username_2 on 2015-08-03. Score: 5

tl;dr: Perfectly full moon lasts, never more than, approximately 2 minutes.

**Estimating an answer from the point of view of Plain Geometry:**

If we approximate, that the sun is at an infinite distance, then exactly half of the moon is illuminated.

Next, how much of the Moon do I see when I look at it from Earth? (This has nothing to do with illumination of the Moon.) Estimate: Radius of moon (Rm), 1740km. Distance to the moon (Dm), 384,000km. Doodle this into a diagram that is VERY much not to scale...

[![enter image description here](https://i.sstatic.net/HIPGI.jpg)](https://i.sstatic.net/HIPGI.jpg)

arctan(Rm / Dm) gives us 0.260 degrees. So as long as the observer is within that angle from the Moon-Sun-centerline, the portion of the Moon which is *visible*, would be a circular disc fully illuminated.

Here, I suspect that the variation in the Moon's distance is the biggest thing I've estimated away.

The question now is: combining the Moon's motion of revolution (aka orbit about the Earth) with the Earth's rotation, how long does it take an observer to perceive the moon moving half a degree (1/4 degree either side of the Moon-Sun-centerline)? Rotation of the Earth gives 15 degrees-per-hour, minus the Moon's 1/2 degree per hour... 14.5 degrees per hour apparent motion.

I get approximately 2 minutes duration when the Moon's shifting of apparent position would only hide/show illuminated portions of it's disc.

**But the lunar orbit is inclined**

So it's only 2 minutes if the Moon is very close to the plane of the Equator. As the Moon's orbit carries it out of the plane, the time would be reduced. Beyond half a degree out of the plane, you would actually never (pure geometry) seen a fully illuminated disc.

So I think the best answer is probably, "Never more than 2 minutes."


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