<p>The moon's orbit is elliptical, and it moves faster when it is closest to the Earth.  This is the case for all elliptical orbits, and was discovered by Kepler, who gave a neat rule for predicting how fast an orbiting body moves. (<a href="https://kids.britannica.com/students/assembly/view/90830" rel="nofollow noreferrer">https://kids.britannica.com/students/assembly/view/90830</a>)</p>
<p><a href="https://i.sstatic.net/Gh6x0.png" rel="nofollow noreferrer"><img src="https://i.sstatic.net/Gh6x0.png" alt="Second Law" /></a></p>
<p>For the moon, this means that when it is largest, it is also moving fastest. So the peaks in the graph (where the moon is large) are narrow, as the moon quickly moves away from periapse.  When the moon is smallest, it is moving slowly, so the troughs in the graph are wide. It changes size slowly when it is far from the Earth.</p>
<p>The bumps are not all the same size because the orbit of the moon is perturbed by the sun.  This changes the amount that the moon changes size. <a href="https://eclipse.gsfc.nasa.gov/SEhelp/moonorbit.html" rel="nofollow noreferrer">https://eclipse.gsfc.nasa.gov/SEhelp/moonorbit.html</a></p>
<p>When the eccentricity is low, there is less difference in size.  When it is large there is a big difference.</p>
<p>This is the cause of &quot;supermoons&quot;, when the full moon occurs at the same time as periapse.  When the eccentricity is at a maximum, the supermoon will appear especially large and bright.</p>
<p><a href="https://i.sstatic.net/WT0xv.png" rel="nofollow noreferrer"><img src="https://i.sstatic.net/WT0xv.png" alt="enter image description here" /></a></p>
<p>The Moon's orbit is one of the most complex orbits in the solar system!</p>

CC BY-SA 4.0
2021-01-02T20:42:54.263Z
supermoons are not related to the sun's influence on the eccentricity, this could be made clearer in this answer
njzk2
CC BY-SA 4.0
2021-01-02T22:22:31.783Z
@njzk2: What are supermoons related to if not the eccentricity?
Peter Mortensen
CC BY-SA 4.0
2021-01-02T22:38:05.460Z
@PeterMortensen *the sun's influence on the eccentricity*. All full moon occurring close to perigee are called supermoon, regardless of how the sun is affecting the eccentricity.
njzk2
CC BY-SA 4.0
2021-01-03T00:37:15.983Z
well "supermoon" is not really a very scientific term.
James K
CC BY-SA 4.0
2021-01-01T19:23:25.543Z
2021-01-03T00:38:01.977Z
2021-01-03T00:38:01.977Z
James K
<p>I know that the orbit of the moon is elliptical.
But I cannot understand this graph:
<a href="https://i.sstatic.net/HlCz3.png" rel="nofollow noreferrer"><img src="https://i.sstatic.net/HlCz3.png" alt="Angular size of the moon vs Time" /></a></p>
<p>The number of 'bumps' in the graph is around 13~14 in a year, so I concluded that the each 'bump' indicates one lunar month.
But why aren't these bumps symmetrical? Since the moon revolves around the Earth once in one lunar month, shouldn't they be symmetrical?</p>

CC BY-SA 4.0
2021-01-01T20:44:05.567Z
There are several kinds of lunar month. The period from one lunar perigee to the next is the anomalistic month. See https://en.wikipedia.org/wiki/Lunar_month Also see https://en.wikipedia.org/wiki/Lunar_precession & https://en.wikipedia.org/wiki/Lunar_theory
PM 2Ring
CC BY-SA 4.0
2022-07-12T11:06:38.790Z
May I ask what program this graph was generated with?
2080
CC BY-SA 4.0
2024-06-18T03:08:04.947Z
@2080 It's Stellarium
DH K
<orbit><the-moon><angular-diameter>
Why does the angular size of the Moon change?
DH K
# Why does the angular size of the Moon change?

(Asked by: username_0 on 2021-01-01)

I know that the orbit of the moon is elliptical.
But I cannot understand this graph:
[![Angular size of the moon vs Time](https://i.sstatic.net/HlCz3.png)](https://i.sstatic.net/HlCz3.png)

The number of 'bumps' in the graph is around 13~14 in a year, so I concluded that the each 'bump' indicates one lunar month.
But why aren't these bumps symmetrical? Since the moon revolves around the Earth once in one lunar month, shouldn't they be symmetrical?

**Tags:** orbit, the-moon, angular-diameter

> **Comments:**
> **username_1**: There are several kinds of lunar month. The period from one lunar perigee to the next is the anomalistic month. See https://en.wikipedia.org/wiki/Lunar_month Also see https://en.wikipedia.org/wiki/Lunar_precession & https://en.wikipedia.org/wiki/Lunar_theory  
> **username_2**: May I ask what program this graph was generated with?  
> **username_0**: @username_2 It's Stellarium  

---

## 1 answers

### Answer by username_3 on 2021-01-01. Score: 14 | Accepted answer

The moon's orbit is elliptical, and it moves faster when it is closest to the Earth. This is the case for all elliptical orbits, and was discovered by Kepler, who gave a neat rule for predicting how fast an orbiting body moves. (<https://kids.britannica.com/students/assembly/view/90830>)

[![Second Law](https://i.sstatic.net/Gh6x0.png)](https://i.sstatic.net/Gh6x0.png)

For the moon, this means that when it is largest, it is also moving fastest. So the peaks in the graph (where the moon is large) are narrow, as the moon quickly moves away from periapse. When the moon is smallest, it is moving slowly, so the troughs in the graph are wide. It changes size slowly when it is far from the Earth.

The bumps are not all the same size because the orbit of the moon is perturbed by the sun. This changes the amount that the moon changes size. <https://eclipse.gsfc.nasa.gov/SEhelp/moonorbit.html>

When the eccentricity is low, there is less difference in size. When it is large there is a big difference.

This is the cause of "supermoons", when the full moon occurs at the same time as periapse. When the eccentricity is at a maximum, the supermoon will appear especially large and bright.

[![enter image description here](https://i.sstatic.net/WT0xv.png)](https://i.sstatic.net/WT0xv.png)

The Moon's orbit is one of the most complex orbits in the solar system!

> **Comments:**
> **username_4**: supermoons are not related to the sun's influence on the eccentricity, this could be made clearer in this answer  
> **username_5**: @username_4: What are supermoons related to if not the eccentricity?  
> **username_4**: @PeterMortensen *the sun's influence on the eccentricity*. All full moon occurring close to perigee are called supermoon, regardless of how the sun is affecting the eccentricity.  
> **username_3**: well "supermoon" is not really a very scientific term.  

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