I know that the orbit of the moon is elliptical. But I cannot understand this graph: Angular size of the moon vs Time

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?


1 Answer 1


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

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

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

  • $\begingroup$ supermoons are not related to the sun's influence on the eccentricity, this could be made clearer in this answer $\endgroup$
    – njzk2
    Jan 2, 2021 at 20:42
  • $\begingroup$ @njzk2: What are supermoons related to if not the eccentricity? $\endgroup$ Jan 2, 2021 at 22:22
  • $\begingroup$ @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. $\endgroup$
    – njzk2
    Jan 2, 2021 at 22:38
  • 1
    $\begingroup$ well "supermoon" is not really a very scientific term. $\endgroup$
    – James K
    Jan 3, 2021 at 0:37

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