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Media coverage of the "super worm equinox moon" (*eyeroll*) has stated that this is the last supermoon of 2019. Since supermoons normally happen every three or four months, how can there be big gaps?

It feels like phenomena like supermoons, which are basically due to the relative phase of orbits, shouldn't have sharp transitions like this.

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    $\begingroup$ That's no superMoon! $\endgroup$ – Carl Witthoft Mar 21 at 15:44
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    $\begingroup$ Maybe what you heard is that we get three or four of them per year? Although actually, I think its more like 2 or 3. $\endgroup$ – Octopus Mar 22 at 4:38
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    $\begingroup$ Have you no respect for a hallowed astronomical institution? The name "Super Worm Equinox Moon" is a proper noun, and as such should be capitalized! ;-) $\endgroup$ – uhoh Mar 22 at 5:10
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    $\begingroup$ @uhoh Guilty, your honour. $\endgroup$ – David Richerby Mar 22 at 9:39
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    $\begingroup$ @uhoh, it's not a proper noun, it was probably capitalized in the title of an article, but it's just a bunch of adjectives and a noun. $\endgroup$ – Octopus Mar 22 at 16:08
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A supermoon does not occur every three or four months. There may be 2 or 3 consecutive supermoons (that is, separated by 1 month) that occur at about the same time each year.

If you ignore the precession of the Moon's orbit, then there is one time of the year when the Full Moon and perigee occur (point 1 in my diagram below). Two weeks later, the New Moon occurs near apogee (point 2). At any other time of the year, the Full Moon occurs at a different point in its orbit around the Earth, so it is farther from the Earth than at perigee. Six months later, the Full Moon is occurring at apogee (point 4), and no one cares about that! Earth's & Moon's orbit (not to scale!)

  1. Full Moon at perigee (closest to the Earth)
  2. New Moon at apogee (farthest from the Earth)
  3. New Moon at perigee
  4. Full Moon at apogee

The Wikipedia article on the Supermoon has a nice graphic showing the Full Moon and distance from the Earth. Depending on how close to the Earth the Moon needs to be to be "super", you can see that there is a "season" when the supermoon occurs. (I have copied the image here, and added a dashed line at 360,000 km to show which moons might be "super" and which ones are not.) enter image description here

Now, if only people would care about the Super First Quarter Moon. Then we would be celebrating on May 12, 2019! (Not really. That is farther than the 360,000 km criteria.)

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  • $\begingroup$ With apsidal precession, the major axis of the Moon's orbit rotates every 8.85 years, making the mean time between supermoon series about 13.53 calendar months. Fred Espenak notes that this is very close to 14 synodic months. $\endgroup$ – Mike G Mar 21 at 17:28
  • $\begingroup$ Does "supermoon" have a technical definition that allows us to say there can be 2 or 3 consecutive ones? Since it is practically impossible for the full moon to occur at exactly the same instant as perigee, I assumed that the next most reasonable definition would be to call a full moon super if it occurs nearer to earth than the previous and next full moons. But that would, by definition, not allow consecutive supermoons. $\endgroup$ – Henning Makholm Mar 22 at 13:31
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    $\begingroup$ @MikeG - 13.94 lunar months by my calculation. $\endgroup$ – David Hammen Mar 22 at 14:20
  • $\begingroup$ @HenningMakholm - The technical description is rather arbitrary. The difference between the Moon's apogee distance and the Earth-Moon distance at full moon has to be 90% or more of the difference between the Moon's apogee and perigee distances to qualify as a supermoon. It's worthy to note that this definition was developed and this term was coined by an astrologer rather than an astronomer. It's also worthy to note that due to variations in the Moon's eccentricity, some non-super full moons are more "super" than are some supermoons. $\endgroup$ – David Hammen Mar 22 at 14:30
  • $\begingroup$ @DavidHammen: The graph in the answer shows an annual variation in eccentricity, which looks like solar tides are deforming the moon's orbit. Are the 90% in percent of the instantaneous apogee-perigee distance, or of difference between maximal and minimal distances over a period of several years? (The graph seems to suggest the latter). $\endgroup$ – Henning Makholm Mar 22 at 14:50

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