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I understand that a "regular supermoon" is a full moon when the moon is at perigee (closest to earth in its orbit).

They seem to happen often, here's a post from one last year. But I've seen news articles saying this is the 'superest' super moon since 1948 and we won't see another super moon this bright until 2034.

What makes the recent super moon so bright?

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  • $\begingroup$ See my severely underrated answer to astronomy.stackexchange.com/questions/19043/… $\endgroup$
    – user21
    Commented Nov 16, 2016 at 6:12
  • $\begingroup$ @barrycarter I don't see how that helps answer this question. At best your answer can be used to determine when a supermoon will occur, but that wasn't the question here. $\endgroup$
    – zephyr
    Commented Nov 16, 2016 at 19:14
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    $\begingroup$ @zephyr Yes, but it also has an image and a link of a spreadsheet showing moon distances during full moon, and showing that this is the 6th closest full moon this century. I think that reasonably answers this question (ie, 1200+ full moons per century and this is #6 on the list) $\endgroup$
    – user21
    Commented Nov 16, 2016 at 20:14
  • $\begingroup$ @barrycarter Feel free to provide this as an answer, it would be great to have hard numbers on this! As it is, I don't know if linking to your other answer makes it easy to find that type of information. $\endgroup$
    – zephyr
    Commented Nov 16, 2016 at 20:21
  • $\begingroup$ @zephyr I hate redundancy, but if someone wants to cut and paste a portion of my answer there as an answer here, I'd be OK with that. I'm not in it for the points :) $\endgroup$
    – user21
    Commented Nov 16, 2016 at 20:45

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The thing you have to really understand is that not all supermoons are equal. The official definition of a supermoon, as you state, is that the full moon must coincide with the moon being at perigee.

However, this never actually happens! The moon being at perigee happens for only a single instance of time - less than a second. The moon being officially "full", again only happens for a single instance of time (i.e., when the Sun, Earth, and Moon all form a single line). The chances of these two instances of time happening simultaneously are almost non-existent.

So, when someone calls a full moon a supermoon, they really just mean that the exact full moon instance is "pretty close" to the exact moment of reaching perigee. The closer the better and the more super your supermoon will be.

If you take a look at the Supermoon which occured on Nov. 14, widely proclaimed to be the "superest" supermoon for a long time, you can see that perigee was reached precisely at 11:23 UTC, while the full moon occurred precisely at 13:52 UTC, two and a half hours later. For pretty much every other supermoon, these two events are close enough for the moon to be called a supermoon, but not two-and-a-half-hours close. The closeness in time between reaching perigee and being truly full made this moon especially big and more important, especially bright.

There is also another thing which made this supermoon the superest and that was the actual distance of the Moon's perigee. As with anything in our solar system, the Moon's orbit is not perfectly elliptical, but instead has small variations and perturbations over time. This means that the perigee of the Moon's orbit is slightly changing over time. The November supermoon occured during a particularly close perigee that won't be matched until 2034 supermoon which you reference.

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  • $\begingroup$ Be-a-utiful answer. I have one follow up question. How close to perigee and a full moon have to be to call it a supermoon? $\endgroup$
    – Coomie
    Commented Nov 17, 2016 at 0:09
  • $\begingroup$ @Coomie That's a good question and I have to admit, I don't know the answer. Cursory googling isn't helping me either. This would be a great question to ask on here though in a new question. $\endgroup$
    – zephyr
    Commented Nov 17, 2016 at 0:42
  • $\begingroup$ Note that the brightness is not just determined by the Moon's distance, but also by that of the Sun and, rather importantly, how close the Full Moon is to Antisol, i.e. the exact opposition to the Sun (barring eclipses). The October 2016 Full Moon was farther from Earth but closer to Antisol, and actually brighter than the November "Supermoon". $\endgroup$
    – AstroFloyd
    Commented Nov 25, 2016 at 18:16

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