What is a Supermoon?

Average full moon vs supermoon
Average full moon vs supermoon
Image by Marcoaliaslama

First, the bad news... The difference between a super-moon and a "normal" full moon is practically imperceptible to humans. Photos show the size difference well and this suggests that we should see a similar difference with the naked eye, but in reality our eyes can't really distinguish one full moon from the next. People get very excited to see a super-moon near the horizon but they're actually seeing the same optical illusion present in every full moon.

For this reason astronomers are somewhat ambivalent about promoting the super-moon (see below), although most would agree that it can be useful as a way to help people understand the Moon's cycles.

A super-moon occurs when two things happen at nearly the same time (it doesn't have to be exactly the same time but the closer the better):

  1. Full moon. This happens once every lunar cycle (approximately once per month). It's when the Sun, Earth and Moon are aligned with each other. Earth is in the middle so we have the Sun on one side and the Moon on the other. We see this when the Moon rises in the east at roughly the same time the Sun is setting on the opposite side. At this time the Moon appears fully lit by the Sun.
  2. Perigee. The Moon's orbit around the Earth is elliptical so sometimes it's closer and sometimes it's farther way. The point where the Moon comes closest to Earth is called perigee.

The full moon doesn't usually coincide with perigee. When it does, it's a super-moon.

How often does a super-moon happen?

Several times per year on average. Each one is slightly different and one of them will be the "best" of the year, meaning that the full moon and perigee will happen closest together in time (perhaps within about half an hour). Most publicity tends to centre around the best super-moon of each calendar year.

How big and bright does the Moon get?

The Moon appears about 14% bigger and up to 30% brighter than it does when it's at its farthest point from Earth (apogee).

Important: This does not mean 14% bigger and 30% brighter than normal, it means bigger and brighter than the other extreme when it's smallest and faintest.

What do astronomers think about super-moons?

Astronomers call a super-moon perigee full moon. The name "super-moon" is unofficial and was originally promoted by astrologers (people who promote the belief that celestial cycles affect life on Earth). Astrologer Richard Nolle claims to have coined the term.

Most astronomers (scientists who study the Universe) don't pay much attention to the super-moon. The "closeness" effect is very minor and doesn't offer any opportunities for study. Observing the Moon through a telescope is not noticeably different, and in any case astronomers don't like observing the full moon because it's too bright and the lack of shadows make it hard to see detail. Also, the full moon pollutes the rest of the sky with light so it's just not a happy time for telescope users.

However the super-moon is appreciated by many in the astronomical community, especially those involved in outreach and education. The super-moon gets people's attention and encourages them to look up at the night sky.

This leads to disagreement about whether astronomers should be promoting super-moons. There are those who think any tool to get people interested in astronomy is good, while others think that exaggerating the super-moon phenomenon is counter-productive and even unethical. My personal feeling is that we must be honest about the visual reality (you probably can't see the difference) but at the same time the super-moon is a real thing and it's still cool to think that you're looking at the "best" full moon of the year. People love it (for now at least), so let's use it as a tool to improve public understanding of our relationship with the Moon.

Can the Super-moon cause earthquakes or other environmental effects?

No. Technically speaking, the Moon moving closer to Earth causes a minute change in the effect of gravity but it's far too small to have any real-world consequences.

See also:

Page information

Author: Dave Owen