Size comparison of a supermoon with an average full Moon
15 August 2024
(See also: blue moon)
Every three or four months we are treated to a spate of news stories about how this month’s full Moon will be a supermoon. And in the summer and fall of 2024, we’ll get four supermoons in a row.
But for the casual observer a supermoon is not a thing. The Moon will not appear significantly different from a regular full Moon. The hype is completely undeserved.
A supermoon is when a full or new Moon is at its closest approach (perigee) to the Earth. While a new Moon can be a supermoon, new Moons are difficult to see, a sliver of a crescent during daylight, so they don’t get the media attention. A full supermoon does have a slightly larger angular size than a normal one, about two arcminutes bigger than an average full Moon—the unaided human eye can discern a difference of about one arcminute, and the full Moon is around 30 arcminutes in size. So the difference to the casual observer (one thirtieth or three percent) is really not noticeable; most people need some kind of instrumentation to discern the difference in size. Also due to an optical illusion, the Moon in any phase will appear significantly larger when it is near the horizon than when it is high in the sky.
The one noticeable difference with a supermoon has to do with the tides. The tidal difference is due to the Sun, Moon, and Earth being in alignment (syzygy), with the gravitational pull of both the Sun and Moon adding to or canceling each other. High tides will be higher at a full supermoon, and low tides will be lower at a new supermoon. If you own a boat or are routinely affected by tidal flooding, then a supermoon might be a concern.
The term supermoon was coined by astrologer Richard Nolle in the September 1979 issue of the magazine Dell Horoscope. Nolle and other astrologers claim that earthquakes, volcanic eruptions, and other natural disasters tend to occur during supermoons. This, like anything else spewed out by astrologers, is simply not true.
I don’t want to dissuade anyone from going out to gaze at the full moon. By all means do so, whether or not it is “super.” The Moon is beautiful and well worth taking your time to view. But if you want to see an impressively large Moon, you don’t need to wait for a supermoon. Go out shortly after moonrise on any clear night. If the Moon is low on the horizon, it will appear impressively bigger than any supermoon.
Sources:
Oxford English Dictionary, third edition, June 2015, s.v. supermoon, n.
Photo credit: Marco Langbroek, 2011. Wikimedia Commons. Licensed under the Creative Commons Attribution-Share Alike 3.0 Unported license.