What is a 'supermoon' and why is it so compelling to us?
This article has been updated.
If it seems like you've been hearing about supermoons a lot these days, it's not just a figment of your imagination. In any given year, there are at least four, and as many as eight, 'super' Moons. Given that there are 12 or 13 Full Moons every year, it may seem strange that we get excited about something that apparently happens so often, but nonetheless they are strangely compelling!
A 'supermoon' is a Full Moon that occurs when the Moon is closer to Earth than it normally is.
The term supermoon was thought up by astrologer Richard Nolle, in 1979.
Nolle defined it as "a new or full moon which occurs with the Moon at or near (within 90% of) its closest approach to Earth in a given orbit (perigee)."
The August 30-31 Perigee Blue Moon is the closest Full Moon of 2023. Credit: Scott Sutherland/NASA's Goddard Space Flight Center
With the Moon closer than normal at this time, it translates into a Super Full Moon appearing slightly larger in the night sky. Without having a specific reference, however, it's not easy to notice a difference in the Moon's apparent size.
What is more noticeable is the brightness of a Super Full Moon. A supermoon can appear around 15 per cent brighter than a normal Full Moon and up to 30 per cent brighter than the apogee Full Moon!
Although the term 'supermoon' is popular with the public, it isn't used very often by astronomers. Still, we can put supermoons into actual astronomical figures if we know the distance between Earth and the Moon at apogee, at perigee, and at the Full Moon, for each orbit it makes. Then it just takes a little bit of calculation.
This diagram reveals how to calculate whether a particular Full Moon is a Supermoon. Credit: Scott Sutherland/Fred Espenak
Fortunately, retired NASA scientist Fred Espenak has done the hard work for us.
However, this brings up a few questions. For instance, why does the Moon's distance to Earth change? Also, how close can it actually get?
If you were able to sit in a spaceship and watch Earth and the Moon from high above, it would appear as though the Moon was tracing out a circle in space as it travelled around the planet. As it turns out, though, if you carefully plot where the Moon is at each point in its orbit, it would be revealed that it actually travels along an ellipse.
This diagram of the Moon's slightly elliptical orbit around Earth shows the closest perigee and farthest apogee distances for 2023. Credit: NASA's Scientific Visualization Studio/Scott Sutherland
The Moon's average distance from Earth (one Lunar Distance) is 384,400 km. During each of its elliptical orbits, though, the Moon spends roughly half of the time a little closer to us than that, and the rest of the time a little bit farther away.
The astronomical term for when the Moon is at its closest distance to Earth during any particular orbit is perigee. The opposite point, when it is farthest from Earth on a particular orbit, is called apogee.
From here on the surface of Earth, we can't watch the Moon travel in the same way, but it's still possible to keep track of this elliptical orbit. We do this by watching the apparent size and brightness of the Moon change from day-to-day and month-to-month. When the Moon is closer, it appears larger and brighter. When it is farther away, it appears smaller and dimmer.
Since the Moon is not only influenced by Earth's gravity, but also the gravitational pull of the Sun, and the other planets in the solar system, each orbit around Earth is slightly different from the last. As a result, the Moon's orbital ellipse changes, month by month, and the timing and distance of the Moon's perigee and apogee change, as well.
This visualization directly compares the sizes of the February 5-6 Apogee Full Moon and August 30-31 Perigee Full Moon. Credit: NASA's Scientific Visualization Studio/Scott Sutherland
As a result, the Moon's perigee distances typically range from between 356,400–370,400 km. However, it can be even closer at times. The closest lunar perigee on record, so far, occurred on January 4, 1912, when the Moon was 356,378 km away. The next time a perigee Full Moon gets closer to us will be on January 1, 2257, when it reaches a distance of 356,372 km.
Apogee, on the other hand, varies between about 404,000–406,700 km. Even these are just averages, though. The absolute farthest apogee on record will be on February 3, 2125, at a distance of 406,718 km.
Of the 13 Full Moons in 2023, four of them are Super Full Moons — one in July, two in August, and one in September.
The Super Full Moons of 2023. Each of these comes to within 90 per cent of the Moon's closest approach to Earth during their respective orbit (July - 96.1%, August - 99.6%, Blue Moon - 99.7%, September 96.4%). Credit: Scott Sutherland/NASA's Goddard Space Flight Center/Fred Espenak
Why the fascination?
Since there are several of them every year, and we can see entire seasons dominated by them, supermoons are not exactly a rare occurrence.
There are definitely more spectacular things to see in the night sky. Meteor showers, lunar eclipses and planetary alignments are just a few that happen on a fairly regular basis. Just seeing the full splendour of a glittering sky of stars, with the Milky Way stretching from horizon to horizon, is absolutely awe-inspiring.
So, why are supermoons so compelling to us?
The Full Moon captured on the night of October 13-14, 2019. Credit: Alex Verville.
Besides the Sun, the Moon is our most common and recognizable sight in the sky, day or night. The thinnest Crescent Moon is an amazing sight to behold, and a Full Moon is a wonder to see. Even when we are standing under the brightest lights of the downtown core of a city, with urban light pollution washing out every other object in the night sky, if the Moon is up, we will see it shining bright.
For this reason, the Moon is a very important part of our lives. This is especially true for those who usually miss out on all the other astronomical events in a year. Quite simply, it is our most common point of connection with the universe beyond our planet.
So, when something about the Moon changes — even when it is just slightly bigger and brighter in the sky — it attracts our attention as something significant.
(Thumbnail image courtesy Greg Cross, who snapped this close-up image of the April 7, 2020 Super Pink Moon from Kelowna, B.C., and uploaded it into The Weather Network's UGC Gallery)