Wednesday, August 18th 2021, 10:00 am - Supermoons are one of the most popular events in astronomy and stargazing.
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 June 24 Supermoon, the last 'super' Full Moon of 2021. Credit: NASA's Scientific Visualization Studio
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 up to one third brighter than a normal Full Moon!
WATCH BELOW: SEE EVERY VIEW OF THE MOON FOR 2021 IN LESS THAN 5 MINUTES
The term 'supermoon' isn't often used by astronomers. Still, to put supermoons into astronomical terms, they occur when the Moon reaches its Full or New phase at a distance from Earth closer than 361,524 km away.
This brings up a few questions. Why does the Moon's distance to Earth change? 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 2021. Credit: NASA's Scientific Visualization Studio/Scott Sutherland
The Moon's average distance from Earth is considered to be 384,400 km. During each of these elliptical orbits, however, 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, and 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.
The Moon's perigee distances from Earth tend to be between 356,400–370,400 km, but they can be even closer at times. The closest lunar perigee on record, so far, was 356,378 km, on January 4, 1912. The next time a perigee Full Moon gets closer will be 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.
This visualization directly compares the sizes of the May 26 Perigee Full Moon and December 19 Apogee Full Moon. Credit: NASA's Scientific Visualization Studio/Scott Sutherland
In 2021, there are technically four Super Full Moons, with the closest perigee Full Moon on the night of May 25-26, when it reaches a distance of 357,309 km. If you're keeping track, though, based on the exact timing, it could be said that there are only two this year.
Of the dozen Full Moons in 2021, those in April and May are the only ones precisely timed to occur while the Moon is close enough to be a supermoon. In March, the exact timing of the Full Moon was slightly early — 2:50 p.m. ET on the 26th, but it crossed into 'supermoon' distance around 10 p.m. ET that night. In June, it's slightly late — Full Moon is at exactly 2:40 p.m. ET on the 24th, but it will have slipped beyond that 'supermoon' distance roughly 25 minutes before, at around 2:15 p.m. ET. Still, in both cases, the Moon can still easily be considered 'full' when it's within the supermoon distance (in March it was still 99.7% full, and in June, it'll still be 100% full, according to NASA).
For comparison, the farthest apogee Full Moon is on December 19, at a distance of 406,007 km.
Not that anyone will see them, since they will be 'lost' in the daytime glare of the Sun, but there are also two Super New Moons in 2021 — on November 4 and December 4.
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, by 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.