General

What is a supermoon?

Considering that the Moon is our closest celestial body, it’s not surprising that we’re always looking at it, investigating it, and defining it. But while seeing the Moon in our night sky is something that unites people across the globe, there are times when it inspires a little extra awe. Every once in a blue moon (ba-dum-ching!), there is a "supermoon." So what makes this lunar event special?

So, what’s a supermoon?

Unlike a blue moon, which has multiple definitions, supermoons are far easier to explain. In short, this event occurs when there’s a full moon and the Moon is also at its closest position to the Earth as it moves through its orbit. It’s easy to spot a supermoon because the Moon looks bigger in the night sky. A supermoon can be as much as 14 percent bigger and 30 percent brighter than a “regular” full moon.

As compared to other lunar events, the term supermoon is fairly new. The term was coined in 1979 by an astrologist, Richard Nolle, but it has since been used by meteorologists and news reports because of its layman's appeal. Astronomers, meanwhile, prefer to call this lunar phenomenon a perigean full moon, or by its most accurate and scientific name, "perigee syzygy"—which, admittedly, doesn't roll off the tongue the same way that "supermoon" does.

What causes a supermoon?

Rising full moon in a desolate desert landscape
Credit: Michael Andrew Just/ Shutterstock

So, supermoons refer to the Moon’s position relative to the Earth while it’s in the full moon phase of the lunar cycle. A lunar cycle is always 29.53 days and represents the length of time it takes the Moon to transition between the various phases (i.e. new moon, crescent moon, etc.). However, the amount of time that it takes the Moon to orbit around the Earth is slightly shorter, at 27 days. But the Moon’s orbit isn’t a perfect circle with a consistent distance. Instead, it’s an off-center ellipse, and this is because of the gravitational influence that the Earth and other celestial bodies like the Sun or other planets have on the Moon.

The Moon’s orbit explained

Within the Moon’s elliptical orbit, there are two specific points that reference when it is at its closest and farthest distances from the Earth. They’re known as apogee and perigee. Apogee is when the Moon is farthest away at 253,000 miles, and perigee is when it’s at its closest point—226,000 miles. But then you can have something called an extreme perigee. And this is where things get tricky because a perigeal full moon isn’t always the same distance.

Wait, what?

Full moon rising over a city skyline
Credit: kyoshino/ iStock

Yes, we know, but stick with us here because it’s just a little tricky. Keep in mind that a lunar cycle is 29.53 days and the Moon’s orbit is 27 days. The two events don’t have identical timelines. Specifically, when the Moon is in perigee or apogee, it’s not just a one-day event but can cover multiple days.

Likewise, it takes time for the Moon to shift into and complete the full moon phase of a lunar cycle. So, not only is this why supermoons aren’t a regular occurrence, but it also explains why when a supermoon or perigeal full moon occurs, the measured distance between the Earth and Moon can vary between events.

Are supermoons common?

Close up of full moon in the pitch black sky
Credit: Onfokus/ iStock

If you’re asking if they happen every month like clockwork, the answer is no. But if you compare them to events like lunar eclipses or blue moons, supermoons are much more common. Typically, in a year with 12 to 13 full moons, as many as three or four can be supermoons. In 2016, a supermoon occurred consecutively for three months in October, November, and December, and in 2019, the first three months of the year featured supermoons.

Can supermoons coincide with other lunar events?

Yes, they can! Supermoons tend to be an anticipated event when they happen by themselves, but there have been times when they coincide with lunar events. One such occasion happened on January 20-21, 2019. Because of the lunar eclipse, the moon appeared reddish-brown (called a "blood moon"), and because the Moon was in perigee, it appeared very large in the sky. To add to those distinctions, full moons in January are often called "wolf moons," so the supermoon that January was officially dubbed a "Super Blood Wolf Moon."