What Is a Supermoon?
How Many Supermoons in 2020?
What is a supermoon? We agree that it’s a catchy word—and anything that encourages us to explore the night sky is positive—but let’s also get our facts straight. Plus, find out how many supermoons will occur in 2020—and when to see the next supermoon!
What Is a Supermoon?
Generally speaking, a supermoon is a full moon that appears larger than a typical full moon due to it being closer to Earth.
However, there’s a lot more to it than that! In fact, there are a couple definitions of “supermoon” out there. Let’s go through the two most popular ones, which we’ll refer to as the “broad” definition and the “strict” definition:
- Broad Definition: A supermoon is a new or full moon that occurs around the same time as when the Moon is also at the point in its monthly orbit where it closest to Earth. By this definition, there can be several supermoons in a year. (We explain this more further down the page!)
- Strict Definition: A supermoon is the closest new or full moon of the year. By this definition, there can be only two supermoons each year (a full moon supermoon and new moon supermoon).
In the media, most people go by the broad definition, since it’s much more exciting to be able to talk about multiple supermoons instead of just one or two!
(Note: In popular usage, “supermoon” tends to refer only to full moons, rather than both full and new moons. Given that new moons are not much fun to look at, this usage makes more sense!)
How Many Supermoons in 2020?
Going by the broad definition of the term (and ignoring new moons), there will be three “supermoons” in 2020—and they’ll happen one right after the other!
- March’s Full Worm Moon: March 9 at 1:48 P.M EDT
- April’s Full Pink Moon: April 7 at 10:35 P.M. EDT
- May’s Full Flower Moon: May 7 at 6:45 A.M. EDT
* Some sources stated February’s Full Moon was a Supermoon but it doesn’t fit our definition.
April’s full Moon is the closest supermoon of the year, sitting at a distance of approximately 221,800 miles from Earth.
Why Do Supermoons Occur?
It all comes down to the fact that the Moon’s orbit around Earth is not a perfect circle—in fact, it’s an elliptical (oval) shape.
Because of this, the Moon’s distance from Earth changes as it travels around our planet. Additionally, Earth doesn’t sit directly in the middle of this elliptical orbit, so there are points in the Moon’s orbit where it is closest and farthest from Earth. These points are called perigee and apogee, respectively.
- Perigee is the point in the Moon’s orbit where it is closest to Earth.
- Apogee is the point in the Moon’s orbit where it is farthest from Earth.
The Moon makes one full orbit around Earth in about 29.53 days, which means that it reaches its perigee and apogee points about once a month. When this occurs at the same time as a full moon, it’s called a perigee syzygy—or, more commonly, a supermoon!
- “Syzygy” is the astronomical term for when three or more celestial bodies (such as the Sun, the Moon, and Earth) line up. When the Sun, Earth, and Moon form a syzygy, we experience a full or new moon, depending on whether the Moon is between the Sun and Earth or Earth is between the Sun and the Moon.
Where Did the Term “Supermoon” Come From?
Although it has been all over the news in recent years, “supermoon” is not an official astronomical term. In fact, it didn’t even exist until astrologer Richard Nolle coined it in 1979!
At the time, Nolle defined a supermoon 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.” This definition is what most people go by today, though we tend to pay attention only to the full moon supermoons, since they’re a lot more interesting to look at!
Does a Supermoon Really Look Bigger?
Given that a supermoon full moon is closer to Earth than a normal full moon, it does appear larger—about 7% larger, technically speaking. This means that the difference between a full moon at perigee and a full moon at apogee can be up to 14%, which is significant.
Here’s the key fact, however: Unless you were somehow able to compare a normal full moon and a supermoon side by side in the sky, it’s nearly impossible to perceive a 7% difference in the Moon’s size.
Even if you could somehow place the year’s biggest possible Moon (the perigee full moon) next to the smallest one (an apogee full moon) in the sky, you’d just barely tell the difference. And that’s with the absolute extreme Moons!
The bottom line is that it’s difficult to truly perceive any difference at all in the Moon’s size from one month to the next, or one night to the next, so don’t get your hopes up about seeing a gigantic Moon out there.
The Moon Illusion
Okay, if you want to be guaranteed of seeing a huge-LOOKING Moon, it’s easy… Simply watch the Moon when it’s rising or setting!
A Moon down near the horizon will always look enormous, thanks to a well-known phenomenon called the Moon illusion, which makes our minds exaggerate the size of objects near the skyline.
Try it! If you want a truly massive supermoon, you can have it—any night!
About This Blog
Welcome to “This Week’s Amazing Sky,” the Almanac’s hub for everything stargazing and astronomy. Bob Berman, longtime and famous astronomer for The Old Farmer’s Almanac, will help bring alive the wonders of our universe. From the beautiful stars and planets to magical auroras and eclipses, he covers everything under the Sun (and Moon)! Bob, the world’s mostly widely read astronomer, also has a new weekly podcast, Astounding Universe!