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. Here’s what a supermoon really is, why they occur, and when to see the next one!
What Is a Supermoon?
Let’s get right to it: 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 orbit closest to Earth.
Of course, there’s more to it than that. Read on to learn more and to find out when the next supermoons will happen.
(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 for the amateur astronomer, this usage of the term makes more sense!)
When Is the Next Supermoon?
Going by the popular definition of the term (i.e., only full moons), there will be three “supermoons” in 2020:
- 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
*April’s full Moon is the true closest supermoon of the year, sitting at a distance of approximately 221,800 miles from Earth.
Why Do Supermoons Happen?
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 on the opposite side of Earth from the Sun.
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 we go by today, though most folks 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 fairly 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 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.
How to See a Massive Moon
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!
Source: Farmer's Almanac - Amazing Sky