What is a Supermoon?

A Full Moon That is Closest to Earth

March 15, 2019
Supermoon Silhouette

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What is a supermoon? We agree it’s a catchy word—and anything that encourages us to explore the night sky is positive—but also let’s get our facts right.

What is a Supermoon?

The term “supermoon” is not an astronomical term. In fact, it did not even exist until someone concocted it recently.

  • “Super” has come to mean that time when the Moon will be closest to Earth in its orbit during the full Moon.
  • It has also meant the closest of two or three Moons each year.
  • More recently, I saw an article quoting, ”4 to 6 supermoons each year.” It was accompanied by a photo of a giant red Moon; they were obviously confused between a totally eclipsed Moon and a merely close Moon. Additionally, they explained that a supermoon “looks 14% larger than normal.”

No, no, and no. Let’s put this to rest. This will also explain why we have not historically used the term “supermoon” in our astronomy calendar in the annual Old Farmer’s Almanac

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Supermoon Facts

The official astronomical name for a supermoon is ”perigee full Moon” or “perigee syzygy.” Not quite as catchy, perhaps.

The word “perigee” refers to the Moon’s closest point to Earth in its monthly orbit, and a “syzygy” is when celestial bodies line up (as the Sun, Earth, and Moon do during a full Moon).

The Moon has an oval or elliptical orbit, not a round one, which means that every night it’s either a bit closer or farther from the Earth than it was the night before.

  • At its closest extreme, the Moon can be 14% closer and thus look 14% larger than at its farthest yearly extreme.
  • Put another way, it varies 7% from its average size

Here’s the key fact: Nobody can perceive a 7% difference in the Moon’s size.

perigee-apogee_full_width.jpg

And if you could somehow place the year’s biggest possible Moon next to the smallest one in the sky, you’d just barely tell the difference. And that’s with the absolute extreme Moons. 

Bottom line: It’s hard to truly perceive any difference at all between the Moon’s size from one month to the next, or one night to the next.

How to see a Huge Moon

OK, 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 the well-known Moon illusion, which makes our minds exaggerate the size of objects near the skyline.

Try it! If you want a Supermoon, you can have it—any night!

Learn about the March 2019 “Super” Moon—the final one of the year.

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

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