The biggest supermoon, as seen from Earth, is compared to the smallest it appears when farthest from us.

upermoons have been around for, oh, I don’t know, roughly 4.5 billion years. They just didn’t used to be called that. In fact the origin of the term is really, really recent and, shall we say, unconventional by normal scientific conventions (as you’ll learn below, the word has nothing to do with science).

And even by the overused definition, they’re not very rare. In fact, there will be three of them this year: Jan. 21, Feb. 19 and March 21. But science journalists and mainstream media love to hype them up. So how big a deal are they?

First, here’s how they work:

The moon’s orbit around Earth is not a perfect circle. The moon is, on average, 238,855 miles (384,400 km) from us, but the distance varies by 26,465 miles. When a full moon happens while the moon is at or near its closest point to Earth (called perigee) it’s called a supermoon. The exact timing and proximity dictates just how super.

A supermoon can be up to 14 percent bigger and 30 percent brighter than a full moon occurring at its farthest point from Earth (apogee). But by the commonly accepted definition, any full moon that is within 90 percent of its closest approach to our home planet gets the blessing.

An airplane flew in front of a supermoon as it rose over the LBJ Presidential Library Nov. 14, 2016, in Austin, Texas. Telephoto lenses and foreground objects can create an effect like this that you don’t see with the naked eye. Photo by Jay Godwin

Supermoon is a catchy term, and a supermoon can be notably bigger to a casual observer, but don’t expect the moon to be dramatically gigantic. Photos that make the moon look positively gargantuan involve telephoto lenses and strategically located foreground objects.

Oh, and about the origin of the term. According to NASA and other astronomy experts, the term “supermoon” was coined in 1979 by astrologer Richard Nolle. Astronomers and astrologers don’t usually see eye-to-eye on the meanings of heavenly objects and occurrences, but on this one they’ve come to agree, at least on terms.

You can think what you will about that.

Explainer of things, independent health and science journalist, author, former editor-in-chief of LiveScience and Space dot com.

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