As we mentioned previously, "supermoon" isn't a scientific term. By several accounts, it was first coined back in the late 1970s by an astrologer, Richard Nolle, who defined it as a new or full moon that occurs when the moon is within 90 percent of its closest approach to Earth in a given orbit — 224,641 miles or 361,524 kilometers from the planet [source: King].
That definition, as astronomers point out, is a pretty easy one to fit. For all the hype that surrounds them, supermoons actually aren't all that rare, occurring between four and six times each year [source: McClure and Byrd].
Supermoons happen because the moon doesn't orbit the earth in a perfect circle, but rather in an elliptical path. That means its distance from the Earth can vary in the course of a month, from 252,000 miles at its farthest point, which is called the apogee, to roughly 225,800 miles at its closest approach, which is called the perigee [source: King].
But the lunar orbit isn't perfectly regular, either, because it's affected by the sun's gravity. So on some occasions, the moon's perigee is closer than others [source: Astropixels.com]
Perigees happen on average every 29.53 days [source:McClure and Byrd].
But a few times a year, the sun, moon and Earth line up just right, so that a perigee coincides with the most dramatic phases of the moon — the new moon, when the orb is just a sliver — or the full moon. That's when we notice because the moon can appear to be slightly bigger and significantly brighter than normal [source: King].
That said, the distinction between a supermoon (what astronomers call a lunar perigree) and a regular-sized moon isn't so great that it's obvious to everyone, especially if you're not a regular gazer at the skies. If you want to make the difference clearer, astronomy writer Bob King suggests fashioning a homemade measuring device that he calls a "supermoon sighter."
Take a pair of scissors, and cut a series of slots of varying widths in an index card. Then hold the card parallel to your face and at arms' length while looking at the moon with one eye, and pick the slot into which the moon fits most snugly. Mark the date next to it. Then, when you hear the announcement of the next supermoon, repeat the procedure, and you should be able to see the difference.