How Supermoons Work

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 the moon's 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 moon's 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].

There are four full supermoons in 2023: one in July (the buck moon), two in August (the sturgeon moon, and making for a blue supermoon!) and one in September (the harvest moon).

That said, the distinction between a full supermoon (what astronomers call a lunar perigee) and a normal full 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.

Since the idea of supermoons was dreamed up by an astrologer, it's not surprising that phenomenon is seen by some as a sign of future events, and perhaps even as a cause of them — a "Moonageddon," as one Australian news website put it.

If you poke around the internet, you'll find seers linking supermoons to earthquakes, volcanic eruptions, monster storms and even wildfires, and noting that various high-profile natural disasters occurred right around the time that a supermoon loomed over the nighttime landscape.

The makers of those claims often stretch the concept of proximity to extremes — 2005's Hurricane Katrina, for example, has been linked in some accounts to supermoon effects, even though it actually occurred more than seven months after a supermoon [source: Portman].

And even when a supermoon does occur close in time to a natural disaster — such as the 9.1 earthquake and tsunami that hit Japan eight days before a supermoon in March 2011 — that isn't necessarily evidence of a connection between two events.

As James Garvin, chief scientist at NASA's Goddard Space Flight Center, explained in a 2011 interview, the earth stores a tremendous amount of energy in its crust, and the relatively small amount of force exerted by the moon's gravitational pull isn't enough to alter the processes that cause earthquakes [source: NASA].

Supermoons don't have that much more effect on the earth than the moon usually does. The only exception is that when the moon is closer to the earth than usual, it does cause tides to be slightly higher — but only by slightly more than an inch, at most [sources: Sessions, ScienceDaily].

The belief that supermoons can affect human behavior is based upon the assumption that a supermoon, because the moon appears bigger than normal, exerts some sort of unusual force upon Earth's inhabitants. But there's no evidence that is so.

When it comes to gravity, for example, the effect of a full moon at perigee is only about three one-thousandths of an ounce (110 milligrams) greater than the moon exerts at apogee — 1/9 of the mass of a paper clip. That effect actually is smaller than you'll encounter when you're near a mountain or even a large building [source: Sessions].

Even though supermoons don't really affect the earth much and aren't considered that important by scientists, they're still an interesting phenomenon to photograph. Here are some tips from photographers on shooting a supermoon.