A supermoon is a full moon that happens when the Moon is at or near perigee, the closest point in its slightly oval orbit around Earth. At its biggest, a full moon at perigee can look up to about 14 percent larger and 30 percent brighter than a full moon at the far end of the orbit, according to NASA. "Supermoon" isn't an official astronomical term, and the extra size is harder to see than the headlines suggest, but the brightness is real.
Why the Moon's distance changes
The Moon's orbit isn't a perfect circle. It is an ellipse, so the Moon swings closer to Earth and then farther away once every orbit. NASA gives the closest point, perigee, as about 226,000 miles (363,300 kilometers) and the farthest, apogee, as about 251,000 miles (405,500 kilometers). Those are typical figures; the exact distances vary a little from orbit to orbit because the Sun's gravity tugs the orbit out of shape.
Here's the part that makes supermoons come and go. The Moon takes about 27.5 days to get from one perigee to the next, but about 29.5 days to get from one full moon to the next. Because the two cycles don't match, the full moon slides gradually around the orbit. For a few months in a row the full moons land near perigee, then they drift away toward apogee, then back again. The full moon and perigee realign roughly every thirteen and a half months, which is why supermoons tend to arrive in runs of three or four and then disappear for most of a year.
Who coined the word
The term came from astrology, not astronomy. The astrologer Richard Nolle introduced "supermoon" in 1979, defining 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. He used it in his own forecasting work, and for decades it stayed a niche word.
It took off in the 2010s, when news outlets and space agencies picked it up because it got people outside looking at the sky. Astronomers would more formally call it a perigee full moon, or a perigee syzygy, syzygy being the word for the Sun, Earth and Moon lining up. Because "supermoon" has no official definition, different publications draw the line in different places, and one source may count three supermoons in a year while another counts four.
Can you actually see the difference?
The brightness, yes. A perigee full moon throws noticeably more light, enough that on a clear night in the country you may see sharper shadows. The size is another matter. With nothing to compare it to, a Moon that is a seventh wider than the smallest full moon of the year looks much the same to most people, and compared with an average full moon the difference is only about half that.
What fools us far more is the Moon illusion. A Moon near the horizon looks enormous next to trees and rooftops, then seems to shrink as it climbs, even though its measured size barely changes. That effect is much bigger than the perigee effect. So if you want the supermoon experience, watch it rise:
- Go out at moonrise. A full moon rises around sunset in the east. Check a local almanac or weather app for the time.
- Find a low horizon. A field, a beach, a hilltop or a long street facing east.
- Give it something to stand next to. A distant water tower or church steeple makes the Moon look huge in a photograph, especially with a zoom lens from far away.
- Don't expect a red Moon. Any orange tint comes from looking through thick air near the horizon, the same thing that colors a sunset.
Tides, earthquakes and other claims
The Moon's pull is the main driver of ocean tides, and it is a little stronger when the Moon is closer. Around a perigee full or new moon, high tides run somewhat higher than average and low tides lower. On their own these are modest. Combined with a storm surge or heavy rain, they can contribute to coastal flooding, which is why weather services sometimes mention them.
Earthquakes are a different story. Seismologists have studied whether tidal stress nudges faults that are already close to slipping, and any effect they have found is small and statistical. Nobody can predict an earthquake from a supermoon, and there's no reason to change your plans because of one. The same goes for claims about supermoons and hospital admissions or mood, which we look at in does the moon affect your mood.
Supermoons in astrology
Because the word was born in astrology, it's no surprise some astrologers still give it weight. In that reading, a supermoon is an amplified full or new moon: whatever the lunation is about, by its sign and the houses it touches in your chart, gets turned up a notch. Plenty of other astrologers don't use the term at all, since traditional astrology has no doctrine of perigee lunations. Either way, the core meaning still comes from the lunation itself, so a supermoon in Taurus is first a Taurus full moon.
There is one perigee-and-apogee idea that is long-standing in modern astrology. The point in space where the Moon reaches apogee is called Black Moon Lilith, and some astrologers place it in birth charts. It's unrelated to supermoons in meaning, but it is the same orbit doing the work.
Our full moon calendar lists the sign of each full moon. If you'd like to mark a bright one, the full moon ritual guide keeps it simple, and The Moon card, all reflected light and instinct, has its own page in our tarot card meanings.
When the labels stack up
You'll sometimes see headlines about a "super blood moon" or a "super blue moon." Each label describes something different, so they can combine:
- Super is about distance: the full moon falls near perigee.
- Blue is about the calendar: a second full moon in a month, or the third of four in a season. See what is a blue moon.
- Blood is usually about shadow: a total lunar eclipse, when the Moon turns copper red. See blood moon meaning.
A super blood moon, then, is a total lunar eclipse that happens near perigee. It is a little larger in the sky than other eclipsed moons, and like all total lunar eclipses it is perfectly safe to watch with the naked eye.
Questions readers ask
How often do supermoons happen?
Usually a few times a year, in a cluster of consecutive full moons. The exact count depends on whose definition you use, since there's no official cutoff for how close to perigee a full moon has to be.
What is a micromoon?
The opposite of a supermoon: a full moon near apogee, the farthest point of the Moon's orbit. It is the smallest and dimmest full moon of its cycle, though, as with supermoons, most people wouldn't notice without a side-by-side photograph.
Is there such a thing as a new moon supermoon?
By Nolle's original definition, yes, since it covered both new and full moons near perigee. You can't see a new moon, though, so it only shows up in tide tables, where perigee new moons bring slightly bigger tides, just as perigee full moons do.
Is a supermoon closer to Earth than usual?
Yes, that's the whole definition. The Moon is somewhere near the low end of its distance range, a few tens of thousands of kilometers closer than it is at apogee. It is still hundreds of thousands of kilometers away.
