Supermoon Meaning and the Hype Around It

To see the difference yourself, photograph a perigee full Moon beside a landmark, then repeat from the same spot with the same settings about seven months later, near apogee, and compare.

A supermoon is a popular label for a full or new Moon that happens when the Moon is near perigee, its closest point to Earth. The Moon's orbit is an ellipse, so its distance changes through each trip around. Perigee is the near point and apogee the far point, and the Moon moves between them about once a month.

The label is not a separate phase or a different kind of Moon. It describes a coincidence of timing, a full or new Moon and a close approach, and astronomers usually say perigee full Moon or perigee new Moon.

The word was coined in 1979 by an astrologer, and news and social media carried it from there. It has no official definition. Where to draw the line for near perigee is a choice, and different sources pick different cutoffs, so lists of supermoons do not always agree.

A label that depends on a chosen threshold works well for headlines and less well for measurement. That does not make the Moon's changing distance unreal. It only means the word is a convenience and not a category.

The distance does change, and astronomers track it precisely. The technical phrase for a full or new Moon near the Moon's closest approach is perigee-syzygy, where syzygy means the Sun, Earth and Moon lined up. The plain-language version is the one most people use when they check a calendar.

NASA cites a supermoon as up to about 14 percent larger across and up to about 30 percent brighter than the smallest full Moon, which falls near apogee. Those are the extremes, measured against the smallest full Moon, so the gap against a typical one is smaller.

An ordinary full Moon is only about half a degree wide, and the eye has no good yardstick for a change of a few percent of that. The Moon also looks larger near the horizon than overhead, an effect of perception that photographs do not show, and it is far more noticeable than the supermoon difference. Brightness is just as hard to judge, because your eyes adjust.

Astrologers who use the term often read a supermoon as an amplified full or new Moon: stronger feelings and bigger turning points. Others use it little or not at all. These are interpretive readings, not measurements, and they work best as a prompt for reflection, such as asking what you would like to look at more closely this week.

News stories add claims that a supermoon brings disasters, earthquakes or odd behavior. These claims have no good support, and a bigger-looking Moon is not a stronger force on people.

One real effect exists. Tides run somewhat higher when a full or new Moon is near perigee, because the Moon's pull on the oceans is a little stronger when it is closer. Tide tables already allow for it, and it is a matter of water levels and not of mood or events.

You do not need equipment, only a clear view and a little time.

1. Check a Moon phase calendar or a source that lists perigee to see when a full Moon is also close.
2. Pick a spot with a clear view of the eastern horizon and go around moonrise.
3. Dress for the weather, watch your footing in the dark and tell someone where you are going.
4. Look at the Moon beside a landmark, such as a tree line, and notice how large it seems.
5. Photograph the same scene from the same spot, with the same camera settings, about seven months later, when the full Moon falls near apogee, and compare the two. A month later the Moon is still near perigee, so you would see little difference.
6. Write one line about what you noticed.

The size, the name and the disaster claims are covered above. These three points rarely make the headline, and each one explains why two supermoon lists can disagree.

Several popular names get mixed up with supermoon. They describe different things, and only two of them have anything to do with the Moon's distance.

The Moon takes about 29.5 days to go from one full Moon to the next, but only about 27.5 days to go from one perigee to the next. The two cycles drift apart by about two days a month, so each full Moon slips a little against the Moon's closest approach and then lines up again after about 14 months.

That is why labeled supermoons tend to arrive in short runs with a long gap between. The exact number in a year depends on the cutoff a source picks for near perigee, which is one more reason published lists differ.

A single photo has nothing to compare it with, so it rarely looks bigger. Side by side, two photos taken from the same place with the same lens and settings do show a difference, though you need a perigee full Moon and an ordinary one taken months apart. A phone camera with default settings tells you little.

A supermoon looks at most about 14 percent larger than the smallest full Moon, and claims that it causes disasters or odd behavior have no good support.