Supermoon and micromoon: the 14% that the photographs do not show
A full moon near perigee is about 14% wider than one near apogee. Here are the 2026 dates, the definition behind the word, and what it does to tides.
A supermoon is a full moon that happens near the closest point of the Moon’s orbit. That is all it is — no special geometry, no alignment, nothing that only happens once a decade. The orbit is an ellipse, the Moon’s distance varies by about 50,000 km over a month, and sometimes a full moon lands near the near end.
The numbers, computed with this site’s own engine for 2026:
| Distance | Apparent diameter | |
|---|---|---|
| Closest approach of the year | 356,688 km (December 24) | 33.49′ |
| Farthest approach of the year | 406,415 km (December 11) | 29.39′ |
| Largest full moon | 356,776 km (December 24) | 33.48′ |
| Smallest full moon | 406,116 km (May 31) | 29.41′ |
Two full moons five months apart, and the near one is 13.8% wider across and about 30% brighter in area. That is the entire phenomenon, and it is worth seeing at least once — but it is a subtle difference, not a transformation, and any photograph that shows otherwise is using a long lens.
Why the difference is so hard to see
Human perception does not measure angles directly; it judges size by comparison. With one Moon in an empty sky there is nothing to compare it against, so a 14% increase registers as “the Moon” rather than “a bigger Moon”. Put the same Moon behind a building at moonrise and the brain suddenly has scale cues — and it overestimates, which is why the illusion produces a far bigger apparent change than the physics does.
That is also why supermoon coverage fills up with photographs that are technically honest and completely misleading: a 600 mm lens compresses the foreground and the Moon into the same frame, and the Moon arrives looking impossible. Nothing in the sky changed.
The year’s smallest full moon is also a blue moon
The May 31, 2026 full moon is the farthest of the year at 406,116 km — a micromoon — and it is also the second full moon in May, which makes it a blue moon. The two labels come from unrelated cycles that happen to overlap: one from the shape of the orbit, the other from the length of the calendar. 2026 gives both at once.
What it does to the tides
The Moon’s tidal force scales roughly as distance cubed, so a perigean full moon raises the predicted tide by a few percent over an average spring tide — a few centimeters in most places, more where the coastal geometry amplifies it.
That is not nothing: NOAA’s coastal flood forecasting treats perigean spring tides as one of the factors that can push a high tide over the threshold, particularly when a storm surge or an onshore wind is superimposed. It is also not a catastrophe — the effect is predicted years in advance and appears in every tide table.
Earthquakes are a different matter. The tidal stress change is around 0.01% of the stress already in a fault, and statistical tests of the hypothesized correlation have not produced a reliable signal. A page that mentions supermoons and earthquakes in the same paragraph is usually selling something.
How to actually see the difference
Two methods that work:
- Photograph it on two dates with the same lens and framing. Take one on the largest full moon and one on the smallest of the same year. Compare the discs side by side; the 14% is obvious.
- Measure it. Hold a ruler at arm’s length: the Moon is about 5 mm across at 57 cm. On a night of a supermoon, a little more; on a micromoon, a little less.
What does not work is going outside and expecting a large Moon. If you want the large-looking Moon, the reliable recipe is the moon illusion — catch it at moonrise, low, with something familiar in the foreground — and that works on any night of the month.
Distances and diameters computed from the lunar theory in this site’s engine and checked against JPL Horizons. No claim is made about supermoons causing events on Earth beyond the tide.
Frequently asked questions
- How much bigger does a supermoon actually look?
- About 14% wider in angular diameter than the smallest full moon of the year, and roughly 30% brighter because brightness scales with area. On the night itself, without a second moon to compare with, almost nobody can detect the difference. The dramatic photographs come from a long lens and a foreground object, not from the Moon changing size.
- Is a supermoon dangerous, or does it cause earthquakes?
- No. The tidal force of a perigean full moon is a few percent stronger than an average full moon, which raises the predicted tide by a few centimeters. That matters for coastal flood forecasting when it coincides with a storm surge, and it is routinely predicted by tide tables. Claims that it triggers earthquakes have been tested repeatedly and do not hold up.
- What is the official definition?
- There is no official one. The term was coined by the astrologer Richard Nolle in 1979, defined as a full or new moon occurring when the Moon is within 90% of its closest approach to Earth in a given orbit. Astronomers use it informally because it has no formal status, and different outlets apply different distance thresholds, which is why the number of supermoons per year varies by source.
- Why does the Moon look enormous when it rises if the size is real and small?
- Because that is the moon illusion, a separate effect. When the Moon is near the horizon the brain scales it against familiar objects and overestimates its size; photographed at the same moment it measures the same as when overhead. The illusion and the distance effect are unrelated, and they are habitually confused in coverage of supermoons.
Sources
Each source is a primary or official reference, checked before publication. Links open the original.
- Meeus, Astronomical Algorithms, 2nd edition, chapters 47 and 54 — the lunar distance series and the relationship between distance and apparent diameter used for the figures on this page
- JPL Horizons (DE441) — the ephemeris against which this site's computed lunar distances are checked, agreement within 150 km over the 2026–2027 test dates (ssd.jpl.nasa.gov/horizons/)
- NASA Science — Moon: the elliptical orbit, perigee at about 363,300 km and apogee at about 405,500 km on average, and the resulting variation in apparent size (science.nasa.gov/moon/)
- NOAA — perigean spring tides and the combined effect of lunar perigee, spring tide and storm surge in coastal flood forecasting (tidesandcurrents.noaa.gov)