"Supermoon" is a term you will see splashed across the news several times a year. It is not an official astronomical term, and the effect is more subtle than the headlines suggest — but there is real science underneath, and knowing it makes the Moon more interesting, not less.

The Moon's orbit is not a circle

The Moon travels around the Earth on a slightly elliptical (oval) orbit, so its distance from us is not constant. Each month it swings between:

  • Perigee — its closest point, roughly 363,000 km away, and
  • Apogee — its farthest, roughly 405,000 km away.

That is a difference of more than 40,000 km, or about 12%. When a full moon happens to fall near perigee, we get a "supermoon"; a full moon near apogee is sometimes called a "micromoon".

How much bigger, really?

A supermoon appears about 14% larger in diameter and up to 30% brighter than a micromoon — the apogee full moon. That sounds dramatic, and in a side-by-side photograph it is obvious. In the real sky it is much harder to notice, for one simple reason: you never see the two full moons side by side. They are six months apart, and the eye has no reference to compare against. A lone supermoon looks, to most people, like a normal full moon.

The "huge moon on the horizon" is a different thing

The genuinely enormous Moon you sometimes see rising over rooftops is not the supermoon effect. It is the Moon illusion — a trick of human perception that makes the Moon look larger when it is near the horizon, next to familiar objects like buildings and trees, than when it is high overhead. You can prove it is an illusion: photograph the Moon low and high on the same night and it measures the same size. The supermoon (a real 14% change) and the Moon illusion (a perceptual effect) are often confused, but they are unrelated.

How to actually enjoy a supermoon

  • Watch it rise. A supermoon near the horizon, magnified by the Moon illusion and tinted orange by the thick air, is the most striking way to see it — even though the extra size at the horizon is the illusion, not the "super".
  • Check the exact moonrise time for your location and face east. Our full moon pages list the upcoming full moons, including which are supermoons, and local moonrise times by city.
  • For photos, include a foreground. A lone Moon in a black sky gives no sense of scale; a Moon rising behind a hill, a building or a landmark is what makes supermoon photographs look huge. A long lens (200 mm+) compresses the scene and exaggerates the effect.

Do supermoons do anything?

They very slightly increase the tides — a "perigean spring tide" a little higher and lower than usual — because the closer Moon pulls marginally harder. Beyond that, the claims you sometimes read about supermoons triggering earthquakes, strange behaviour or disasters have no scientific support. A supermoon is a lovely, ordinary full moon that happens to be a bit closer. For how full moons fit into the monthly cycle, and where terms like "blue moon" come from, see moon phases explained.