Supermoon: definition, perigee, real effects and upcoming dates

A word born in astrology

The term supermoon was coined in 1979 by the American astrologer Richard Nolle. Its definition: a new or full moon that occurs when the Moon is "at or near" (at least 90%) its closest approach to the Earth along its orbit. The term is not official in astronomy. Astronomers prefer the technical term perigee-syzygy: the perigee is the point of the orbit closest to the Earth, and syzygy is the alignment of the Sun, the Earth and the Moon (new or full moon).

As there is no official threshold, the number of supermoons per year varies with the definition used: NASA is often cited for a threshold of about 362,000 km for a full moon; other sources are stricter.

Why the distance changes

The Moon's orbit is an ellipse, not a circle. Its distance from the Earth therefore varies from about 357,000 km at closest to more than 406,000 km at farthest, a difference of nearly 50,000 km. The average is about 384,400 km. This variation depends on the orbit and also changes from one month to the next under the influence of the Sun: not all perigees are alike.

Full moons fall at perigee only every so often, roughly every 13 to 14 months on average, because the phase cycle (29.53 days) and the perigee cycle (27.55 days) do not coincide.

What we actually see

A full moon at perigee is up to 14% wider than a full moon at apogee, and about 30% brighter. These figures are extreme differences, measured against the most distant one. Compared with an ordinary full moon, the difference is much smaller, and the human eye, which has no standard to compare against, almost never notices it without measurement.

What really strikes people, however, is the rising Moon: near the horizon, any full moon looks giant, because of the Moon illusion. This illusion is much stronger than the effect of distance, and its cause is not agreed upon. To feel the real difference, you have to photograph the Moon with the same camera several months apart.

Tides and earthquakes: what is true and what is not

A full moon close to perigee slightly strengthens the tides: these are called perigean spring tides, or "king tides". The effect is small on a planetary scale, but it can be felt locally, for example in beach erosion. On the other hand, no link has been found between supermoons and earthquakes or volcanic eruptions: this idea, regularly circulated, has been refuted.

A few date markers

A few examples from published lists: the full supermoon of 14 November 2016 was the closest since 1948; we will have to wait until 25 November 2034 for a full moon to come closer, and until 6 December 2052 for the closest of the 21st century. The next full moons close to perigee are given for 24 December 2026 and 22 January 2027 (UTC times). These dates should be checked on an ephemeris site before planning an evening, as they vary with the time zone and the definition used.

You sometimes hear expressions such as "blood supermoon" or "blue supermoon": these are overlaps (total eclipse, calendar blue moon, perigee) that occur by chance and are rare.

Contemporary symbolism: intensity

In practices inspired by esotericism, the supermoon is read as an "amplified" full moon: a moment of stronger emotions, clarity, gratitude or letting go. No study confirms this. It is a modern reading: for many people, it mainly serves as a pretext to go out and look at the sky, which is already a lovely thing.

• Watch the Moon rise from a clear horizon, with a landmark (church tower, hill).
• Photograph the Moon with the same settings at each full moon to compare.
• Make the evening a time of gathering or gratitude, as for any full moon.

Frequently asked questions

Also read

The meanings mentioned belong to tradition and symbolism. They are neither promises of results nor medical advice. Back to Guide to the moon.