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How does a supermoon happen?

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Quick answer

A supermoon occurs when the full moon coincides with the moon's closest point to Earth in its elliptical orbit, called perigee. Because the moon can be up to about 50,000 km closer than at its farthest, it appears up to 14% wider and 30% brighter than a full moon at apogee.

A supermoon happens when a full moon coincides with the moon's closest approach to Earth in its elliptical orbit, making it appear slightly larger and brighter. Because the moon's distance varies by up to about 50,000 km, a full moon near perigee can look up to 14% wider and 30% brighter than one at its farthest point.

Step by step

  1. The moon travels around Earth in an elliptical orbit, so its distance from Earth changes continuously.
  2. The closest point in that orbit is called perigee, and the farthest is apogee.
  3. A supermoon happens when the moon reaches perigee at the same time it is in its full phase.
  4. During this alignment, the moon appears slightly larger and brighter than a full moon at average distance.
  5. The effect is most noticeable when the moon is near the horizon, where the moon illusion exaggerates its size.

Key things to understand

  1. The moon orbits Earth in an ellipse, so its distance changes from about 363,000 km at perigee to 405,000 km at apogee.
  2. A supermoon occurs when the full moon phase aligns with perigee, the closest point in that orbit.
  3. At perigee, the moon appears roughly 14% larger and 30% brighter than at apogee.
  4. The alignment of full moon and perigee does not happen every month because the moon's orbital orientation shifts.
  5. Supermoon is not a formal astronomical term; it was popularized by astrologers in 1979 and later adopted by media.

Labelled diagram

  1. Earth

    The observer's position at the center of the moon's elliptical orbit.

  2. Perigee

    The moon's closest point to Earth, about 363,000 km away.

  3. Apogee

    The moon's farthest point from Earth, about 405,000 km away.

  4. Full moon

    The phase when the moon is opposite the sun, fully lit.

  5. Supermoon

    When full moon and perigee coincide, making the moon appear larger.

How a supermoon aligns

Real-world examples

  • The supermoon of November 2016 was the closest full moon since 1948, appearing noticeably larger in photographs.
  • The supermoons of April and May 2021 fell in consecutive months, showing how supermoons tend to cluster.
  • The supermoon of May 2021 coincided with a total lunar eclipse, visible from the Pacific Ocean and parts of the Americas.
  • Photographers often capture supermoons rising behind landmarks like the Statue of Liberty or the Taj Mahal to show the size difference.

Common mistake

✗ Myth: A supermoon is a rare, dramatic event that makes the moon look enormous.

✓ Fact: A supermoon is only about 14% wider and 30% brighter than a full moon at its farthest. The dramatic size often comes from the moon illusion near the horizon, not from the supermoon itself.

People also ask

What is a supermoon and when is the next one?
A supermoon is a full moon that occurs when the moon is at its closest point to Earth in its orbit. The next supermoon depends on the year; they happen roughly three to four times annually, often in consecutive months.
How much bigger does a supermoon look?
A supermoon appears about 14% wider and 30% brighter than a full moon at its farthest distance. However, without a reference, most people cannot tell the difference with the naked eye.
Does a supermoon affect tides?
Yes, a supermoon can raise tides slightly because the moon is closer and its gravitational pull is a bit stronger. The effect is small, adding only a few centimeters to normal tidal ranges, and is not dangerous.
Does a supermoon look bigger to the naked eye?
The size difference is subtle to the naked eye, often only noticeable when compared side by side with a normal full moon. The moon illusion, where the moon looks larger near the horizon, can make any full moon appear huge.
How often do supermoons occur?
Supermoons occur roughly three to four times a year, but the exact number depends on how strictly you define the term. They tend to happen in clusters because the moon's perigee and full moon align for a few consecutive months.
Do supermoons cause natural disasters?
No. Supermoons have a slightly stronger gravitational pull, which can raise tides by a few centimeters, but they do not cause earthquakes, volcanic eruptions, or tsunamis. The extra tidal force is small compared to normal variations.
Why is a supermoon brighter than a regular full moon?
A supermoon is brighter because it is closer to Earth, so more of its reflected sunlight reaches us. The increase is about 30% compared to a full moon at its farthest point, though the difference is hard to detect without instruments.

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