What Is a Supermoon? The Science Behind the Biggest and Brightest Full Moon

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Scientific Definition

The term "supermoon" sounds informal, but it has a specific definition. The original definition, coined by astrologer Richard Nolle in 1979, states that a supermoon occurs when a Full Moon or New Moon is within 90% of its closest approach to Earth in a given orbit. In astronomical terms, this is called a perigee-syzygy — "perigee" meaning closest approach, and "syzygy" meaning the alignment of the Sun, Earth, and Moon.

NASA uses a slightly simpler working definition: a supermoon is a Full Moon that occurs when the Moon is within 90% of its minimum distance from Earth. The Moon's average distance from Earth is about 238,855 miles (384,400 km). During a supermoon, the Moon is typically closer than 224,000 miles (360,000 km) from Earth's center.

It is worth noting that "supermoon" is not a strictly technical astronomical term. The International Astronomical Union (IAU), the official body that names celestial phenomena, does not formally recognize the term. However, it has become widely adopted by NASA, media outlets, and the public because it is catchy and accurately describes a real phenomenon.

The Two Key Requirements

For a supermoon to occur, two things must happen simultaneously:

1. The Moon must be full — the Sun, Earth, and Moon must be aligned with Earth in the middle.

2. The Moon must be near perigee — the Moon must be at or near its closest orbital approach to Earth.

The Moon's orbit is elliptical, so its distance from Earth varies continuously. Perigee (closest point) and apogee (farthest point) occur once each per orbit, roughly every 27.3 days. A Full Moon happens every 29.53 days. Because these two cycles have different periods, a Full Moon does not always coincide with perigee — but when it does, we get a supermoon.

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How Supermoons Happen: The Moon's Elliptical Orbit

The Moon does not orbit Earth in a perfect circle. Its orbit is an ellipse — a slightly elongated oval shape. This means the distance between Earth and the Moon changes throughout each orbit.

Perigee vs. Apogee

• Perigee is the point in the Moon's orbit where it is closest to Earth. The average perigee distance is about 225,600 miles (363,300 km), though it can be as close as 221,500 miles (356,500 km) in extreme cases.
• Apogee is the point where the Moon is farthest from Earth. The average apogee distance is about 252,000 miles (405,500 km), and can reach 252,700 miles (406,700 km).

The difference between perigee and apogee is roughly 26,500 miles (42,600 km) — about 11% of the Moon's average distance. This is enough to produce a noticeable (though not dramatic) difference in the Moon's apparent size and brightness.

Why the Orbit Is Elliptical

No orbit in the solar system is perfectly circular. According to Kepler's laws of planetary motion, orbits are ellipses with the parent body at one focus. The Moon's orbit is particularly influenced by the gravitational pull of the Sun, which causes the eccentricity (the amount of elongation) to vary over time. The Moon's orbital eccentricity ranges from about 0.026 to 0.077 over an approximately 8.8-year cycle.

The elliptical orbit also means the Moon's orbital speed varies. It moves faster at perigee (closer to Earth) and slower at apogee (farther from Earth), following Kepler's second law of equal areas. This is one reason why the exact timing and distance of supermoons vary from month to month and year to year.

The Precession of the Lunar Orbit

The Moon's orbit does not remain fixed in space. The major axis of the ellipse slowly rotates (precesses) over an 8.85-year cycle called the apsidal precession. This means perigee does not always occur at the same point in the Moon's orbit relative to the stars. Additionally, the line of nodes — where the Moon's orbital plane crosses the ecliptic — precesses over an 18.6-year cycle. These long-period variations affect when and how closely a Full Moon coincides with perigee.

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How Much Bigger and Brighter Is a Supermoon?

The numbers often cited — 14% bigger and 30% brighter — are accurate but require context. These figures compare a supermoon (Full Moon at perigee) to a micromoon (Full Moon at apogee). This is the maximum possible difference.

Size Comparison

The Moon's apparent angular diameter varies from about 29.43 arcminutes at apogee to about 34.07 arcminutes at perigee. The difference of roughly 4.6 arcminutes represents about a 14% increase in apparent diameter. In practical terms, if you held a ruler at arm's length, the difference in the Moon's width would be less than a millimeter.

Brightness Comparison

The Moon's brightness follows an inverse square law with distance. Because the supermoon is closer, more reflected light reaches Earth per unit area. A perigee Full Moon is roughly 30% brighter than an apogee Full Moon — but this is the extreme comparison. Compared to an average Full Moon, a supermoon is only about 15% brighter.

Supermoon vs. Average Full Moon vs. Micromoon

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Can You Actually See the Difference?

Here is the honest truth: most people cannot tell the difference between a supermoon and a regular Full Moon just by looking at it.

The 14% size increase sounds significant, but human perception of size is notoriously poor when there is no reference object for comparison. The Moon floating alone in a vast sky gives you nothing to compare it against. You would need to see a micromoon and a supermoon side by side, or photograph them with identical camera settings and overlay the images, to clearly see the difference.

The 30% brightness increase is slightly more noticeable. A supermoon genuinely does illuminate the ground more than an average Full Moon — enough to read large print by moonlight in clear conditions. However, atmospheric conditions, light pollution, and the Moon's elevation affect perceived brightness far more than the 30% difference.

Many people report that a supermoon looks enormous when it rises near the horizon. However, this is almost entirely due to the Moon illusion — a well-known optical illusion that makes the Moon appear larger near the horizon regardless of its actual distance. A supermoon rising looks big, but a regular Full Moon rising looks just as big for the same psychological reason.

How to Actually Compare

If you want to see the difference, the best method is photographic comparison:

1. Photograph the Moon at perigee (supermoon) and at apogee (micromoon) using the same camera, lens, and settings.

2. Ensure the Moon is at the same elevation angle for both shots (to minimize atmospheric effects).

3. Overlay or place the images side by side.

When presented this way, the size difference becomes clearly visible. Without this controlled comparison, you are unlikely to notice.

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Supermoon vs. Micromoon

The micromoon is the opposite of a supermoon — a Full Moon that occurs near apogee. Because the Moon is at its farthest point from Earth, a micromoon appears smaller and dimmer than average.

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Supermoon and Tides

The Moon's gravitational pull is the primary driver of Earth's tides, and a supermoon has a measurable — though modest — effect on tidal ranges.

Perigean Spring Tides

When a supermoon occurs near a Full Moon or New Moon (both produce spring tides), the resulting tides are called perigean spring tides or colloquially "king tides." Because the Moon is closer to Earth than usual during a supermoon, its gravitational pull is slightly stronger, and the tidal bulge is slightly larger.

Perigean spring tides are typically about 1 to 2 inches (2 to 5 cm) higher than normal spring tides. This may sound trivial, but in coastal areas that are already prone to flooding — especially during storms or high sea levels — that extra inch or two can make the difference between a dry road and a flooded one.

Coastal Flooding Risk

Coastal communities that experience "sunny day flooding" or "nuisance flooding" — flooding that occurs at high tide even without rain — are most affected by perigean spring tides. Cities like Miami, Norfolk (Virginia), Charleston (South Carolina), and Venice regularly see enhanced flooding during supermoon events, particularly when the supermoon coincides with a storm surge or heavy rainfall.

The National Oceanic and Atmospheric Administration (NOAA) monitors and forecasts these perigean spring tide events, issuing coastal flood advisories when conditions warrant.

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Supermoon and Earthquakes: No Scientific Link

A persistent claim circulates online that supermoons trigger earthquakes, volcanic eruptions, or other geological disasters. The logic seems reasonable at first glance: if the Moon's gravity is strong enough to move oceans (tides), surely a closer, stronger gravitational pull could stress Earth's crust enough to trigger quakes.

However, extensive scientific research has found no statistically significant link between supermoons and seismic activity. A comprehensive 2016 study by researchers at the University of Tokyo analyzed over 10,000 earthquakes of magnitude 5.5 or greater over 20 years and found no correlation with lunar distance or tidal stress.

The reasoning is straightforward: while the Moon's gravity does create tidal forces in Earth's solid crust (called "solid Earth tides"), these forces are extremely small — on the order of a few centimeters of surface displacement. Earthquakes are driven by the movement of tectonic plates and the buildup of stress over years, centuries, or millennia. The tiny additional stress from a supermoon is negligible compared to the forces already at work in the crust.

The USGS (United States Geological Survey) has explicitly stated that there is no connection between supermoons and earthquakes. The idea persists mainly because of confirmation bias — when a major earthquake happens near a supermoon, people remember the coincidence, but they do not notice the many supermoons that pass without any seismic event.

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Photography Tips for Supermoons

Supermoons are popular photography subjects, and with good technique, you can capture striking images. Here are practical tips:

Equipment and Settings

Composition Tips

• Include foreground: A supermoon behind a building, mountain, or tree is far more dramatic than the Moon alone in a black sky.
• Shoot at moonrise: The Moon appears largest (due to the Moon illusion) and warmest (due to atmospheric scattering) when it is near the horizon.
• Use apps to plan: Apps like PhotoPills, The Photographer's Ephemeris, and Stellarium can tell you exactly where and when the Moon will rise.
• Expose for the Moon: If you want detail on the lunar surface, expose for the Moon itself. If you expose for the foreground, the Moon will be a blown-out white circle. Bracket exposures or plan to composite images.

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How Often Supermoons Occur

Supermoons occur roughly 3 to 4 times per year, always in consecutive months. Because the Moon's perigee and the Full Moon cycle drift relative to each other, the supermoon "season" shifts gradually from year to year.

Recent and Upcoming Supermoons

The closest supermoon of the 21st century will occur on December 6, 2052, when the Moon will be approximately 221,475 miles (356,429 km) from Earth — only about 1,000 miles farther than the extreme perigee limit.

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FAQ

Q: Is a supermoon actually super?

A: "Super" is a relative term. A supermoon is genuinely closer and brighter than an average Full Moon, but the difference is modest — about 7% larger and 15% brighter than average. It is only "super" compared to a micromoon (14% larger, 30% brighter). Most observers cannot distinguish a supermoon from a regular Full Moon without instruments.

Q: Who coined the term "supermoon"?

A: The term was coined by astrologer Richard Nolle in 1979. He defined it as a Full Moon or New Moon occurring when the Moon is within 90% of its closest approach to Earth. Despite its astrological origin, the term has been adopted by NASA and major media outlets because it is catchy and describes a real phenomenon.

Q: Can a supermoon cause natural disasters?

A: No. Scientific studies have found no link between supermoons and earthquakes, volcanic eruptions, or tsunamis. The Moon's gravitational effect on Earth's solid crust is extremely small, and the additional force from a supermoon is negligible compared to the tectonic forces that drive earthquakes.

Q: Does a supermoon affect tides?

A: Yes, but only slightly. Supermoons produce perigean spring tides that are about 1 to 2 inches (2 to 5 cm) higher than normal spring tides. In flood-prone coastal areas, this small increase can contribute to nuisance flooding, especially when combined with storms or high sea levels.

Q: How can I photograph a supermoon?

A: Use a telephoto lens (200mm+), mount your camera on a tripod, and expose for the Moon as if it were a daylight subject (try f/11, ISO 100, 1/100s as a starting point). Shoot when the Moon is near the horizon for the most dramatic compositions with foreground elements. Use manual focus on the Moon's edge for the sharpest results.

Q: Is every supermoon equally close?

A: No. The Moon's orbit is perturbed by the Sun's gravity, so perigee distance varies from orbit to orbit. The closest supermoons — sometimes called "extreme supermoons" — occur when perigee coincides almost exactly with the Full Moon. The difference between the closest and farthest supermoons can be several thousand miles.

Q: Can a New Moon be a supermoon?

A: Yes, by the original definition. A New Moon that occurs near perigee is technically a supermoon — but since the New Moon is invisible (the lit side faces away from Earth), you cannot observe it directly. The term is most commonly applied to Full Moons because those are the ones people can actually see.

Q: Why do supermoons come in clusters?

A: Because the synodic month (29.53 days) is slightly longer than the anomalistic month (27.55 days, the time between successive perigees), they drift relative to each other slowly. When a Full Moon coincides with perigee, the next few Full Moons will also be relatively close to perigee, producing a series of 3 to 4 supermoons in consecutive months.