The sky — astronomy

Supermoon: Is the Moon Really Bigger?

A supermoon is a full (or new) Moon that occurs near perigee, the closest point of the Moon’s elliptical orbit around Earth. According to NASA, it can appear up to about 14% larger in diameter and up to about 30% brighter than a full Moon at apogee, but the difference is hard to notice without a comparison.

The short version

The supermoon is one of the most talked-about lunar sights, yet the idea behind it is simple geometry. The Moon does not circle Earth in a perfect circle, so its distance changes. When a full Moon coincides with a relatively close point in that orbit, we call it a supermoon. The effect is real, but subtle.

The Moon’s elliptical orbit

The Moon’s orbit is an ellipse (a slightly flattened oval), not a circle. The average distance between the centers of Earth and the Moon is about 384,400 kilometers, but over each month it varies by tens of thousands of kilometers. The orbit’s eccentricity (a measure of how stretched it is) is about 0.055: small, yet enough to change the apparent size of the lunar disk.

Earth sits at one focus of the ellipse, not at its center. So the Moon moves closer and farther during every revolution, which takes about 27.5 days measured from one closest approach to the next. For more on our satellite, see the Moon.

Perigee and apogee

The two extremes of the orbit have their own names:

• Perigee (from Greek, “near Earth”) is the closest point, roughly 360,000 kilometers away, and a little under 357,000 in the closest cases.
• Apogee is the farthest point, around 405,000 kilometers, and slightly over 406,000 in extreme cases.

A full Moon near apogee is sometimes called a micromoon, meaning a smaller-looking, more distant full Moon. Neither “micromoon” nor “supermoon” is an official astronomical term, so there is no single rule for how close is close enough.

According to NASA, a full Moon at perigee can look up to about 14% larger and up to about 30% brighter than one at apogee. The extra brightness comes from the Moon’s larger angular size, which means more reflected sunlight reaches us.

Where the word “supermoon” comes from

The word does not come from astronomy. It was coined in 1979 by the astrologer Richard Nolle for a new or full Moon that occurs when the Moon is at or near perigee. The name caught on widely, but astronomers do not use it formally. The scientific name for the situation is a perigean full Moon (perigean syzygy).

Once the media adopted the word, it became the popular label for the closest full Moons. That is why different sources may count a different number of supermoons in the same year: it depends on the threshold chosen for “near.” Supermoons often come in runs of a few consecutive full Moons, because the time between full Moons (about 29.5 days) is a bit longer than the time between perigees (about 27.5 days). For the phases, see Moon phases.

How big is the difference to the naked eye?

The Moon’s average angular diameter is about half a degree. A difference of up to about 14% between the farthest and closest full Moon is a very small change in angle. Without a reference object and without photos taken with identical settings, it is hard to sense. This is why many people who go out to see a supermoon see a beautiful, bright full Moon rather than a “giant” one. The extra brightness is often easier to notice than the size, though it too is modest.

To compare for yourself, photograph the full Moon with the same lens at perigee and at apogee, then overlay the frames. The difference becomes clear and measurable.

Some supermoons coincide with other events, such as a lunar eclipse or a so-called blue Moon. See also the blue Moon.

The Moon illusion near the horizon

The most striking effect at moonrise is not about distance at all. The Moon near the horizon looks much larger than when it is high in the sky, yet photographs show its size is practically the same. This is the Moon illusion: a perceptual effect, not a physical change. In fact, the Moon at the horizon is slightly farther from us than when it is high, because when it is overhead we are closer to it by nearly one Earth radius (about 6,400 kilometers).

Scientists have not fully agreed on the cause. One often discussed idea is that the brain compares the disk with objects on the ground, such as trees, buildings and hills, and so perceives it as larger. Another concerns how we perceive the distance to the sky’s dome. The illusion tends to be stronger when landmarks are in view.

A simple test: hold a small coin at arm’s length and compare it with the Moon at the horizon, then again when the Moon is high. You will find the size is the same.

Enjoying a supermoon

The sight is most impressive at moonrise, when the illusion and a near-perigee Moon add to each other. Choose a spot with a clear horizon and little light pollution. More tips: how to watch the sky.

A supermoon is a pleasant reason to look up and remember that celestial bodies move along ellipses, not perfect circles.

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