Supermoons Explained: Lunar Perigee & Apogee

A supermoon is a full moon that occurs near perigee, the closest part of the Moon’s orbit around Earth. It can look somewhat larger and brighter than an average full moon, but the difference is usually subtle.

Supermoon isn’t an official astronomical classification, and different sources draw the line in slightly different places. We use the common 90-percent rule, which we explain below.

When is the next supermoon?

The next supermoon reaches its full phase on November 24 at 9:54 a.m. EST, or November 24 at 6:54 a.m. PST. The fact sheet above shows the corresponding time for your location, along with the Moon’s distance from Earth.

Depending on your time zone, the local calendar date may differ from the Eastern or Pacific date. We calculate the localized date before deciding which night offers the better moonrise view.

You don’t need to catch the exact minute to see it. The Moon will appear nearly full on the evenings before and after the full phase, although moonrise often provides the most striking view.

Supermoon dates by year

What makes a full moon a supermoon?

The Moon doesn’t travel around Earth in a perfect circle. Its elliptical orbit means it is sometimes nearer to Earth and sometimes farther away.

The nearest point in each orbit is called perigee, while the farthest is called apogee. When the full phase falls close enough to perigee, the result is commonly called a supermoon.

How we define a supermoon

We use the common 90-percent rule. At the exact full phase, the Moon must fall within the closest 10 percent of the perigee-to-apogee distance range used for the calculation.

Other publishers may use a fixed distance or a set number of hours around perigee. That’s why two otherwise reliable supermoon lists may not include exactly the same dates.

Some definitions also count a new moon near perigee as a supermoon. Our supermoon calendar includes full moons only, since a new supermoon is lost in the Sun’s glare and isn’t visible in the night sky.

How much larger and brighter is a supermoon?

At the closest end of its orbit, a full moon can appear up to about 14 percent wider and 30 percent brighter than a full moon near apogee. Those figures compare the two extremes, not a supermoon with an average full moon.

In practice, the change is hard to judge without photographs taken under the same conditions. NASA’s side-by-side images from perigee and apogee make the difference easier to see.

A supermoon can look especially large as it rises, but that involves another effect. The Moon illusion makes any moon near the horizon appear larger when trees, buildings, or hills give your brain something familiar to compare it with.

The opposite: a micromoon

A micromoon is a full moon that occurs near apogee, the farthest part of the Moon’s orbit. It appears somewhat smaller and dimmer than average, although the difference is just as difficult to recognize without a comparison.

The terms make a useful pair because the largest quoted supermoon figures compare these opposite ends of the orbit. A supermoon isn’t 14 percent wider than the full moon you saw last month unless that earlier moon happened to be close to apogee.

How often do supermoons happen?

There are usually three or four full supermoons in a year, and they tend to arrive in a row. The sequence happens because the cycle from one full moon to the next doesn’t quite match the cycle from one perigee to the next.

Occasionally, a supermoon is also a blue moon or coincides with a total lunar eclipse, producing the popular name super blood moon. These combinations sound rare, but each term describes a different part of the event.

Do supermoons affect tides?

Full and new moons already produce larger tidal ranges because the Sun, Moon, and Earth are aligned. When that alignment occurs near perigee, it can contribute to the higher high tides and lower low tides known as perigean spring tides.

The effect varies from one coast to another and doesn’t guarantee flooding. Local geography, weather, wind, atmospheric pressure, and sea level can matter more than the Moon’s extra proximity.

A supermoon also doesn’t make an earthquake predictable. The U.S. Geological Survey has found that tidal stresses may slightly alter the probability of certain earthquakes under particular conditions, but the absolute probability remains very small.

How to see a supermoon

Start with the local moonrise time and direction in the fact sheet, then find a clear view of that part of the horizon. You won’t need a telescope, although binoculars can bring out lunar plains, crater rays, and other surface details.

For the most useful comparison or photograph:

• Watch near the best local moonrise shown above, when the Moon is close to the horizon.
• Include a distant foreground object such as a building, tree line, or ridge for scale.
• Lower the exposure to preserve detail on the bright lunar surface.
• If you want to compare two full moons, use the same camera, focal length, crop, and shooting location.