What Is a Supermoon

How much bigger can a supermoon look?

A near-perigee full Moon can appear up to about 14 percent wider than the smallest full Moon of the year. That comparison uses two extremes. For a photo comparison, hold the lens and image scale constant and measure the Moon’s diameter in pixels.

Calendar terms and scope

Perigee is the closer end of the Moon's elliptical orbit around Earth. Full Moon describes the phase, when the near side appears nearly fully illuminated. The informal term supermoon refers to a full Moon near perigee.

An example using local conditions

Check the time of full phase and compare it with perigee. Then look up local moonrise for a viewing plan. If you photograph two full moons to compare apparent size, keep camera and lens settings fixed. For a photo comparison, note the date, time, camera settings and whether the Moon was near the horizon. A visual impression alone can be influenced by foreground objects and framing. Compare measurements or matched photos if apparent size is the question. The Moon can appear unusually large near the horizon because of the viewing context. Foreground buildings and framing can affect the impression. A matched photograph or angular measurement supports a more reliable comparison.

Use the local moonrise time and choose a place with a clear view. For comparison photographs, keep the same lens and image scale. A Moon photographed near a familiar building can look different because of framing even when the change in apparent size is small.

Photograph the Moon with the same camera, focal length and output resolution on both nights. Keep the crop fixed and measure the diameter in pixels. If one picture includes buildings or hills, their size relative to the Moon can make the scene look more dramatic without giving you a comparable lunar measurement. Keep that impression separate from the actual pixel diameter. Use the full-phase and perigee times to explain why one full Moon can appear larger, and use local moonrise information to choose when you can see it from your location.

A second example

For observation, set up a camera at one location and compare two full moons with the same lens and crop. A horizon photograph can be visually dramatic, but changes in foreground or image scale can overwhelm the relatively small apparent-size difference. If a calendar reports a date in another time zone, convert the moment of full phase before comparing it with your local date. Then check when the Moon will actually be above your horizon.

Separate phase from orbital distance

A full Moon is a phase determined by the relative geometry of the Sun, Earth and Moon. Perigee describes the Moon's distance along its orbit. A supermoon describes a coincidence of those conditions under an informal closeness rule. The label does not define a new phase.

Compare published dates

Check the time zone for the full phase and the rule used for closeness to perigee. Two lists can select different events while using the same astronomical data. The Moon’s distance varies over its orbit, and supermoon is an informal label for the full phase near perigee.

Perigee and the full phase

Perigee is the closest point in the Moon’s orbit around Earth. A supermoon is a full Moon that occurs near that point. Compare the times of the full phase and perigee before checking local moonrise. The first two describe the event itself, while moonrise tells you when you can observe it from your location.

Plan a practical Moon observation

Check the full-Moon time, perigee time and local moonrise for the date you want to observe. The first two describe the lunar orbit and phase. Moonrise tells you when the Moon becomes visible from your location. Weather and an unobstructed horizon will affect what you can see. If you want to compare sizes in photographs, use the same lens, focal length and image scale for both photographs. A large Moon near buildings can look dramatic, but changes in zoom, cropping and nearby landmarks complicate a visual comparison.

Compare apparent size using consistent observations

The Moon’s apparent diameter changes with its distance. If you compare an image of one full Moon with another, fix the focal length, image resolution and crop. Measure the Moon’s diameter in pixels in both frames. This removes some of the variation introduced by photographing at different zoom levels. A visually striking Moon close to the horizon can still make an excellent picture, especially with a foreground building or ridge. Keep that compositional observation separate from a measurement of lunar size. Planning for a clear view of moonrise is a different task from checking when the Moon reaches perigee.