Moon ExactlyMoon Exactly Learn
What Is a Supermoon?
Supermoon is a popular name for a Full Moon that occurs relatively near the Moon’s closest orbital distance to Earth. It is useful in conversation, but it is not an official astronomical phase.
The Moon follows an elliptical orbit, so its distance varies. A nearby Full Moon can appear modestly larger and brighter than a distant one, though the difference is often hard to judge without a side-by-side comparison.
Perigee, apogee, and Full Moon
Perigee is the point in an orbit nearest Earth; apogee is the farthest. These distances vary from orbit to orbit because solar and planetary gravity perturb the Moon’s path. When Full Moon occurs near perigee, popular media often call it a supermoon.
There is no single universal cutoff. One frequently used definition counts a Full Moon within 90 percent of its closest possible approach, while other lists use different distance or timing rules. Two reputable calendars can therefore disagree on how many supermoons occur in a year.
How dramatic is the difference?
A near-perigee Full Moon is physically larger in angular diameter and brighter than a near-apogee Full Moon. Without a fixed reference, however, most people will not perceive a startling size change. Photographs can exaggerate the effect when a long lens frames the Moon beside a distant foreground object.
Do not confuse it with the Moon illusion
The Moon often looks enormous near the horizon even when its measured angular size has barely changed. That is the Moon illusion, a perception effect influenced by foreground context. A supermoon concerns actual Earth–Moon distance; the horizon illusion concerns how the brain interprets the scene.