A full or new moon that occurs when the moon is near perigee, the closest point in its elliptical orbit to Earth, making it appear up to about 14 percent larger and 30 percent brighter than a full moon at its farthest point, or apogee. The effect is subtle enough that most observers cannot reliably distinguish a supermoon from an ordinary full moon without a side-by-side comparison, and the term originated in astrology before being adopted more broadly in popular astronomical reporting.
Facts
PeriodicityOf the twelve or thirteen full or new moons that occur in a year, usually three or four qualify as supermoons under common definitions 1 ScaleA full moon at perigee appears roughly 14 percent larger in diameter and about 30 percent brighter than a full moon at apogee, its farthest point from Earth 1 First DescribedThe term supermoon was coined by astrologer Richard Nolle in 1979 in Dell Horoscope magazine 1 Identification
Distinguishing AppearanceMost observers cannot reliably tell a supermoon apart from an ordinary full moon without a side by side comparison; the dramatic size many report is usually the Moon illusion seen near the horizon rather than the supermoon effect itself 1 Sources
1. Wikipedia - Supermoon
Frequency section
Of the possible 12 or 13 full (or new) moons each year, usually three or four may be classified as supermoons, as commonly defined.
Brightness and size section
A full moon at perigee appears roughly 14% larger in diameter than at apogee.
Moon illusion section
Many observers insist that the Moon looks bigger to them. This is likely due to observations shortly after sunset when the Moon appears near the horizon and the Moon illusion is at its most apparent.
Etymology section
The name supermoon was coined by astrologer Richard Nolle in 1979, in Dell Horoscope magazine
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