What is a supermoon?
A supermoon is a full moon that happens when the Moon is at or near its closest point to Earth in its orbit. It looks slightly bigger and brighter than an average full moon.
The Moon's orbit is an ellipse, not a perfect circle, so its distance from Earth changes during each month. The closest point is called perigee, about 363,000 kilometers away, and the farthest is apogee, about 405,000 kilometers. When a full moon happens near perigee, it is called a supermoon.
A supermoon can appear up to about 14 percent larger and 30 percent brighter than a full moon at apogee, sometimes called a micromoon. In practice, the difference is hard to notice without comparing photos side by side. Moons near the horizon often look huge because of an optical illusion, not because they are supermoons.
Where the name came from
The term supermoon was coined in 1979 by astrologer Richard Nolle. It is not an official astronomical term, but it has become popular, and NASA and other agencies now use it. There are usually three or four supermoons in a row each year.
Did You Know?
Supermoons cause slightly higher high tides and lower low tides than usual, called perigean spring tides, but the effect is usually only a few centimeters.