What is a supermoon?
Quick Answer: A supermoon is a full (or new) moon that happens when the Moon is near perigee, the closest point to Earth in its slightly elliptical orbit, making it appear a little larger and brighter than usual.
Because the Moon's orbit is not a perfect circle, its distance from Earth varies. At perigee the Moon is roughly 356,500 km away, while at apogee it is about 406,700 km away. When a full moon coincides with perigee, it can look up to around 14% larger and 30% brighter than a full moon at apogee, which is sometimes called a micromoon. The difference is real but subtle, and it is easiest to notice when the Moon is low on the horizon.
The word supermoon comes from popular and astrological writing rather than formal astronomy, so there is no single official definition. In practice, a few supermoons occur each year, and they are a great excuse to step outside and enjoy the night sky.