March 22, 2015 by Keith Grice
A supermoon occurs when the moon is both ‘full’ and at its closest point to Earth.
Astronomers call this phenomenon a ‘perigee moon,’ which describes the full moon that occurs when the moon is at its closest point to Earth each year.
Perigee marks the closest point a body reaches as it orbits around its host object such as Earth, while apogee marks the farthest point from its parent body.
Almost every orbit has an apogee and perigee because orbits tend to be elliptical rather than perfectly circular, so even Earth has an apogee and perigee around the sun.
It occurs when a full moon or a new moon coincides with its closest approach to Earth. The moon’s distance from Earth varies between 222,000 miles (357,000km) and 252,000 miles (406,000km).
This accounts for a difference in brightness of about 30 per cent when the moon is closest and furthest away.
However, despite what many people think, the supermoon does not make the moon seem noticeably bigger in the night sky. In fact, the difference is almost impossible to spot with the naked eye.
Also, for this supermoon, it is occurring during a ‘new moon’ – the opposite of a full moon, which means the moon is mostly dark, with the sun on the opposite side, which also explains how the eclipse took place.
Several supermoons occur every year, with three more due in August, September and October 2015.
Author Keith Grice is an amateur astronomer and astro-photographer living in Scunthorpe, North Lincolnshire, UK. He is a founding member of the Scunthorpe and District Astronomical and Astrophotography Group, a Facebook group of like-minded amateur astronomers using modest equipment in our back yards. Keith is also a member of the North Lincs Astronomical Society based in Barton Upon Humber.