Why are some moons called supermoons?

If you are a person who pays attention to the night sky or astronomical events, you may have noticed the last three full moons this year have been supermoons. You might have even thought the full moons looked bigger than usual... and you're correct, they are larger in the sky than a standard full moon.

The term "supermoon" was coined in 1979 by Richard Nolle. A supermoon is when the full or new moon coincides with the moon's closest approach (also known as 'perigee') to Earth in its orbit. According to Nolle, the moon must be within 90% closeness to the Earth in order for the moon to be called a supermoon. 90% of the moon’s closest approach to Earth can be figured by the year’s perigee (moon’s closest point to Earth for the year) and the year’s apogee (moon’s farthest point from Earth for the year). In 2020, the perigee is 221,772 miles (356,907 km) and the apogee is 252,707 miles (406,692 km). So – reckoning it this way – any full moon or new moon coming closer than 224,865 miles (361,885 km), (as measured from the centers of the Earth and moon), counts as a supermoon in 2020.

The moon's orbit around the Earth is not a perfect circle. It has an average distance of 238,000 miles (382,900 km) from Earth, but its apogee (furthest) and perigee (closest) approaches from Earth change every lunar month. In order to get a supermoon, the moon needs to be at its closest approach, or perigee, to the Earth in its 27-day orbit. The moon also needs to be at the full phase, which happens every 29.5 days when the sun fully illuminates the moon. Supermoons may appear brighter and closer than normal (although the difference is usually hard to spot with the naked eye, or so they say). 

It's not just the size and brightness of the moon that will be impacted. The closer-than-average full moons (or closer-than-average new moons) ... supermoons ... elevate the tides even more than usual. These extra-high spring tides climb up especially high, and, on the same day, low tides plunge especially low. Experts call these perigean spring tides, in honor of the moon’s nearness. Makes me wonder how that impacts the bioluminescent waves!

The supermoon of November 2016 was the closest supermoon in 69 years, although a closer supermoon will rise in November 25, 2034 and the closest full moon of the 21st century will fall on December 6, 2052. While a supermen is likely to occur every year, it doesn't always, and like we've seen this year sometimes there are three in a row!