A supermoon is a moon that is full when it is also at or near its closest point to Earth in its orbit. The term “supermoon” was coined by astrologer Richard Nolle in 1979.
The scientific name for the phenomenon is “perigee-syzygy of the Earth-Moon-Sun system”. The Moon reaches perigee when it is closest to Earth, and syzygy occurs when the Sun, Earth, and Moon are aligned. If the alignment happens during a full moon, we get a supermoon!
Supermoons usually occur once every 13 months. However, there can be as many as three in a row – like we had in January 2018! This happens because the lunar orbit is elliptical and sometimes the Moon orbits faster and catches up to perigee. When this happens, you can get two full moons within one calendar month – which we call a blue moon. And if the second blue moon of that month also happens to be a supermoon… well then you’ve hit the trifecta!
So why does a supermoon look so much bigger than a regular full moon? It’s an optical illusion caused by something called the Moon illusion. Our brain tricks us into thinking that objects on the horizon are larger than they actually are. So when the Moon rises above the horizon, it looks huge! But once it’s high up in the sky, it appears to shrink back down to its normal size.