Tonight we’ll be treated to the astronomical phenomenon known as a “supermoon” â this is the term for the occasion in the moon’s orbit around the Earth, when a full moon, and the moon’s closest position to the Earth (its perigee, in technical terms) coincide. The supermoon phenomenon is a real thing â like any orbit, the moon’s orbit around the Earth is an ellipse, and has a nearest point to the Earth (perigee) and furthest point from the Earth (apogee). When perigee and a full moon happen at the same time, the moon can seem both bigger and brighter.
Technically the phenomenon is known as a “perigee-syzygy of the Earth-Moon-Sun system” â a syzygy, in addition to being the most irritating Scrabble word ever, refers to a line up of three astronomical bodies in a single gravitational system. In the case of a supermoon, the moon, Earth, and sun are all in line, at the same time the moon is at its orbital perigee.
The combination of closeness to the Earth, and fullness of the moon, means that a supermoon is considerably brighter than normal. The distance from the Earth to the moon can vary by as much asÂ 357,000 kilometers (222,000 mi.) at its perigee to 406,000 km (252,000 mi.) at apogee. A supermoon therefore can look almost 30% bigger in area, and shed 30% more light, than an average full moon.
An interesting complication would be a watch that shows you when a supermoon is happening. The orbital mechanics for the moon are such that a supermoon will, on average, occur once every 14 full moons. However, the actual cycle is not a whole number; the full moon cycle, from one supermoon to another, isÂ 13.9443 months, which means (as with the Gregorian calendar) eventually any supermoon complication will become de-synchronized with the actual supermoon cycle. Still, though, it would be a fun complication and one I don’t think anyone’s actually ever done.