IF THE WEATHER IS CLEAR, go outside this Monday night and look up at the moon. It is the second of three “supermoons” this year, and it will be bigger and brighter than it has been since 1948. The term “supermoon” is an astrological one. Astrology isn’t really a real thing, but they are much better at naming things: the more scientific astronomical term for a supermoon is “perigee syzygy.” Which just doesn’t roll off the tongue in the same way.
A supermoon occurs when a few things happen in our solar system. The moon’s orbit around earth is elliptical, so there are some times when it is closer to earth than others. This point in the orbit is called the “perigee.” If the moon aligns directly with the earth and sun, it’s a “syzygy.” What this means is that the moon is full when it is closest to the earth, which makes the moon bigger and brighter in the sky.
Supermoons aren’t particularly rare: there was one last month, there will be another next month. But the two events don’t usually line up exactly. And in this case, the moon becomes completely full within two hours of hitting its perigee. That hasn’t happened since January 1948. It will not get this close again until November 2034.
Supermoons are about 14% bigger and 30% brighter than apogee moons (full moons that occur when the moon is its furthest from earth). The best time to see a supermoon is when the moon is close to the horizon. This causes an optical illusion where, when juxtaposed with buildings, trees, and mountains, the moon looks huge. But this may not be in the cards for you. The position of the moon in the sky when it becomes full mostly depends on where you live.
Regardless, it should be a really cool show Monday night. You can learn more about supermoons over at NASA.