The moon doesn’t care what term we use to describe its location, but there’s a bit of a debate here on Earth about what the full moon on September 16th should be called. Because it’s the closest full moon to the autumnal equinox (September 22nd), it’s a Harvest Moon, but some also think it’s a so-called supermoon. The disagreement comes down to an interesting quirk of orbital measurements.The term “supermoon” isn’t a scientific one, but it’s a word people have latched onto because it describes a phenomenon you can observe just by looking up. Astrologer (the one with horoscopes, not science) Richard Nolle coined the term. It is now understood to refer to any full moon where the moon is at or near (90% or greater) its closest approach to Earth. The closest approach in an orbit is called perigee, and it can make the full moon look as much as 14% larger in the sky. At the other end of its orbit (apogee) the moon would look noticeably smaller.Nolle maintains a table of supermoons, and he does not list the September 16th full moon on that list. However, former NASA astronomer (the one with science and no horoscopes) and eclipse expert Fred Espenak has his own table of supermoons. He says the upcoming full moon will indeed be a rare conjunction of a supermoon and harvest moon. The difference of opinion comes down to how you measure the moon’s orbit.Since the arbitrary definition of a supermoon as laid out by Nolle requires the moon to be at 90% or greater of its closest approach to Earth, we’re talking about a difference measured in tens of thousands of kilometers. Nolle is using the closest perigee and farthest apogee for all of 2016, meaning the full moon will only be at 89% of perigee by that measurement. Espenak bases his table on the perigee and apogee of each month’s orbit. Using that metric, the September 16th full moon will be at 93% of perigee, so it’s a supermoon.Both versions have their defenders in the geekier internet comment sections. Whichever version you think sounds right, the full moon this week will be a big one. Of course, it won’t actually be bigger, just closer.
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