Friday, January 05, 2018

Supermoons are over-hyped

All the media hype about so-called supermoons in recent years is getting a bit tedious. I'm pretty sure the moon itself has not changed, but now it seems like almost every month's full moon is a supermoon. For example, the January 1st full moon was a supermoon, but then so was the previous one on December 3rd, and now apparently so will the next one, on January 31st (which, being the second full moon in a calendar month, also makes it a blue moon).
So, what actually is a supermoon? A supermoon is just what used to be described as a full moon at its perigee (i.e. the closest point to the earth in its orbit), or more technically the perigee syzygy of the Earth-Moon-Sun system. Because it is closer to the earth than average, it appears slightly bigger than average full moon, and therefore slightly brighter.
Technically, only about one in every fourteen full moon will be at perigee, and thus a real supermoon. But the modern definition of a supermoon was arbitrarily set (in 1979, by Richard Nolle, an astrologer, not an astronomer) as being when a full moon is within 90% of perigee, which makes the phenomenon more common (and less super, I would say). More recently still, even full moons within one or two days of pedigree are often referred to as supermoons.
Anyway, my point is that the operative word in the description above is "slightly", a point that a recent article on also made. The difference in size between a full moon at perigee and a full moon at apogee (i.e. at its furthest point from the earth, sometimes referred to as a micromoon, but how often do you read about THAT in the news?) is just 14%. So, a full-blown supermoon is only 7% bigger than an average full moon, a difference that you would probably not even notice unless alerted to it by the press. As so often, Neil DeGrasse-Tyson may have said it best when he asked, "If you turned a 14-inch pizza into a 15-inch pizza, would you call it a super-pizza?" And bear in mind that there is also the "moon illusion" phenomenon, whereby the moon always looks larger when it is low in the sky (partly because our brains assume it is farther away than when it is overhead, and partly because it is magnified by the earth's atmosphere).
Likewise, supermoons are often quoted as being up to 30% brighter than an average full moon, which sounds impressive but is actually equivalent to just a 0.1 or 0.2 magnitude difference in brightness, which again is unlikely to be spotted by the average observer.
I guess that the recent media fixation with supermoons has probably had the admirable effect of encouraging more people go out and commune with nature to observe the wonders of the universe. But its "super" branding is probably something of a misnomer, or at the very least over-used.

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