Full moons have a hold over me, and not just in a vague gravitational sense.
I get entranced by them — cool silver or fiery gold, low on the horizon or high among the stars, but especially, to echo what I wrote a couple of years ago during a lovely Buck Moon, when they come as the moon is essentially at perigee, its closest point to Earth in its elliptical orbit. (As I'm sure we all learned once upon a time, neither the orbit of our moon around Earth nor that of Earth around the sun is an exact circle; rather, they are very slight ovals.)
What I'd never heard until yesterday, though, is the term "supermoon" — or as Richard Nolle, who coined the term in 1979, writes it, SuperMoon. It was invoked on my local news broadcast of choice, since today we are enjoying what Nolle calls an extreme supermoon, sending me to Google in a happy moment of Internet connectivity to get the full story.
I didn't have the time, energy, or signal strength to do a lot of research, but here's a
bit of further study for you: a NASA video; an article on space.com dealing in statistics; a nifty infographic that accompanies it; an article written by Nolle at his website, Astropro; and an article for Universe Today that takes issue with some of Nolle's more astrologically oriented interpretations — although we're still talking geologic repercussions rather than your usual horoscope fare.
The gist of it all is that when the moon is both at perigee and seen in full rather than partially or new (i.e., entirely in shadow), it is larger and brighter and more impactful upon the tides to the extent that it is, per Nolle, worthy of the name SuperMoon. And given that today the moon is 5,000 miles closer than it has been to Earth in 18 years (granted, with an average perigee of 226,425 miles and a flat-out average Earth-to-moon distance of roughly 238,000 miles, everything's relative), it's apparently considered extreme.
Nolle applies the SuperMoon label to a full moon that occurs within 90% of its orbital perigee, but I'm unclear on what bumps up a given instance to the status of extreme supermoon. My guess is either the aforementioned nearer-than-normal perigee, as even perigees and apogees fluctuate over time given slight changes in orbits, or a supermoon conjunction that's particularly close, say 95% or more of the way towards total unity, as today the exact perigee occurs only 50 minutes after the brief moment that the moon is officially full. I suppose that it could also be due to a documented increase in the number of shirtless young men who think that supermoons are so totally rad that they're compelled to make devil fingers and rope-swing through waterfalls.
Human perception of the moon as full lasts for at least a day, which is fortunate for those of us for whom the supermoon hour falls during daylight; it happens at 3 p.m. EDT on March 19th, about 9 hours after this post is scheduled. I don't know whether, come darkness, the moon will still entirely be the estimated 30% brighter and nearly 15% larger that it will be at the hour of perigee than the full moon that will coincide with apogee in October, and certain sources I skimmed insisted that the naked eye either couldn't register the increases anyway or would compensate for them as nothing special. The forecast is for clear skies in my area, however, so I'll definitely be looking up and beaming.
Photo: NASA — Sean Smith.
Updated and revised January 2019
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