The full moon will be will be about 221,802 miles from Earth tonight, which is about 15,300 miles (roughly 7%) closer than average, and therefore is making its way into the popular consciousness as a "supermoon." Wikipedia sniffs that it's really called a perigee-syzygy, and that indeed all full moons are really just plain old syzygys. ("Syzygies," which probably is more correct, lacks orthographical style and balance.) I don't think the snooty name is going to catch on; it's hard to rhyme and it doesn't scan worth a hoot. Although "perigee" and "syzygy" aren't bad dactylic oblique rhymes for each other, they wouldn't make a satisfying limerick.
There's been some crazy talk lately about how much stronger the tidal forces will be, or how big and bright the moon is going to look, or even what social paroxysms may be observed. Newspapers tend to say it will be "14% bigger and brighter," whatever that means. A disk area of 14% greater size, I suppose? That sounds bigger than it looks to the naked eye. Here's what a 12% increase looks like, from 2011's perigee-syzygy-superdupermegamoon:
As for tides, the effect hasn't been that great in the past. Supermoons happen pretty often, about once a year. The variation results from our satellite's elliptical orbit. Although the moon's orbit has a period (obviously) of one month, the "bump" of the ellipse is out of synch with the full-new-moon-phase cycle by a couple of days, so it takes a little over a year to repeat a line-up of the full moon with the short end of the ellipse. Solar eclipses (not to be confused with ellipses) also are affected by how close the moon is to the Earth, as well as how close the Earth is to the sun; that can make the difference between a really dark eclipse and one with a bright ring of sun peeking out all around the dark moon. Eclipses, however, exhibit much longer cycles than supermoons, because eclipses also are affected by the fact that the moon's orbit around the earth is about 5 degrees off of the plane of the earth's orbit around the sun, a retrogressing wobble that makes the plane of the moon's orbit cross the plane of the Earth's orbit in a direct line between Earth and sun only every 18 years or so.
It's been so cloudy here that I'm not sure we're going to see the supermoon at all, but we'll make sure the guns are loaded anyway, in case of a zombie apocalypse.
By Texan99 on Saturday, May 05, 2012