Supermoon: what it is, and what it definitely isn’t (a repost)

A repost, apropos of this weekend’s supermoon and the fact that people are going bugnut over it… yet again… and Taslima seemed lonely in being the only other FtBer covering this one. My original post is here, published March 17, 2011.

Look up in the sky! It’s a bird! It’s a plane! No, it’s a SUPERMOOOOOOON!!

I have written at some length about the moon, with its wobble called libration, and how its elliptical orbit means that it varies in its distance to us between roughly 360,000km and 406,000km. That’s a difference of ~46,000km, or about ten percent of its distance at apogee. Apogee is what you call the moon’s furthest point in its orbit, and perigee the closest. As the moon orbits us about once a month (thus the lunar cycle), that means that during a predicted perigee, the moon is about two weeks away from apogee.
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