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Friday, March 18, 2011

Perigee Moon

Tomorrow evening's full moon will be within an hour of its perigee, meaning that it will be the closest and biggest-looking full moon since March of 1993. Around the time of sunset and moonrise tomorrow the diameter of the apparent disk of the moon will be 33.6' of arc. The average is about 30', which is about half a centimeter if you were to hold a ruler out at arm's length (if that number seems small to you, measure it yourself, or just try to cover the disk of the full moon with your pinky-nail and you'll see what I mean). The moon looks bigger because it's 50,000 km closer tomorrow than when it is at the farthest away part of its ellipse.

This happens to correspond well to the vernal equinox, which will be at 1621pdt on Sunday the 20th. At the latitude of Seattle, our most extreme seasonal tides occur at the new or full moon nearest the solstices. But in Anchorage AK, for example, the most extreme seasonal tides occur at the new or full moon closest to the equinoxes. So the combination of the moon's perigee, the equinox and the normal monthly spring-tide of the full moon means that the range of tide in Anchorage on Sunday morning will be 36.9 feet, or 11.25 meters. That's a five story building, more or less.

Anyway, it looks like the skies in Seattle should clear up a bit for us to see the moon tomorrow night!

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