*smooch* (ldy) wrote,
*smooch*
ldy

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When the moon hits your eye like a big pizza pie, that's proxigee at perihelion

No, the moon wasn't playing tricks on your eyes last night; last night's moon was AMAZING.1

Now, here's why it was so amazing.

Our Sister Moon circles us once monthly on an eccentrical orbit, which means its orbit is more of an oval than a circle. The eccentricity is increased during the full and new moon, because the sun's gravitational field pulls on it more strongly.

When the moon passes closest to us in this eccentric orbit, it is called "perigee."2 When it is at its most distant, it is said to be at "apogee."3

A full moon at perigee is considerably brighter than a full moon at apogee.4 It looks bigger, too.

But not all perigees are created equal.

Because the orbit is eccentric, perigee does not occur at the same distance every cycle. It varies quite a bit, in fact-- to the tune of some 50,000 km!5 The closest perigee is called a "proxigee." The moon usually reaches proxigee within hours of a full or new moon.

Well, you guessed it-- the moon is not merely at perigee, it's at proxigee. And it's full.

The next time we have a full moon at proxigee will be on June 3, 20046 . But it won't be quite as bright as last night's.

You see, the earth circles the sun, also on an eccentric orbit. When the earth is nearest to the sun, it's called "perihelion."6 This occurs during the first few days of January (again, it varies because of our eccentricity7).

Because the full moon is actually a reflection of the sun's light, full moons in the months near perihelion are especially bright.

So, if anyone asks you tomorrow what was up with that crazy full moon, just look at them and ask "well, what do you expect when a proxigee full moon occurs in the months near perihelion?"

Or better yet, just howl ;D

Oh, and Rune, my icon was taken at a full moon in October of 2000.

Sweet dreams, moonbeams.

1 Unless your sky was overcast, as was mine ;P
2 Peri, meaning near, and gee meaning wow.
3 Apo, meaning distant, and gee meaning wow.
4 About 30% brighter, in fact.
5 The same applies to far-wows.
6 Next year's proxigee will fall within half an hour of a new moon on November 23. However, another close perigee will occur early the morning of April 17, during the full moon.
6 Peri, meaning near, and helion meaning misspelled evil dog, or sun.
7 I'm eccentric. I blame it on the planets.
8 This website is a good resource if you wish to learn more:
http://www.fourmilab.ch/earthview/moon_ap_per.html
And they have a nifty calculator for determining perigees and apogees, and full and new moons for any given year:
http://www.fourmilab.ch/earthview/pacalc.html
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