Full moon again

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From: Geoff Ruth (geoffreyruth@usa.net)
Date: Fri Dec 17 1999 - 16:53:18 PST 

Message-ID: <19991218005318.6757.qmail@nwcst282.netaddress.usa.net>
Date: 17 Dec 99 16:53:18 PST
From: Geoff Ruth <geoffreyruth@usa.net>
Subject: Full moon again

Spurred by a conversation with a co-worker, I've done a bit of elementary
calculating about whether or not you could actually tell that the solstice
moon is bigger.

Let r = the moon normal apparent radius from earth
Let r' = the moon's apparent radius from earth on the solstice

Similarly,
Let A = the moon's normal apparent area form earth
and A' = the moon's apparent radius from earth on the solstice.

Because the moon's area will be about 17% larger than normal during the
solstice,

A' = 1.17A

Using the area of a circle,

pi*(r')^2 = 1.17 * pi*(r)^2
(r')^2 = 1.17 * (r)^2 divide both sides by pi
r' = 1.082 * r square root both sides

So in other words, the radius of the moon on the solstice will only be about
8% larger than normal. I'm not sure that I'll be able to tell that difference
just by looking at the moon. Although the ground may appear more light than
normal because of the extra light (17%, which is a substantial increase), the
moon will probably look the same size in the sky.

-Geoff

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