[EMAIL PROTECTED] wrote:
>
> In regard to the excitement over the close perigee occuring when the earth is
> closest to the sun (very close to the December solstice.) My local paper
> made a comment that the Moon will appear 17% larger than a full moon last
> summer (that is in June-July for you down underlings), but the full moon in
> January will appear 40% larger. I suspect it was a misprint, but I thought
> I'd send it out so you could chew on it.
> ~troy
Hello Troy,
Well, let's find out.
Full Moon June '99:
06/28 21:38 UTC
Earth-Moon distance: 401955.3 km (249764.0 miles)
True Equatorial RA: 18h 26m 34.5s Dec: -20°10'09"
Topocentric coordinates: RA: 18h 26m 58.8s Dec: -20°25'17"
Full Moon Dec.'99:
12/22 17:33 UTC
Earth-Moon distance: 356733.3 km (221664.4 miles)
True Equatorial RA: 06h 01m 47.3s Dec: +20°33'07"
Topocentric coordinates: RA: 05h 59m 38.1s Dec: +19°45'39"
The calculated ratio between the full moon distances above is
1.1267. However, to keep things straight note that the June '99 full
moon is about three days after apogee and the Dec '99 full moon is very
close to perigee, i.e., approx. 6hrs after. We'll see that this really
doesn't matter much though and will keep things relative to the June '99
full moon.
The intensity of light varies as the inverse square of the
distance between a light source and the observer. Therefore, the
intensity difference between these two distances is 1.2696 or
approximately 27%. This is not trivial by any means. The trivial part is
as follows. There has been some recent clamor about the claim of the
Dec. 22 full moon not being the "brightest" full moon in 133yrs. Well,
the fact is that those denying this are correct but what is the real
magnitude of such a disclaimer? Well, let's take the same June '99 full
moon distance, i.e., 401955.3 km and see what value we get when we
divide it by the closest perigee in the years 1750 through 2125, which
was 356375 km on January 4, 1912 and as it turns out was very close to
being entirely full. So, 401955.3 km / 356375 km = 1.1279 and 1.1279^2 =
1.2722. Drum roll please, (1.2722/1.2696) * 100 = 0.2%. Is anyone
impressed? A lot of disclaimer for 0.2%!
The question of apparent size relates to the subtended angle of
the moon at its various distances and is just the 2 * arc_sin(lunar
radius / lunar distance). The lunar distances should be calculated as
topocentric distances to be as accurate as possible.
My suggestion, ENJOY IT! For a much more detailed treatment
visit the following URL:
http://www.fourmilab.ch/earthview/moon_ap_per.html
Best,
Luke
