Brightness of the Moon
Edward S Light (light@argoscomp.com)
Wed, 20 Oct 1999 21:00:12 -0400 (EDT)
This morning, Rob Matson asked ...
> This brings up a question that perhaps someone out there
> can answer: just what ~is~ the magnitude of the Moon at
> first-quarter phase? Clearly it is much less than half
> the brightness of the full moon (-12.3), and last night's
> gibbous moon falls somewhere in between. --Rob
>
In "Astrophysical Quantities" by C. W. Allen, 3rd Edition, 1973,
on page 143, in the section on Photometry of Planets and Satellites,
is a table giving the Moon's phase law which includes the data:
Phase Mag-M(f) Phase Mag-M(f) Phase Mag-M(f)
------- -------- ------- -------- ------- --------
0 deg 0.00 50 deg 1.35 110 deg 3.48
5 deg 0.08 60 deg 1.62 120 deg 3.93
10 deg 0.23 70 deg 1.91 130 deg 4.44
20 deg 0.51 80 deg 2.24 140 deg 5.07
30 deg 0.79 90 deg 2.63 150 deg 5.9
40 deg 1.06 100 deg 3.04 160 deg 7.5
Notes:
(1) Phase is defined in the usual astronomical sense (the angle at the
moon between the sun and earth; hence 0 degrees corresponds to a
full moon, 90 to first or last quarter, and 180 to new moon.
(2) Mag-M(f) is the difference between the magnitude at the given phase
and that at full moon, which is typically -12.73 (according to the
book - more modern values may be slightly different).
(3) The above table clearly ignores differences in albedo between
different parts of the moon's visible surface. Also, the varying
earth-moon distance obviously effects the apparent magnitude.
Clear and dark (and un-moonlit) skies!
Ed Light
Lakewood, NJ, USA
N 40.1072, W 074.2317, Hgt +21 m (69 ft)
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