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Re: Moon Light
I came to the same problem on a conference two days ago. Walking through a
forest I remembered that Moon can be perhaps as bright as -12.5 mag and
that 0 mag means 2.54E-6 lm/m2. -12.5 mag means exactly 10^5 times
brighter, i.e., 2.5E-1 lm/m2.
The Moon can be still some 0.3 mag brighter, when close to us, however its
direct light is diminished by 0.2 mag at least by the atmosphere. Part of
it gets as a diffuse light down to the ground, so perhaps 3.05*10^-01
lm.m^-2 is an absolute maximum for the terrestrial (direct + diffuse)
clear-sky illumination by the Moon. With appropriate clouds, snow cover,
and a Moon in a hole between the clouds, it can be probably thrice higher.
One-week illumination maxima are less, over 0.1 lux with the Moon high in
the sky they reach plus minus three days around full Moon.
I adapted my old ephemeris program "planet" to compute the illumination of
a horizontal surface as an alternative. (The photometric data for Moon are
probably not very good, as I took their phase dependence the same as for
Mercury, as I see the source now.) For computing just the Moon, the line
should read (- before parameters is not needed):
planet -c10 -k4 -i -nb
and of course date and another parameters can be given, e.g., for
planet d18.12.1999 k4 t23:50 c10 n6 f40 i nb
I've obtained:
---------------------------
Planet topocentric (Long.= 16.600, Lat.= 40.000 degrees)
coordinates on
1999-12-18 at 22:50: 0, expressed in UTC, or
1999-12-18 at 23:50: 0 (Saturday)
expressed in the used time, which is 1.00 h ahead of the UTC:
Julian date : 2451531.4514 Local sidereal time : 5:44:50
Planet from S ang.height far L-LS illum. b PA long. PhA
-------- ------- ---- ---- ----- ---- ----- --- ---
degrees degrees AU degr. lux d e g r e e
Ea.R. col.
Moon 70.62 32.15 57.1 126 3.3E-2 7.1 -19.8 -6.8 43.2
1999-12-19 at 23:50: 0 (Sunday) of the used time:
Moon 63.25 44.69 56.1 140 6.7E-2 6.9 -15.6 -5.2 55.3
1999-12-20 at 23:50: 0 (Monday) of the used time:
Moon 51.24 56.76 55.4 154 1.2E-1 6.2 -10.2 -3.1 67.4
1999-12-21 at 23:50: 0 (Tuesday) of the used time:
Moon 28.43 66.68 55.0 169 2.0E-1 5.2 -3.8 -0.8 79.5
1999-12-22 at 23:50: 0 (Wednesday) of the used time:
Moon -10.98 70.12 55.0 -177 2.6E-1 3.8 3.0 1.6 91.6
1999-12-23 at 23:50: 0 (Thursday) of the used time:
Moon -46.03 64.26 55.4 -162 1.6E-1 2.1 9.5 3.8 103.8
1999-12-24 at 23:50: 0 (Friday) of the used time:
Moon -64.63 53.59 56.0 -148 9.7E-2 0.4 15.3 5.7 116.0
> far < gives distance of the body from the Earth (in Earth radii for Moon),
> L-LS < difference of the ecliptical longitude of the body
from that of the Sun (if positive, the body is east from the Sun),
> old < for Moon is (L-LS / 360 degrees) * 29.53059 days
> b < planetographic latitude of the subterrestrial point
> PA < position angle of the central meridian of the planet
> long < planetographic longitude of the subterrestrial point
> PhA < "phase angle", i.e., angle Sun-body-Earth.
> col. < for Moon is the so-called colongitudo (its PhA=abs(L-LS))
---------------------------
So the horizontal direct-light illuminances through clear
atmosphere from 1999-12-18 to -24 at 23:50 of UTC+1h at the longitude of
Brno (but 9 degrees southward) would range from 0.033 lux to 0.26 lux).
For an atmosphere with zenith extinction of 0.4 mag (parameter -ze40), the
values would be from 0.023 lux to 0.22 lux.
The programme planet (source files in Pascal and binaries for PCs under
DOS and Linux) is available on
http://astro.sci.muni.cz/pub/hollan/programmes
(Formatting for an illumination option should be repaired, and an option
for non-horizontal surfaces added, next week perhaps. Also the Moon
photometry should be checked.)
clear skies,
Jenik