[AZ-Observing] SQM and crescent Moon

     For about 20 years I've used the rule-of-thumb that if the Moon
is less than 15 percent illuminated there was no significant effect
for visual deep-sky viewing.  Last night at Anderson Mesa I had the
opportunity to make some SQM measurements of the effect of the late-rising
Moon, which was 14 percent illuminated.  Here's the run of values taken
hourly dusk-to-dawn (almost):


UT Date  hhmm  LST   site   mu    pv    dp   remarks
                                 (mm)  (C)
20060918 0300  1920  Mesa  21.50   3.5  -1
20060918 0400  2020  Mesa  21.47   3.5  -1
20060918 0500  2120  Mesa  21.42   3.5   0
20060918 0600  2220  Mesa  21.39   3.5   1
20060918 0700  2320  Mesa  21.42   3.5  -2
20060918 0800  0020  Mesa  21.51   3.5  -3
20060918 0900  0120  Mesa  21.55   3.5  -3
20060918 0937  0200  Mesa  21.55   3.5  -2   14%- Moon at 0deg alt
20060918 1000  0220  Mesa  21.54   3.5  -3   14%- Moon at 5deg alt
20060918 1100  0320  Mesa  21.46   3.5  -3   14%- Moon at 16deg alt


...where the column 'mu' gives the SQM readings, which are nominally 
V magnitudes per square arcsecond.
     Normally I would expect the sky brightness to decline slowly through the
night, so the bump in late evening was unexpected---and not the least
noticeable visually.  The evening (5pm local) and morning (5am) upper-air
balloon soundings gave identical total water-vapor, so the atmosphere was
stable overnight.  The brightness did decline after midnight, and I took
readings right at Moonrise and later at the hourly intervals.  As you can
see, the sky got a bit brighter by 11h UT = 4am (local astronomical twilight
was about 4:45am).  But though the Moon obviously changed the nocturnal
scene, its effect on the zenithal sky brightness was modest, even over the
large area taken in by the SQM meter, and still darker than earlier in the
night.  I have seen a similar increase at this season without Moonlight
simply from the winter Milky Way rising together with the prominent
northern-autumn zodiacal light crossing it as a large, diffuse X in the
pre-dawn sky.

\Brian
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