On 12 Sep 2004 23:40:22 -0700, Canopus wrote:

While driving back home after an all night observing session on
Friday-night Saturday-morning, I noticed the Moon rising over the
eastern horizon about 5:00 am MT on 9/11/2004 ( 12:00 pm 9/11/2004
UTC). The moon was close to being new - about 25-26 days old; only a
thin eastern crescent remained. Unlike many other moonrises, this one
in particular caught my eye because of the brightness of dark portion
of the disk facing Earth. On rising, the dark portion of the disk was
steely blue and quite bright. Mare Imbrium and Sinus Iridium were
easily seen on the dark portion of the Moon's face. As the Moon rose
beyond 5-10 degrees of the horizon, this effect lessened. The Sun
rose at around 7:00 am local time (2:00pm 9/11/2004 UTC).

I am assumming this was Earthshine reflecting off the Moon's surface,
but I have not seen this effect before.

Is this effect somehow related to the Moon's closeness to the
ecliptic, or does it occur every month about this time during the
lunar cycle? The Moon is getting closer to the ecliptic as the total
October 28 lunar eclipse approaches.

- Canopus


Here are some *possible* explainations:
1. Earthshine - blue from the oceans, white from clouds and snow
2. Color contrast - atmospheric dust reddened the brighter part of the
cresent. They eye compensated for the darker portion making it seem
more blue, less red.
3. In the past, volcanic dust and aerosols made the whole moon look blue.

Cheers,
Larry G.


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