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Re: Ultraviolet radiation from the moon



Regarding Jim Harris' question:

Dear Lurkers

A question came up today in class...Does reflected light from the moon
contain ultraviolet frequencies. We were discussing the effect of UV light
on the skin - albino conditions, etc. and a student asked me if it were safe
for albino people to swim (for example) in the moonlight.

Jim, are you sure you want to ask the lurkers? You may not get an answer
from them.

I agree with Bill Larson's assessment:

Gee, I suppose the Moon more or less reflects everything, but
down by a HUGE amount, so, sure it's safe.

Let's look at the numbers.

I certainly do not know the relevant UV reflectivity from the lunar
surface, but if it is comparable to the moon's visible albedo (i.e. 0.12)
this means that the intensity of moonlight at the Earth's surface is
about 4.1 x 10^5 times less intense for a full moon than for sunlight at
the same altitude in the sky. (Recall that the mean distance between
the Earth and Moon is about 221 times greater than the lunar radius, and
the reflected intensity goes proportional to this ratio squared times the
albedo.) At this rate an exposure of some 4 hours to direct moonlight
would be equivalent to an exposure of about *35 ms* from direct sunlight
at the same altitude in the sky. If I had to guess the actual UV
reflectivity of the lunar surface I would expect it to probably be
somewhat lower than that for visible light anyway. *Even* if the lunar
surface was a perfect reflector of skin-burning UV light (very unlikely),
then this worst case intensity would still give a 4-hour exposure to full
moonlight equivalent to an exposure of some 0.29 s of sunlight. For some
reason I doubt that even the most fair-skinned albino has anything to
worry about concerning UV exposure from a skinny-dip in the moonlight.

David Bowman
David_Bowman@georgetowncollege.edu