Re: Ultraviolet radiation from the moon

[Rondo Jeffery <RJEFFERY@WEBER.EDU>]

Re David's answer and Jim's question:


> > > David Bowman <David_Bowman@GEORGETOWNCOLLEGE.EDU> 12/06/99 02:51PM >>>
> 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.

<snip>

> 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

First, the lunar albedo David cites is much higher than any astro book gives.  The usual value is 0.07.  This makes the reflected light lower by 7/12, or only 58% of David's figures.

Second, a quicker calculation would be to take the apparent magnitude of the full moon, about -12, compared with the magnitude of the sun, about -26.7.  The difference is then about 15 magnitudes, or a brightness ratio of 1 million.  In other words, the full moon is about a million times fainter than the sun.  Even supposing that the UV reflectivity is the same as for average visual wavelengths, the light intensity is a million times lower.  (And I think you could argue that UV reflectivity would be considerably lower, though I don't have any hard numbers right at hand.)  5 hours under full moon would be roughly equal to 18 ms of direct sun exposure.  Not a problem.

One of our local TV stations ran a piece a couple of weeks ago about a child with a rare skin disorder that causes her to be hyper sensitive to sunlight.  The only time she has been able to go outside is at night.  Recently they have come up with a completely light tight suit and helmet that the story said was the same as NASA uses.  This has allowed this child to venture out in daylight.  The night outing shots they had on the piece seemed to show some moonlight, I believe.  Or, at least they would have had to have the TV camera lights on. :-)  Also, what about interior house lighting?  Do such sensitive people have to worry about flouescent lighting versus incandescent?  There is quite a difference in UV content of the those lights.

Rondo Jeffery
Weber State University
Ogden, UT 84408-2508
rjeffery@weber.edu