Why paintthemoon won't work

Dan Kohn dan@dankohn.com
Thu, 9 Aug 2001 08:04:00 -0700

This is from a well-respected physicist who will go unnamed. It's a
shame, though. Paintthemoon is, in theory, a great idea:

>There's a movement afoot to get enough people to shine laser
>pointers at the moon all at once to make a visible spot.
> http://www.paintthemoon.org/

You would need many billions, moreover with very steady hands on the
part of 
all concerned.  The Moon is about 8 milliradians wide, and laser-pointer

spots are ~0.3 milliradians in width.  They output at most 5 milliwatts
and most are 2 or less, and you would need to overwhelm Earthshine from
Moon, which is roughly 0.1 W/sq.meter.  Your common spot on the Moon
would be 
about 1/30 of a lunar diameter, or ~130 km in width, or ~10,000 sq.km or
billion sq.meter in area.  Thus, you would need a billion watts of total

laser-pointer illumination to overwhelm Earthshine, or the output of 500

billion laser-pointers.  Of course, if you were to look at the Moon with

spectral filter glasses which just matched the ~2 nm spectral-width of
laser-pointers, you would cut this illumination requirement by
~500-fold, so 
that only one-sixth of the human race -- one-third of those who could
at the Moon at any given moment -- would have to point to the same spot
the non-Sunlighted portion of the Moon in order to see a doubling of the

apparent illumination there -- which is not greatly in excess of human 
visual-intensity threshold at low light levels. 

		- dan

In theory, there is no difference between theory and practice. But, in
practice, there is. - Jan L. A. van de Snepscheut 

Dan Kohn <mailto:dan@dankohn.com>
<http://www.dankohn.com/>  <tel:+1-650-327-2600>