James Downey «Paint the Moon» 2001.

Paint the Moon, 2001

What if you could get millions of people to point their little laser pointers to the Moon all at the same time. Would it be possible to Paint the Moon? That's exactly what this project is all about. Created by artist James Downey, Paint the Moon's mission is: To unite millions of people in an effort to 'paint' a red spot on the dark portion of a first-quarter moon using common laser pointers during a five-minute period this autumn.

Will it work? Many experts in physics say no but that's no reason not to at least try. After all, many people thought landing on the Moon was impossible.

Paint the Moon 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. 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 the 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 10 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 the 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 point at the Moon at any given moment -- would have to point to the same spot on 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 Kohn.

source: space.com
For more information see: afineline.org/paint