# ‘Pillock’ chastised for trying to paint dark side of moon red

## Scientists come out in force

Tim Richardson's story about one man's stunt aimed at colouring the moon red by using laser pointers caused a few raised eyebrows among the more scientific members of our readership.

Could this moon-decorating stunt work? Here's Domonic Fulford's thoughts on the matter:

This man is clearly a pillock.

if:

minmum distance to moon = 350000km
angular dispersion of HeNe laser ~ .01 degrees (conservative)
power of pen laser << 1 Watt very conservative

we have

350,000 * tan (.01) = radius of spot on moon

This works out at 3500 km

if six billion people with very powerful laser pointers shone them at the moon that would be 6 billion watts over an area of 12 million sq km (3500 square (forget the pi)

or about 500 watts per square kilometer. Or .0005 Watts/sqm ! Sounds pretty damn bright to me.

Muppets

Meanwhile, many thanks to Matthew Leigh, who provided the following analysis.

Hi, felt I had to share my opinion on the artist who plans to paint the moon red...

A few calculations, based on some simple optics theory. All assumptions are weighted such that reality will cause a worse outcome (in other words, this is a best case analysis).

Assume that the laser pointers have 1 degree of divergence (real pointers can have up to 30 degrees). Shining a point source with 1 degree of divergence from the earth to the moon (380,000 km on average) gives a spot radius of 6,600 km, which gives a spot area of 138,216,000,000,000 m^2. It will actually be more than this because laser beams diffract with a Gaussian envelope, not a linear envelope.

Now the key factor for visiblity is intensity, which is optical power divided by spot size. A typical laser pointer has 1mW of power - we'll assume 1W, and also assume no attenuation throughout the atmosphere. That gives 7*10^-15 W/m^2 of intensity per person on the moon's surface. Then, if you assume that the moon's surface is perfectly reflecting and that the
spreading factor is the same on the way back, we have 10^-30 W/m^2 of intensity hitting the observer on the ground, per person.

Everyone still following this?

I'm not 100% sure what the minimum intensity visible by the human eye is, but if every person on earth shone a laser pointer at the moon, the reflected intensity would be in the order of 10^-21 W/m^2, and that is definitly not visible. In other words, the guy is dreaming.

You probably stopped reading a long time before, but there has been a laser beam bounced off the moon before. It didn't cause a reddish hue, but it was detected by a very sensitive photodetector. The Apollo astronauts left a total internal reflector on the surface of the moon, and some physicists focused a laser through an enormous telescope, the sort used for looking at distant galaxies. Even then it was incredibly difficult to detect the received photons.