I have a book stating:
When astronauts of the Apollo missions visited the moon, they placed a mirror on its surface, facing the earth. Then scientists on the earth sent a strong laser beam, which was reflected by the mirror on the moon and received back on the earth. The size of the reflected laser beam and the time taken for the round trip were measured. This allowed a very accurate determination of (a) the extremely small divergence of a laser beam and (b) the distance of the moon from the earth.
I am quite okay with (a), but (b) confuses me a lot, I am not able to understand, how:
- They directed the laser directly on the mirror (may be the mirror was a gaint)
- They kept the mirror perfectly adjusted and stationary (well, this one is probably easy)
- (Hardest one, I think) They were present at the location where laser returned