# How can someone calculate distance between Earth and Moon using a laser?

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
• It wasn't a mirror. It was a retroreflector. Commented Mar 3, 2019 at 18:20
• why should they be present. the mirror would not walk away.. back and forth distance is the same. Commented Mar 3, 2019 at 18:30
• why would you need the astronauts to be there while the measurement was taken? I think I see from your answer to Solomon.you thought they should orient the mirror (if it were a simple mirror). But this could be arranged for a specific earth location at a specific passage of the full moon. with the retroreflector that complexity becomes unnecessary. Commented Mar 3, 2019 at 19:23
• Laser lunar ranging experiment Commented Mar 3, 2019 at 19:34
• The laser beam diverges a lot. By the time it returns to earth it’s just a few photons that actually make it back. Commented Mar 3, 2019 at 21:11