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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
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    $\begingroup$ It wasn't a mirror. It was a retroreflector. $\endgroup$
    – Solomon Slow
    Commented Mar 3, 2019 at 18:20
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    $\begingroup$ why should they be present. the mirror would not walk away.. back and forth distance is the same. $\endgroup$
    – anna v
    Commented Mar 3, 2019 at 18:30
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    $\begingroup$ 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. $\endgroup$
    – anna v
    Commented Mar 3, 2019 at 19:23
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    $\begingroup$ Laser lunar ranging experiment $\endgroup$
    – Farcher
    Commented Mar 3, 2019 at 19:34
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    $\begingroup$ The laser beam diverges a lot. By the time it returns to earth it’s just a few photons that actually make it back. $\endgroup$
    – Lambda
    Commented Mar 3, 2019 at 21:11

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Two key facts provide an answer to your question. The first is that we know the speed of light, so if we know how long it takes for a pulse of light to travel from a telescope on the Earth to a retroreflector on the Moon, then back to the telescope, we know how far the light traveled.

The second important fact is that the Moon's surface is not highly retroflective. If we shine a laser at the Moon and look for scattered light that returns to us, we receive practically zero light back. The retroreflector placed on the Moon by astronauts is highly efficient at sending light back to its source, and allows us to receive enough light back at the telescope to actually detect it.

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