At 3.8 centimetres a year, the Moon is about two metres further away than it was when the last mirrors were set up on it in 1972.

That is the whole measurement, and it is the reason the mirrors matter. Bouncing a laser off them times the round trip to a few millimetres, so a drift far too small to notice any other way shows up as a clean, steady line on a graph that now runs for over fifty years.