Astronauts left mirrors on the moon so that laser beams can be bounced off of them from Earth to measure the distance with precision.

Prior to the 20th century, the best way to determine the distance to the Moon was to use parallax. The Moon's coordinates in the sky are slightly different from different places on Earth. The Moon forms a skinny triangle with the two locations on Earth. The small angle is the angular distance between the two measured lunar positions. Its opposite side is the physical distance between the two places on Earth. Using trigonometry, you can thus find the distance to the Moon.
 
With all the technology of RADAR, much more specific determinations on the lunar length started to be feasible. Also, due to the actual Apollo astronauts, there exists a significant reflector for the Moon's area, away from that beam of light lighting may be bounced coming from Globe. Through measuring any time the actual beam of light lighting takes to produce the actual rounded visit to the actual celestial satellite as well as returning, it is fast length may be measured to be able to within just one half inch.