Longest optical tape measure

Reflectors left on the Moon's surface by the astronauts of Apollo 11, 14 and 15 missions have provided scientists around the world with the ability to measure the distance of the Moon from the Earth since 1969. Taking a 2.5-second round trip, light from lasers based in the USA, France and Russia has been providing accurate measurements of the distance of the Moon, which averages 384,567 km (238,897 miles). This makes this four-decade-old experiment the oldest and longest optical tape measure. Considering that the Moon is moving away from the Earth at a rate of 38 mm per annum, this record is one that will increase with time.