As tightly focused beams of light, lasers found a use in the scientific community for more than just cutting. Precise measurements, for example, were conducted with the beams by measuring the travel time from beam output to detection. In similar fashion, atmospheric and gravitational forces could measured. Laser technology proved particularly useful for performing some of measurements of the moon.

On this day, May 9, in 1962, a pulse light laser beam sent by a team of scientists from MIT successfully bounced off the moon, the first lunar laser ranging experiment. Later similar experiments used a reflector left on the moon’s surface by the Apollo 11 mission to increase accuracy.

The Laser Ranging Retroreflector mirrors were also deployed on Apollo, 14, and 15 missions and a similar device was also included on the Soviet Union’s Lunakhod 2 spacecraft. But the original Apollo reflector is the only one still operational. Readings collected over the years have shown the moon is slowly spiraling away from the earth, at the rate of around 38 millimeters per year, and that is contains a liquid center, which makes up about 20% of its radius.