Laser Reflectors

On three of the Apollo landings, 11, 14 & 15, Laser Light Reflectors were left behind for experiments from observatories around the world. These were placed on the moon by hand and aligned with the earth in the sky. As the moon is in a captured rotation, the earth doesn't move in the moon's sky. Lasers are fired toward the three sites on the moon and a reflection is made back to the telescope every time. If you aim the laser at any other part of the moon, nothing comes back. 


A list of experiments using the Earth-Moon laser ranging technique are listed in the book.

How does it work?

Since the 1950’s astronomers at Jodrell Bank Observatory have been bouncing radio signals off the moon to directly measure its distance. It’s a simple radar technique. They measured its distance by timing how long it took the echo to come back by using an atomic clock. They divided the total return time in seconds by two, then by the speed of light. The earth & the moon are moving during the experiments, but both motions direction & speed are well understood and taken into consideration.

Formula Echo Timing ÷ 2 x c = Distance (c is speed of light).

Echo from transmission to receive –

2.5644408 seconds ÷ 2 = 1.282204 x 299,792.458 = 384,395.07km to the Moon.

As the wavelength of light is thousands of times shorter than radio, the accuracy with a laser is thousands of times better.