Accurate measurements of the distance of the Moon from the Earth as a function of time are used to test theories of gravity and to investigate the distribution of mass within the Earth and Moon. These measurements are made using the retroreflectors that were placed on the Moon in 1969-71 by Apollo 11, 14 & 15 astronauts. A retroreflector reflects incoming light back along its direction of incidence for any angle of incidence. The distance of the moon is obtained by firing intense 200 picosecond pulses of laser light and measuring the time delay of the reflected pulse.