May 9, 1962: Laser beam first used to measure distance to the moon

In 1962, laser technology was a new and exciting science. Lasers produce a light that is intense, coherent, and monochromatic. The beam of light emitted by a laser is also extremely narrow. It would be impossible to bounce a flashlight beam off the moon, as the light disperses too much to travel any distance. But a laser beam is so narrow that it can make the roughly 239,000 mile journey to the moon and still be detected back on Earth. The first time this was done, MIT scientists using a ruby laser to bounce a light beam off the moon in a series of pulses, estimated that its area on the moon's surface was just four miles in diameter. Later they were able to reduce this to under 2 1/2 miles.

But why would anyone want to bouce light off the moon? This technique is used because it provides the most accurate measurement of the moon's distance from Earth. In fact, the measurement has become even more accurate as a result of reflectors left by astronauts on the moon's surface. By bouncing light off these reflectors, scientists can easily measure the moon's distance to within a few meters. And, as laser technology becomes still more sophisticated, that range of error is decreasing.