Answer: Lunar Distance

Outside of the University of Texas McDonald Observatory there is a small and unassuming white outbuilding. Inside that outbuilding is a powerful laser that forms one element of the world’s most accurate long-range laser measuring project. Scientists at the observatory monitor the distance between the Earth and the Moon by firing the laser at reflectors placed on the moon by the crews of Apollo 11, Apollo 14, Apollo 15, and the unmanned Soviet missions Lunokod 1 and Lunokod 2.

The returned laser light is observed in the nearby observatory and used to calculate the distance between the Earth’s surface and the lunar surface with impressive accuracy. The measurements performed at the McDonald Observatory are accurate to less than half an inch–an amazing feat given that the surface of the moon is, on average, 239,000 miles from the Earth. To put that distance and accuracy into a more terrestrial scale it is equivalent to measuring the distance between blades of grass in New York City and Los Angeles to one hundredth of an inch.

As a result of the accurate and prolonged measurements provided by their laser ranging station, we’ve learned quite a few new things about the Moon. The Moon is moving away from the Earth at a rate of 3.8 centimeters per year. Both Newton and Einstein have had their work confirmed and reinforced by the laser ranging experiments; the precise measurements have confirmed that the universal force of gravity is very stable and consistent, and the orbit of the moon as predicted by the General Theory of Relativity falls directly into the range measured by the laser station.