The Moon drifts away about as fast as a fingernail grows

On July 20, 1969, Apollo 11's Armstrong and Aldrin set up a panel about 46 centimeters square in the Sea of Tranquility on the Moon. The panel holds 100 small glass reflectors. No matter which direction light comes from, each reflector sends it straight back the way it came.

It needs no electricity, so it is still in use today. The Apollo 14 and 15 crews each left a panel of the same kind. The Apollo 15 panel holds 300 reflectors, so the light that returns from it is the strongest.

When an observatory on Earth aims a short laser pulse at one of these panels, the light takes about 2.5 seconds on average to reach the Moon and come back. The speed of light is known, so multiplying it by half the travel time gives the distance. On August 1, 1969, the Lick Observatory in the United States was the first to hit the panel and see the light return.

Today's equipment is better, and the distance between Earth and the Moon can be measured to within a few millimeters. Still, by the time the beam reaches the Moon it has spread to more than 2 kilometers wide, so very few particles of light come back.

Decades of measurements show the Moon moving away by about 3.8 centimeters a year, or roughly 3 millimeters a month. The cause is Earth's ocean water. The Moon's pull raises two bulges in the oceans, and those bulges tug the Moon slightly forward along its orbit. In exchange, Earth's rotation slows down very gradually.

The Moon is about 385,000 kilometers away on average, so 3.8 centimeters is nowhere near enough to make the Moon look smaller in the sky. It is a number that shows change building up over a very long time. The Moon's ground also stretches under Earth's pull, so the panels rise and fall by up to about 15 centimeters over a month.