Is it true that the Moon is moving away from Earth?

The figure doesn't come from a theoretical calculation but from a measurement repeated for more than half a century. Among the instruments the Apollo 11 crew installed in 1969 was a retroreflector: a square 46-centimeter aluminum panel holding one hundred fused-silica prisms, each 3.8 centimeters in diameter, cut so that they return any beam of light exactly in the direction it came from. It needs no electricity or maintenance, which is why it is still operating: thirty years after its installation it was still working without a single failure.

From several Earth-based observatories, laser pulses are fired at that panel and the round trip is timed. Of the countless photons sent out, only a handful return, but that is enough to fix the distance between Earth and the Moon with a precision of about two centimeters. The relative accuracy is better than one part in ten billion, comparable to measuring the distance between Los Angeles and New York with an error of one hundredth of an inch. Accumulating measurements year after year reveals the trend: a recession of about 3.8 centimeters a year.

The cause lies in the gravitational interaction between the two bodies. The tides the Moon raises in Earth's oceans do not stay aligned with it: the planet's rotation drags them slightly ahead, and that displaced bulge pulls the Moon forward, pushing it into a somewhat higher orbit, while almost imperceptibly slowing Earth's spin. It is a bookkeeping exchange: the Moon gains orbital energy and Earth loses rotational speed. Over a human lifetime the effect goes unnoticed, but on geological timescales it has visible consequences, such as total solar eclipses gradually becoming harder to produce.