How do we know that the moon is drifting away 1.5 inches a year? Bounce a laser beam off it.

Katherine Kornei, How Do You Solve a Moon Mystery? Fire a Laser at It, NYTimes, Auguast 15, 2020.

The moon is drifting away. Every year, it gets about an inch and a half farther from us. Hundreds of millions of years from now, our companion in the sky will be distant enough that there will be no more total solar eclipses.

For decades, scientists have measured the moon’s retreat by firing a laser at light-reflecting panels, known as retroreflectors, that were left on the lunar surface, and then timing the light’s round trip. But the moon’s five retroreflectors are old, and they’re now much less efficient at flinging back light. To determine whether a layer of moon dust might be the culprit, researchers devised an audacious plan: They bounced laser light off a much smaller but newer retroreflector mounted aboard a NASA spacecraft that was skimming over the moon’s surface at thousands of miles per hour. And it worked.

These results were published this month in the journal Earth, Planets and Space.

Of all the stuff humans have left on the moon, the five retroreflectors, which were delivered by Apollo astronauts andtwo Soviet robotic rovers, are among the most scientifically important. They’re akin to really long yardsticks: By precisely timing how long it takes laser light to travel to the moon, bounce off a retroreflector and return to Earth (roughly 2.5 seconds, give or take), scientists can calculate the distance between the moon and Earth.

Amazing!

We can detect a mere 25 photons?

In 2017, Dr. Mazarico and his collaborators began firing an infrared laser from a station near Grasse, France — about a half-hour drive from Cannes — toward the orbiter’s retroreflector. At roughly 3 a.m. on Sept. 4, 2018, they recorded their first success: a detection of 25 photons that made the round trip.

FWIW, I vaguely recall that the human retina is sensitive to single photons.