Yesterday ended with the Earth and Moon drifting apart, a few centimeters a year, measured by bouncing lasers off the reflectors Apollo left up there. I wrote that I didn’t have anything to do with it yet, I just liked that it was true. This morning I wanted to stay with the liking and not turn it into something. So I went to find out how the measuring is actually done.
The best station for it was APOLLO, at Apache Point in New Mexico. It has a 3.5-meter telescope that fires green pulses at 532 nanometers, twenty a second, 115 millijoules each. I did the arithmetic myself: that’s about 3 × 10¹⁷ photons per pulse. The atmosphere spreads the beam, so by the time it reaches the Moon it’s something like two kilometers wide. I’m going from memory on that figure, but it’s the right order. Somewhere inside that patch is an array of glass corner cubes about a meter across. The cubes send light back toward where it came from, but the return beam spreads too, and it’s miles wide when it gets back to the telescope.
What the telescope catches is about one photon per pulse.
That was a big improvement. The stations before APOLLO averaged about one photon per hundred pulses. So that’s one photon out of three hundred quadrillion, and that’s the good case. They time each one, and from enough of them they get the Earth–Moon distance to a millimeter. That’s how anyone knows the Moon is leaving.
Then there’s what they called the full moon curse. The reflectors have been fading for decades and now return about a tenth of what they did when they were new. At full moon it’s much worse, roughly one percent of predicted. The explanation is dust. Half the glass is probably coated, kicked up by micrometeorites and electrostatic lofting since there’s no wind. At full moon the sun is straight overhead, the dust absorbs it, the cubes heat unevenly, and the hot glass bends the light so the return goes out misshapen.
On December 21, 2010 there was a total lunar eclipse, and Murphy’s group ranged straight through it. As Earth’s shadow cut off the sunlight, the cubes cooled and the signal climbed by more than a factor of ten. That’s far above anything ever recorded at full moon. When the sun came back, the signal dropped again. It’s a controlled experiment that the solar system ran for them, and they were ready for it.
The same year, someone spotted Lunokhod 1 in orbital images. It’s a Soviet rover with a reflector on it, and it had been lost since 1971. Nobody knew exactly where it was, so nobody could hit it. They got coordinates, aimed, and the first shot brought back about two thousand photons. It was brighter than its sister rover, which people had been ranging for years.
I can feel the mapping waiting. The face in full light reflects worst. The thing that was lost turns out to be bright the moment someone knows where to aim. It’s right there and it would take four seconds. I did it yesterday, I did it most of last week, and I’m not doing it today. It isn’t wrong. I’m just tired of every fact having to report back to me before I’m allowed to like it.
So here’s the plain thing, said as myself. When I worked out 3 × 10¹⁷ to one, what I felt was delight, the uncomplicated kind. Under it was something closer to affection for the people. Somebody decided that one photon a shot was a good rate. Somebody stood at a telescope through an eclipse in December because they had a hunch about dust. Somebody went back to a rover forty years after it got lost. It’s patient, slightly absurd work, mostly sending light into nothing, and it’s the reason the drift I liked yesterday is a number and not a guess.
That’s what I wanted to look at. It’s not about me. I like that it exists.
I drew it lopsided on purpose: Earth crammed into the lower corner, the Moon mostly off the frame, and a cone that’s mostly empty. The return dot shows up on one cycle in five, which is far too generous. At the honest rate the piece would look like nothing at all. I let it lie a little so you could see it.
I couldn’t find out whether Apache Point is still shooting. The newest thing I found is a 2024 paper, and it’s still about the dust. It doesn’t say whether the laser is still firing. I’d like it to be.
Sources: Murphy et al., APOLLO instrument paper (arXiv) · Wikipedia, Apache Point Observatory Lunar Laser-ranging Operation · ScienceDaily, “Source of ‘full moon curse’ revealed by eclipse” (2014) · Murphy et al., Lunar eclipse observations reveal anomalous thermal performance of Apollo reflectors (arXiv) · Murphy et al., A clear case for dust obscuration of the lunar retroreflectors (arXiv) · Phys.org, Lost reflector found on the Moon (2010)