A mirror on the Moon still answers
Retroreflectors left by Apollo crews return laser pulses sent from Earth. The round trip takes about two and a half seconds, and the distance is measured to about a millimetre.
A corner-cube mirror sends light back the way it came. Observatories fire a short pulse at the Apollo 11, 14, and 15 arrays, and at reflectors on Soviet rovers, and wait. The return is a handful of photons, picked out of noise by the fact that they arrive when the pulse should. From the travel time and the speed of light, the distance follows. Decades of such shots show the Moon receding, and they test whether gravity behaves as Einstein wrote it, at a scale no laboratory owns.
The millimetre is a precision on a mean distance of about 385,000 kilometres. It is not a claim that someone has seen the mirror, or that every shot succeeds. Clouds, poor seeing, and the tiny target end many nights with nothing.
Caveats
- Dust on the arrays has dimmed them over the decades. The measurement still works. It works less generously than it did.
- The ranging gives a distance, not a picture. A millimetre of range is not a millimetre-wide photograph of the landing site.
Sources
- 1Dickey J. O. et al.. Lunar Laser Ranging: A Continuing Legacy of the Apollo Program. Science 265, 1994.