Eli5: Scientists tell that moon is moving away from the Earth at rate of 3.78 cm per year. How do they measure this small distance so accurately for these large bodies?
In: Physics
My understanding is that astronauts left mirrors on the moon in a certain location so that lasers could be beamed at the mirror from earth, and the time it takes for the laser to go from earth to the moon, reflect off the mirror, and come back to earth where the laser beam originated from, can be accurately measured to see small changes over a period of time.
They bounce a laser off it.
The Apollo missions left a laser retroreflector on the moon, we can bounce laser beams off it and measure the transit time to get extremely accurate distance measurement.
As said, this has been measured directly:
> The motion of the Moon can be followed with an accuracy of a few centimeters by lunar laser ranging (LLR). Laser pulses are bounced off mirrors on the surface of the Moon, emplaced during the Apollo missions of 1969 to 1972 and by Lunokhod 2 in 1973.[22][23] Measuring the return time of the pulse yields a very accurate measure of the distance. … From the period 1970–2012, the results are: …
+38.08 ± 0.04 mm/yr in the mean Earth–Moon distance[24]
However, this was not a surprise:
> Edmond Halley was the first to suggest, in 1695,[1] that the mean motion of the Moon was apparently getting faster, by comparison with ancient eclipse observations, but he gave no data. (It was not yet known in Halley’s time that what is actually occurring includes a slowing-down of Earth’s rate of rotation: see also Ephemeris time – History. When measured as a function of mean solar time rather than uniform time, the effect appears as a positive acceleration.) In 1749 Richard Dunthorne confirmed Halley’s suspicion after re-examining ancient records, and produced the first quantitative estimate for the size of this apparent effect
https://en.wikipedia.org/wiki/Tidal_acceleration
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