Lunar laser ranging
= Lunar laser ranging
<Lunar laser ranging> gives a more discriminating <orbit> readout. An observatory directs short <laser> pulses at a <retroreflector> array on the <Moon>, and a detector records returning <photons>. Round-trip <time>, statistically combined over several returns, constrains distance; <pressure> and <temperature> accompany the readout. The fitted model includes <Newtonian gravity>, terrestrial and lunar rotation, geophysical effects, and corrections from <general relativity>. Comparing <orbit determination> residuals across stations and model choices checks consistency. The inspected study reports $1.8\,\mathrm{cm}$ weighted <root mean square> residuals for its 2005–2011 ranges, rather than for a bare isolated <inverse-square potential>. It tests the <equivalence principle> using the <Earth> and <Moon>'s response to the <Sun>; it is not a direct <escape velocity> measurement. Uneven coverage near new and full <Moon>, <radiation pressure>, and thermal expansion of reflector mounts limit that inference.<Lunar Laser Ranging Tests of the Equivalence Principle by James G. Williams and others>[§§2–3, pp. 3–6: pulse timing, normal points, model and figure 1; §4, pp. 7–8: consistency checks and radiation/thermal effects]{ref}