expansion of the universe. APOLLO will place much tighter bounds on any such variations. Other tests At this level of accuracy, general relativity is needed to predict the orbit of the Moon. Current tests measure geodetic precession to a 0.35% level of precision, gravitomagnetism at the 0.1% level, and checks whether gravity behaves as 1/r as expected. APOLLO will improve on all these measurements. Principles of operation APOLLO is based on measuring the time-of-flight of a short-pulse laser reflected from a distant target—in this case the retroreflector arrays on the Moon. Each burst of light lasts 100 picoseconds (ps). One millimeter in range corresponds to only 6.7 ps of round-trip travel time. However, the retroreflectors on the Moon introduce more than one millimeter of error themselves. They are not usually at an exact right angle to the incoming beam, so the different corner cubes of the retroreflectors are at different distances from the transmitter. This is because the Moon, although it keeps one face to the Earth, does not do so exactly—it wobbles from side to side and up and down, by as much as 10° in magnitude. (See libration.) These librations occur since the Moon rotates at constant speed, but has an elliptical and inclined orbit. This effect may seem small, but it is not only measurable, it forms the largest unknown in finding the range, since there is no way to tell which corner cube reflected each photon. The biggest array, the Apollo 15 reflector, can have a corner-to-corner range spread of ≈ 1.2 sin (10°) m, or 210 mm, or about 1.4 ns of round-trip time. The root-mean-square (RMS) range spread is then about 400 ps. To determine the distance to the reflector to 1 mm precision, or 7 ps, by averaging, the measurement needs at least (400/7)2 ≈ 3000 photons. This explains why a much larger system is needed to improve the existing measurements—the pre-APOLLO 2 cm RMS range precision required only about 10 photons, even at the worst-case orientation of the retroreflector array. APOLLO attacks this problem by using both a bigger telescope and better astronomical seeing. Both are considerably improved over existing systems. Compared to McDonald Observatory ranging station, the Apache Point telescope has a factor of 20 greater light-collecting area. There is also a big gain from better seeing—the APO site and telescope combined can often achieve one arcsecond seeing, compared to the