measuring the change in gravity as the observatory moves closer to or farther from the Earth's center. Using all these measurements, scientists try to model and predict the exact location of the telescope, and the delays through the atmosphere, so they can compensate for them. The tides are fairly predictable, and the Earth's rotation is measured by the IERS and can be accounted for. Atmospheric delay is fairly well understood, and is dominated by the pressure measurement alone. Early models had uncertainties in the 5–10 mm range for reasonable elevation angles, though more recent efforts have produced a model claiming 3 mm accuracy down to 10 degrees above the horizon, and sub-millimeter performance above 20–30° elevation. The weather is perhaps the biggest error source. Atmospheric loading is estimated from the barometric pressure at the telescope and the average pressure within a radius. Ocean loading has been handled strictly by empirical models, and ground water has been largely ignored. APOLLO will probably require improvements in all these models to reach the full accuracy of the measurements. Discoveries In April 2010, the APOLLO team announced that, with the aid of photos from the Lunar Reconnaissance Orbiter, they had found the long-lost Lunokhod 1 rover and had received returns from its laser retroreflector. By the fall of 2010, the location of the rover had been trilaterated (using range measurements from different points in the Earth's rotation and the Moon's libration) to about a centimeter. The location near the limb of the moon, combined with the ability to range the rover even when it is in sunlight, promises to be particularly useful for determining aspects of the Earth-Moon system. The APOLLO collaboration has discovered that the optical efficiency of the lunar reflectors decreases at full moon. This effect was not present in measurements from the early 1970s, was visible but not strong in the 1980s, and is now quite significant; the signal is about 10-fold less during a full moon. The cause was suspected to be due to dust on the arrays, leading to temperature gradients, distorting the returned beam. Measurements during the total lunar eclipse of December 2010 confirmed thermal effects as the cause. The sudden cut off and restoration of the light allowed thermal time constants of the effect to be observed. Status APOLLO has been up and working to various degrees since October 2005, with science-quality data beginning April 2006. By