measured, and then their position would be known thereafter. This assumption is not too bad for the Moon, which is a quiet environment. But for the Earth, the stations move quite a bit on this scale: The Earth's polar axis moves and the Earth's rotation is irregular. The polar axis moves due to various causes, some predictable (the Moon exerts a torque on Earth's tidal bulge) and some variable (rocks are rebounding from the last ice age, weather). Weather also affects the Earth's rotation, by moving large masses of water around. These effects, important to many other science projects as well, even have their own agency to keep track of them—the International Earth Rotation and Reference Systems Service. The stations move due to tides. The Moon, since it is tidally locked to the Earth, has relatively small and repeatable tides of about 10 cm. The solid Earth has larger tides, oscillating about 35 cm peak-to-peak, every 12 hours. The Earth's crust changes in response to long term fluctuations such as post-glacial rebound and loading caused by sediment transport. The Earth's short-term weather can also affect the location of the telescope, primarily vertically. Various weather effects can load local regions of the Earth's crust, depressing the crust by a few millimeters. These effects come from the atmosphere (high pressure systems press on the Earth's surface), and the ocean (water piles up on the coast depressing the crust). Ground water fluctuations, caused by rain, can also affect the telescope location. The pressure of sunlight pushes the Moon's orbit slightly off center. This is a small effect, about 3.65 mm, but it is particularly important since it mimics the effect of an EP violation. Even continental drift must be compensated for. In addition, the Earth's atmosphere causes an additional delay, since the speed of light is slightly slower through the atmosphere. This amounts to about 1.6 meters when looking straight up at Apache Point. This delay is also affected by weather, primarily atmospheric pressure, which determines just how much air there is above the site. Since many of these effects are weather-related, and also affect the more common satellite laser ranging, ranging stations traditionally include weather stations, measuring local temperature, pressure, and relative humidity. APOLLO will measure all these, plus measure local gravity very precisely, using a precision gravimeter. This instrument is capable of sensing vertical displacements as small as 0.1 mm, by