Sinus Medii (Moon)

Sinus Medii (Latin sinus mediī "Middle Bay") is a small lunar mare. It takes its name from its location at the intersection of the Moon's equator and prime meridian; as seen from the Earth, this feature is located in the central part of the Moon's near side, and it is the point closest to the Earth. From this spot, the Earth would always appear directly overhead, although the planet's position would vary slightly due to libration. During the Apollo program, Sinus Medii was designated Apollo Landing Site 3 (ALS3). Flight operations planners were concerned about having the optimum lighting conditions at the landing site. Hence, alternative landing sites moved progressively westward, following the terminator. A delay of two days for weather or equipment reasons would have sent Apollo 11 to Sinus Medii instead of ALS2, Mare Tranquillitatis (where it actually landed); another two-day delay would have resulted in ALS5, a site in Oceanus Procellarum, being targeted.