Geological Chronicle
Sinus Medii sits directly at the intersection of the Moon's equator and prime meridian, centered near 1.6°N 1.0°E on the central near side. Spanning 286.67 kilometers across, the mare joins Mare Insularum to the west and Mare Vaporum to the north. From its central location, Earth appears directly overhead, shifting only slightly with lunar libration.
Highland regions border the bay to the north and south, framing several flooded craters. Along the eastern section, rille systems include the 220-kilometer-long Rima Ariadaeus and the Rimae Triesnecker system. Along the northern border lie the impact craters Murchison and Pallas, while flooded craters including Flammarion, Oppolzer, and Réaumur line the southern margin.
Early observers gave the feature several names, beginning with English astronomer William Gilbert, who called it Insula Medilunaria. Michael van Langren introduced Sinus Medius on his 1645 map, which inspired its modern name, while Johannes Hevelius designated it Mare Adriaticum in 1647 and Giovanni Riccioli named it Sinus Aestuum in 1651. In November 1967, the Surveyor 6 mission landed to the west-southwest of Bruce crater inside Sinus Medii, following the crash of Surveyor 4 earlier that year.
During the Apollo program, planners designated Sinus Medii as Apollo Landing Site 3. Flight operations planners sought optimal lighting conditions at the landing site, requiring alternative sites to move progressively westward with the terminator. A two-day launch delay would have sent Apollo 11 to Sinus Medii instead of Mare Tranquillitatis, where the mission ultimately landed.