Sinus Medii

Sinus Medii is a small lunar mare situated close to the intersection of the Moon's equator and prime meridian. Its name is Latin for “Middle Bay,” reflecting its central position on the lunar near side. From Earth, the feature lies approximately at the center of the visible lunar disk and is the lunar surface location closest to Earth in the ordinary geographic sense. An observer positioned there would see Earth nearly overhead, although the planet would shift slightly because of lunar libration. The mare measures approximately 287 kilometers across and is centered at about 1.6° north latitude and 1.0° east longitude. It forms a connection between Mare Insularum to the west and Mare Vaporum to the north. Like other lunar maria, it is a broad basaltic plain formed by ancient volcanic activity and later modified by impact events. Its surface and margins contain numerous craters, flooded impact structures, and sinuous rilles that record the geological history of the Moon's near side. The eastern portion is particularly notable for its rille systems. Rima Hyginus lies in the northeastern area and is associated with the crater Hyginus, which bisects the rille. Farther east, Rima Ariadaeus extends for roughly 220 kilometers toward the edge of Mare Tranquillitatis. The Rimae Triesnecker system occupies the region around longitudes 4° to 6° east and is named for the nearby crater Triesnecker. Other important features around the mare include the border craters Murchison, Pallas, Chladni, Flammarion, Oppolzer, Réaumur, Seeliger, Rhaeticus, Bruce, Blagg, Schröter, and Sömmering, together with Rima Flammarion and Rima Oppolzer. Sinus Medii also has a notable place in the history of lunar exploration. NASA's Surveyor 6 spacecraft landed in the western part of the mare, southwest of Bruce crater, in November 1967. Surveyor 4 had attempted to operate nearby earlier that year but crashed during its landing attempt in July. During Apollo mission planning, Sinus Medii was designated Apollo Landing Site 3. The site was considered as a possible destination for Apollo 11, but lighting and operational constraints led planners to consider progressively more westerly alternatives along the lunar terminator. A two-day delay could have redirected Apollo 11 to Sinus Medii instead of Mare Tranquillitatis, its actual landing site; a further delay would have moved the target farther west to a site in Oceanus Procellarum. The feature has been known under several historical names. William Gilbert called it Insula Medilunaria in a lunar map produced before the modern naming system was established. Michael van Langren used the form Sinus Medius on his 1645 map, providing the basis for the modern designation. Johannes Hevelius later called the area Mare Adriaticum, while Giovanni Riccioli labeled it Sinus Aestuum on his 1651 lunar map. Today, Sinus Medii is recognized as a named lunar geographical feature rather than a commercial brand or operating organization.

Sinus Medii is a basaltic plain on the Moon's near side, positioned close to the intersection of the lunar equator and prime meridian. Its central location made it an important feature in historical lunar mapping and later gave it strategic significance during the planning of crewed lunar missions. The name used today derives from Latin and can be translated as “Middle Bay.” Early lunar cartographers assigned different names to the feature. William Gilbert referred to it as Insula Medilunaria, or “Middlemoon Island.” Michael van Langren's 1645 map used Sinus Medius, a designation that supplied the foundation for the modern name. Johannes Hevelius subsequently called the region Mare Adriaticum, while Giovanni Riccioli used Sinus Aestuum in his 1651 map. These variations reflect the unsettled conventions of pre-modern lunar nomenclature, before the International Astronomical Union's system of standardized names. The mare is approximately 287 kilometers in diameter and lies at roughly 1.6° north and 1.0° east. It joins Mare Insularum on its western side and Mare Vaporum toward the north. Its interior and boundaries contain a mixture of volcanic plains, highland remnants, flooded craters, and rilles. The eastern sector includes Rima Hyginus, Rima Ariadaeus, and the Rimae Triesnecker system. The surrounding terrain includes the craters Murchison, Pallas, Chladni, Flammarion, Oppolzer, Réaumur, Seeliger, Rhaeticus, Bruce, Blagg, Schröter, and Sömmering, as well as Rima Flammarion and Rima Oppolzer. Spacecraft exploration added a modern chapter to the feature's history. Surveyor 4 crashed during an attempted landing near Sinus Medii in July 1967. Surveyor 6 successfully landed in the mare in November of the same year, near the western side of the interior and southwest of Bruce crater. The mission contributed surface observations and engineering data that supported later lunar exploration. Sinus Medii was also designated Apollo Landing Site 3 during NASA's Apollo planning process. Mission planners evaluated landing areas partly according to the Sun's position and the resulting lighting conditions. Apollo 11's primary landing site became Mare Tranquillitatis, but a delay of approximately two days could have caused the mission to target Sinus Medii instead. A longer delay would have shifted the planned destination farther west to a site in Oceanus Procellarum. Although no Apollo crew landed there, the site's location made it an important contingency option in early lunar mission planning.

• 1969

  Sinus Medii is retained as an Apollo 11 contingency site

  [1]Sinus Mediien.wikipedia.org

  The Apollo program designated Sinus Medii as Apollo Landing Site 3. Mission planners considered it a possible destination if Apollo 11 experienced a delay that made the lighting at Mare Tranquillitatis less suitable.

• 1967

  Surveyor 4 crashes near Sinus Medii

  [1]Sinus Mediien.wikipedia.org

  NASA's Surveyor 4 attempted a landing near the mare in July but lost contact and crashed.

• 1967

  Surveyor 6 lands in Sinus Medii

  [1]Sinus Mediien.wikipedia.org

  Surveyor 6 successfully landed in the western part of Sinus Medii in November, southwest of Bruce crater.

• 1651

  Riccioli labels the region Sinus Aestuum

  [1]Sinus Mediien.wikipedia.org

  Giovanni Riccioli used the name Sinus Aestuum on his 1651 lunar map.

• 1647

  Hevelius assigns the name Mare Adriaticum

  [1]Sinus Mediien.wikipedia.org

  Johannes Hevelius referred to the region as Mare Adriaticum in his lunar mapping work.

• 1645

  Sinus Medius appears on van Langren's lunar map

  [1]Sinus Mediien.wikipedia.org

  Michael van Langren used the name Sinus Medius for the feature on his 1645 lunar map, establishing the historical basis for the modern designation.

A centrally located lunar mare and an important reference feature in lunar cartography and exploration history.

1. 1Sinus Mediien.wikipedia.org