Sinus Medii is a small lunar mare

Lunar mare

The lunar maria are large, dark, basaltic plains on Earth's Moon, formed by ancient volcanic eruptions. They were dubbed maria, Latin for "seas", by early astronomers who mistook them for actual seas. They are less reflective than the "highlands" as a result of their iron-rich compositions, and...

 that is located at the intersection of the Moon

Moon

The Moon is Earth's only known natural satellite,There are a number of near-Earth asteroids including 3753 Cruithne that are co-orbital with Earth: their orbits bring them close to Earth for periods of time but then alter in the long term . These are quasi-satellites and not true moons. For more...

's equator

Equator

An equator is the intersection of a sphere's surface with the plane perpendicular to the sphere's axis of rotation and containing the sphere's center of mass....

 and prime meridian

Prime Meridian

The Prime Meridian is the meridian at which the longitude is defined to be 0°.The Prime Meridian and its opposite the 180th meridian , which the International Date Line generally follows, form a great circle that divides the Earth into the Eastern and Western Hemispheres.An international...

. As seen from the Earth

Earth

Earth is the third planet from the Sun, and the densest and fifth-largest of the eight planets in the Solar System. It is also the largest of the Solar System's four terrestrial planets...

, 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

Libration

In astronomy, libration is an oscillating motion of orbiting bodies relative to each other, notably including the motion of the Moon relative to Earth, or of Trojan asteroids relative to planets.-Lunar libration:...

.

This plane was given the latin

Latin

Latin is an Italic language originally spoken in Latium and Ancient Rome. It, along with most European languages, is a descendant of the ancient Proto-Indo-European language. Although it is considered a dead language, a number of scholars and members of the Christian clergy speak it fluently, and...

 name for "Central Bay" by Johannes Mädler. The selenographic coordinates are , and the diameter is 335 km. It joins Mare Insularum

Mare Insularum

Mare Insularum is a lunar mare located in the Insularum basin just south of Mare Imbrium. The basin material is of the Lower Imbrian epoch, with the mare material of the Upper Imbrian epoch. The mare is bordered by the craters Copernicus on the east, and Kepler on the west...

 in the west with Mare Vaporum

Mare Vaporum

Mare Vaporum is a lunar mare located between the southwest rim of Mare Serenitatis and the southeast rim of Mare Imbrium. The lunar material surrounding the mare is from the Lower Imbrian epoch, and the mare material is from the Eratosthenian epoch. The mare lies in an old basin or crater that is...

 to the north.

The eastern part of this area is notable for a series of rille

Rille

Rille is typically used to describe any of the long, narrow depressions in the lunar surface that resemble channels. Typically a rille can be up to several kilometers wide and hundreds of kilometers in length...

 systems. In the far northeast is the Rima Hyginus, which is bisected by the crater Hyginus

Hyginus (crater)

Hyginus is a small lunar caldera located at the east end of the Sinus Medii. Its rim is split by a long, linear rille Rima Hyginus that branches to the northwest and to the east-southeast for a total length of 220 kilometers. The crater is deeper than the rille, and lies at the bend where they...

. At the far eastern end is the 220-km long Rima Ariadaeus

Rima Ariadaeus

Rima Ariadaeus is a linear rille on the Moon at . It is named after the crater Ariadaeus, which marks its eastern end. Over 300 kilometers long, it is thought to have been formed when a section of the Moon's crust sank down between two parallel fault lines . It is a relatively young lunar feature,...

 rille which continues eastward to the edge of the Mare Tranquillitatis

Mare Tranquillitatis

Mare Tranquillitatis is a lunar mare that sits within the Tranquillitatis basin on the Moon. The mare material within the basin consists of basalt formed in the intermediate to young age group of the Upper Imbrian epoch. The surrounding mountains are thought to be of the Lower Imbrian epoch, but...

. Crossing the prime meridian longitude is the Rimae Triesnecker rille system, named after the crater Triesnecker

Triesnecker (crater)

Triesnecker is a prominent lunar impact crater that is located in the Sinus Medii, near the central part of the Moon's near side. It is located to the north-northwest of the crater Rhaeticus, and to the east-southeast of the flooded Murchison....

 just to the west.

The northern edge of the Sinus Medii is formed by a highland region, with the impact craters Murchison

Murchison (crater)

Murchison is a lunar crater on the north edge of the Sinus Medii, named in honour of the geologist Sir Roderick Murchison. It shares a section of rim with the crater Pallas. To the southeast on the mare is the circular crater Chladni, and to the northeast is Ukert. Farther to the east is the...

 and Pallas

Pallas (crater)

Pallas is a heavily eroded lunar crater located to the north of the Sinus Medii. To the northwest is the smaller but less worn crater Bode. Pallas shares a low wall with the crater Murchison that is attached to the southeast, and there are two gaps in the shared rim.The outer wall of Pallas is...

 along the border. Near the northern border on this mare is the cup-shaped Chladni

Chladni (crater)

Chladni is a small lunar impact crater that lies near the northwest edge of Sinus Medii, in the central part of the Moon. The rim of the crater is roughly circular, and there is a small central floor at the mid-point of the sloping inner walls. This feature has a higher albedo than the surrounding...

.

Another highland region lies to the south and southeastern edge of the Sinus Medii. Several flooded craters lie along this border, with Flammarion

Flammarion (lunar crater)

Flammarion is a lunar crater on the south edge of Sinus Medii. It is located between the crater Mösting to the northwest and Herschel to the southeast. The bowl-shaped Mösting A intersects the western rim of Flammarion....

 near the western edge, then Oppolzer

Oppolzer (crater)

Oppolzer is the remnant of a lunar crater that is located on the southern edge of Sinus Medii, along the meridian of the Moon. It is located within one crater diameter of the origin of the selenographic coordinate system at 0° N, 0° W. Attached to the surviving remnants of the southeast rim is the...

, Réaumur

Réaumur (crater)

Réaumur is the remains of a lunar crater located on the southern edge of Sinus Medii. It lies to the northwest of the large walled plain Hipparchus, and east of Flammarion. To the south is Gyldén, and farther to the south-southwest is Ptolemaeus....

, and Seeliger

Seeliger (crater)

Seeliger is a relatively small lunar impact crater that lies near the southeastern edge of Sinus Medii. This is a circular, cup-shaped feature that has not been appreciably worn by impact erosion....

 further east. The Rima Flammarion and Rima Oppolzer rille

Rille

Rille is typically used to describe any of the long, narrow depressions in the lunar surface that resemble channels. Typically a rille can be up to several kilometers wide and hundreds of kilometers in length...

s lie along the edge of the mare near their corresponding craters. Also along the southeast border and bisecting the prime meridian is the crater Rhaeticus

Rhaeticus (crater)

Rhaeticus is a lunar crater that lies astride the equator of the Moon, on the southeast edge of the Sinus Medii. To the north-northwest is the crater Triesnecker, and due south can be found the worn remnant of the walled plain Hipparchus...

.

In the western half of the interior are the small craters Bruce

Bruce (crater)

Bruce is a small lunar impact crater located in the Sinus Medii. It lies to the west-northwest of the irregular crater Rhaeticus, and just to the west of the even smaller Blagg....

 and Blagg

Blagg (crater)

Blagg is a tiny lunar impact crater located on the Sinus Medii. It is a circular, breast-shaped crater with no appreciable erosion. To the east-southeast is the irregular crater Rhaeticus, and to the northeast lies Triesnecker....

. To the west-southwest of Bruce on the mare floor are the landing sites of the Surveyor 4

Surveyor 4

Surveyor 4 was the fourth lunar lander in the American unmanned Surveyor program sent to explore the surface of the Moon.*Launched July 14, 1967; landed July 17, 1967*Weight on landing: 625 lb...

 and Surveyor 6

Surveyor 6

Surveyor 6 was the sixth lunar lander of the American unmanned Surveyor program that reached the surface of the Moon.*Launched November 7, 1967; landed November 10, 1967*Mass on landing: 299.6 kg Surveyor 6 landed on the Sinus Medii...

 missions. Near the western end of the mare are the flooded craters Schröter

Schröter (lunar crater)

Schröter is a lunar crater near the mid-part of the Moon, on the eastern Mare Insularum, and named after German astronomer Johann Hieronymus Schröter. It lies to the north of the craters Sömmering and Mösting. To the southeast of the crater rim is a rille named the Rima Schröter...

 and Sömmering

Sömmering (crater)

Sömmering is the lava-flooded remains of a lunar crater on the eastern edge of the Mare Insularum. To the southeast is the crater Mösting, and north of Sömmering lies a similar flooded crater designated Schröter...

.