Réaumur is the remains of a lunar

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...

 crater

Impact crater

In the broadest sense, the term impact crater can be applied to any depression, natural or manmade, resulting from the high velocity impact of a projectile with a larger body...

 located on the southern edge of Sinus Medii

Sinus Medii

Sinus Medii is a small lunar mare that is located 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...

. It lies to the northwest of the large walled plain Hipparchus

Hipparchus (lunar crater)

Hipparchus is the degraded remnant of a lunar crater. It is located to the southeast of Sinus Medii, near the center of the visible Moon. To the south is the prominent crater Albategnius, and to the southwest lies Ptolemaeus, a feature of comparable dimensions to Hipparchus. Horrocks lies entirely...

, and east of 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....

. To the south is Gyldén

Gyldén (crater)

Gyldén is the remnant of a lunar crater that is located to the northeast of the walled plain Ptolemaeus on the Moon. It lies along the prime meridian of the selenographic coordinate system, and less than 150 km south of the lunar equator...

, and farther to the south-southwest is Ptolemaeus

Ptolemaeus (lunar crater)

Ptolemaeus is an ancient lunar impact crater close to the center of the near side. To the south-southeast Ptolemaeus is joined to the rim of the crater Alphonsus by a section of rugged, irregular terrain, and these form a prominent chain with Arzachel to the south...

.

The rim of Réaumur is heavily eroded, and the northern half consists of little more than a series of low ridges in the 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...

. The southern rim is notched and indented by small crater impacts. The interior floor is relatively smooth and flat with no significant features.

The broken northern rim intersects a 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...

 designated Rima Oppolzer. This feature is about 110 kilometers long and follows a course from the north-northeast to the south-southwest. To the east of the crater rim is a smaller rille designated Rima Réaumur. Farther to the east is the small 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....

.

Satellite craters

By convention these features are identified on lunar maps by placing the letter on the side of the crater mid-point that is closest to Réaumur.

Réaumur	Latitude	Longitude	Diameter
A	4.3° S	0.2° E	16 km
B	4.2° S	0.9° E	5 km
C	3.4° S	0.2° E	5 km
D	0.2° S	2.8° E	4 km
K	3.8° S	1.0° E	7 km
R	3.5° S	2.1° E	14 km
W	3.2° S	2.8° E	3 km
X	2.9° S	0.6° W	5 km
Y	1.3° S	0.6° E	3 km