Oppolzer is the remnant 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...

 that is 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...

, along the meridian

Meridian (geography)

A meridian is an imaginary line on the Earth's surface from the North Pole to the South Pole that connects all locations along it with a given longitude. The position of a point along the meridian is given by its latitude. Each meridian is perpendicular to all circles of latitude...

 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...

. 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 crater 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....

. To the west-southwest is the lava

Lava

Lava refers both to molten rock expelled by a volcano during an eruption and the resulting rock after solidification and cooling. This molten rock is formed in the interior of some planets, including Earth, and some of their satellites. When first erupted from a volcanic vent, lava is a liquid at...

-flooded walled plain 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....

.

This formation consists of little more than an irregular arc of mounts along the south, and a ring of scattered ridges along the northern rim. There is a particularly wide gap in the rim to the northeast where no ridges survive to mark the crater perimeter. The interior floor has been resurfaced by basalt

Basalt

Basalt is a common extrusive volcanic rock. It is usually grey to black and fine-grained due to rapid cooling of lava at the surface of a planet. It may be porphyritic containing larger crystals in a fine matrix, or vesicular, or frothy scoria. Unweathered basalt is black or grey...

ic lava

Lava

Lava refers both to molten rock expelled by a volcano during an eruption and the resulting rock after solidification and cooling. This molten rock is formed in the interior of some planets, including Earth, and some of their satellites. When first erupted from a volcanic vent, lava is a liquid at...

.

A 110-km-long 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 passes through the southern part of the crater floor, and continues to the east and west of the crater.

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 Oppolzer.

Oppolzer	Latitude	Longitude	Diameter
A	0.5° S	0.3° W	3 km
K	1.7° S	0.3° W	3 km