Janssen is an ancient impact 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 in the highland region near the southeastern 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...

 limb. The entire structure has been heavily worn and is marked by many lesser crater impacts. The outer wall is breached in multiple locations, but the outline of the crater rim can still be observed. The wall forms a distinctive hexagonal shape upon the rugged lunar surface, with a slight curvature at the vertices

Vertex (geometry)

In geometry, a vertex is a special kind of point that describes the corners or intersections of geometric shapes.-Of an angle:...

.

The prominent crater Fabricius

Fabricius (crater)

Fabricus is a lunar impact crater that is located within the northeast part of the walled plain Janssen. Attached to the north-northwest rim is the slightly larger cater Metius. Fabricius has multiple central peaks that rise to 0.8 km, with a rugged rise to the northwest running north–south. The...

 lies entirely within the outer wall, in the northeast quadrant

Circular sector

A circular sector or circle sector, is the portion of a disk enclosed by two radii and an arc, where the smaller area is known as the minor sector and the larger being the major sector. In the diagram, θ is the central angle in radians, r the radius of the circle, and L is the arc length of the...

 of the floor. A number of other lesser, but still notable craters mark the crater floor. Connected to the northeast rim is Metius

Metius (crater)

Metius is a lunar impact crater located in the rugged highlands to the southeast of the Moon's near side. To the southwest the rim is attached to the crater Fabricius. Offset to the east-northeast is the heavily worn crater Brenner...

, and to the north is the heavily worn Brenner

Brenner (crater)

Brenner is an old lunar crater that lies in the rugged southeastern part of the Moon's near side. It is named after the Serbian astronomer Spiridon Gopčević and is located within one crater diameter northwest of the crater pair Metius and Fabricius.This ancient formation has been deeply eroded by...

. Southeast of Janssen are the co-joined craters Steinheil

Steinheil (crater)

Steinheil is a lunar impact crater that is located in the rugged highlands in the southeastern part of the Moon. It forms a prominent crater pair with the similar-sized Watt, which it partly overlies to the southeast. To the northwest is the large walled plain Janssen. Due to the location of this...

 and Watt

Watt (crater)

Watt is a lunar crater that is located in the southeastern part of the Moon. The northwestern third of the crater rim has been completely overlain by the same-sized Steinheil, leaving much of the interior floor covered with the outer rampart of ejecta from the latter formation...

. Astride the southwest wall is the smaller Lockyer

Lockyer (lunar crater)

Lockyer is a lunar crater that is located along the western wall of the large walled plain Janssen. This crater is roughly circular with a slight outward bulge along the eastern side. The edge of the rim is only lightly eroded, with an indentation in the side to the north-northwest...

. Further to the east, although appearing nearby due to elongation, is the huge Vallis Rheita

Vallis Rheita

Vallis Rheita is a linear valley on the near side of the Moon. It is located in the southeastern quadrant, and is oriented radially to Mare Nectaris. This valley appears to share a common origin with the Vallis Snellius to the northeast, as both are oriented radially with Mare Nectaris.The center...

.

In the southern two-thirds of Janssen can be discerned the remains of a large, concentric

Concentric

Concentric objects share the same center, axis or origin with one inside the other. Circles, tubes, cylindrical shafts, disks, and spheres may be concentric to one another...

 crater, the wall of which is overlaid by Fabricus. The floor of this inner depression contains 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...

system named the Rimae Janssen. The rille curves from the rim of Fabricius to the southeast of the outer wall of Janssen, extending for a distance of up to 140 kilometers.

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 Janssen.

Janssen	Latitude	Longitude	Diameter
B	43.2° S	34.4° E	22 km
C	42.8° S	34.9° E	7 km
D	48.5° S	41.1° E	29 km
E	48.8° S	39.9° E	25 km
F	49.7° S	41.9° E	36 km
H	45.3° S	41.7° E	11 km
J	43.4° S	36.6° E	30 km
K	46.1° S	42.3° E	16 km
L	45.9° S	43.4° E	12 km
M	41.8° S	35.4° E	16 km
N	41.4° S	32.2° E	5 km
P	45.3° S	39.7° E	5 km
Q	46.2° S	39.4° E	5 km
R	48.1° S	38.7° E	17 km
S	50.4° S	41.9° E	8 km
T	48.8° S	42.2° E	31 km
X	42.9° S	33.3° E	24 km