Jansen is a lunarimpact crater in the north part of the Mare Tranquillitatis. The diameter is 24 km. It was named after 17th century Dutch optician Zacharias Janssen.[1] It is located to the east-southeast of the crater Plinius. The rim of Jansen is low and narrow, with a notch along the western edge. The interior is relatively level, which may indicate it has been covered by lava. To the south-southwest a small but prominent crater lies on the crater floor, halfway between the center and the rim.
To the northwest of the rim is a rille named Rima Jansen, and to the east are wrinkle ridges in the mare surface. A low ridge runs from near the southeast rim in a southeasterly direction.
Menzel, D. H.; Minnaert, M.; Levin, B.; Dollfus, A.; Bell, B. (1971). "Report on Lunar Nomenclature by the Working Group of Commission 17 of the IAU". Space Science Reviews. 12 (2): 136–186. Bibcode:1971SSRv...12..136M. doi:10.1007/BF00171763. S2CID122125855.