Nielsen is a bowl-shaped formation that lies astride a small ridge that runs north-northwestwards towards the Mons Rümker. The latter is an unusual raised formation of lunar domes.
In 1973, the crater was named jointly after the Danish astronomer Axel Nielsen (1902-1970) and the Danish–American physicist Harald Herborg Nielsen (1903-1973), by the IAU.[1]
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.