Nielsen is a lunar 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...

 on the Oceanus Procellarum

Oceanus Procellarum

Oceanus Procellarum is a vast lunar mare on the western edge of the near side of the Earth's Moon. Its name derives from the old superstition that its appearance during the second quarter heralded bad weather...

. It is located north-east of Montes Agricola

Montes Agricola

Montes Agricola is an elongated range of mountains near the eastern edge of the central Oceanus Procellarum lunar mare. It lies just to the northwest of a plateau containing the craters Herodotus and Aristarchus....

 on the western hemisphere of the Moon. To the east-southeast is the crater Wollaston

Wollaston (crater)

Wollaston is a relatively small lunar impact crater located in the Oceanus Procellarum. To the northwest is the similar Nielsen. To the southeast is the somewhat larger Krieger. There are several small rilles to the southwest of Wollaston, forming part of the Rimae Prinz.This is a circular,...

.

Nielsen is a bowl-shaped formation that lies astride a small ridge that runs north-northwestwards towards the Mons Rümker

Mons Rümker

Mons Rümker is an isolated volcanic formation that is located in the northwest part of the Moon's near side, at selenographic coordinates 40.8° N, 58.1° W. The feature forms a large, elevated mound in the northern part of the Oceanus Procellarum. The mound has a diameter of 70 kilometres, and...

. The latter is an unusual raised formation of lunar dome

Lunar dome

A lunar dome is a type of shield volcano that is found on the surface of the Earth's Moon. They are typically formed by highly viscous, possibly silica-rich lava, erupting from localized vents followed by relatively slow cooling. Lunar domes are wide, rounded, circular features with a gentle slope...

s.

The crater is named jointly after the Danish astronomer Axel Nielsen

Axel Nielsen

Axel Vilfred Nielsen was a Danish astronomer at the Ole Rømer Observatory in Aarhus from 1927 until his death in 1970....

 (1902-1970) and the Danish–American physicist Harald Herborg Nielsen

Harald Herborg Nielsen

Harald Herborg Nielsen was an American physicist.He was born in Menominee, Michigan to immigrant parents who had moved to the United States from Denmark. His college education was attained at the University of Michigan, where he earned a Ph.D. He then spent a year at the Bohr's Institute in...

(1903-1973).