51 (number) - Definition and Overview

51 is the natural number following 50 and preceding 52.

In mathematics

Fifty-one is a pentagonal number as well as a centered pentagonal number (one of the few numbers to be both) and an 18-gonal number. It is also the 6th Motzkin number, telling the number of ways to draw non-intersecting chords between any six points on a circle's boundary, no matter where the points may be located on the boundary.

In science

• The atomic number of antimony

In astronomy,

Messier object M51, a magnitude 8.0 spiral galaxy in the constellation Canes Venatici, also known as the Whirlpool Galaxy

The New General Catalogue object (http://www.ngcic.org/) NGC 51, a spiral galaxy in the constellation Andromeda

The Saros number (http://sunearth.gsfc.nasa.gov/eclipse/SEsaros/SEsaros1-175.html) of the solar eclipse series which began on -1407 September 2 and ended on 108 February 29. The duration of Saros series 51 was 1514.5 years, and it contained 85 solar eclipses.

The Saros number (http://sunearth.gsfc.nasa.gov/eclipse/LEsaros/LEsaros1-175.html) of the lunar eclipse series which began on -1105 June 13 and ended on 211 August 12. The duration of Saros series 51 was 1316.2 years, and it contained 74 lunar eclipses.

In other fields

Fifty-one is also:

• the code for international direct dial phone calls to Peru.
• the model number of the P-51 Mustang WWII fighter aircraft
• part of the title of a collection of stories by Lord Dunsany, Fifty-One Tales.
• Area 51, a parcel of U.S. military-controlled land in southern Nevada, apparently containing a secret aircraft testing facility.
• The year AD 51, 51 BC, and 1951.