50 ("fifty") is the number following 49 and preceding 51.

In mathematics

Fifty is the smallest number that can be written as the sum of two squares in two distinct ways: 50 = 1² + 7² = 5² + 5². It is also the sum of three squares, 50 = 3² + 4² + 5². It is a Harshad number.

There is no answer to the equation φ(x) = 50, making 50 a nontotient. Nor is there an answer to the equation x - φ(x) = 50, making 50 a noncototient.

In science

• The atomic number of tin
• The fifth magic number in physics

In astronomy,

Messier object M50, a magnitude 7.0 open cluster in the constellation Monoceros

The New General Catalogue object (http://www.ngcic.org/) NGC 50, a spiral galaxy in the constellation Cetus

The Saros number (http://sunearth.gsfc.nasa.gov/eclipse/SEsaros/SEsaros1-175.html) of the solar eclipse series which began on 1201 February 11 BC and ended on 97 April 1. The duration of Saros series 50 was 1298.1 years, and it contained 73 solar eclipses.

The Saros number (http://sunearth.gsfc.nasa.gov/eclipse/LEsaros/LEsaros1-175.html) of the lunar eclipse series which began on -1134 July 3 and ended on 164 August 20. The duration of Saros series 50 was 1298.1 years, and it contained 73 lunar eclipses.

In other fields

Fifty is also:

• The traditional number of years in a default jubilee period.
• The number of states in the USA (since 1959, still true in 2004).
• A calibre of ammunition (0.50 inches: see .50 BMG)
• In millimeters, the focal length of the normal lens in 35mm photography.
• the percentage equivalent to one half, so that the phrase "fifty-fifty" commonly expresses something divided equally in half, or an event of probability one half
• In U.S. dollars, the denomination of the Federal Reserve note with Ulysses S. Grant's portrait, the denomination of the savings bond with George Washington's portrait, and the denomination of treasury bonds with Thomas Jefferson's portrait.
• In cents of the U.S. dollar, the denomination of the coin with John F. Kennedy's portrait.
• In years of marriage, the gold or "golden" wedding anniversary.
• part of the name of 50 Cent, a rapper.
• One of the years: AD 50, 50 BC, or (for example) 1950.