61 (number) - Definition and Overview

61 (sixty-one) is the natural number following 60 and preceding 62.

In mathematics

It is the 18th prime number. The previous is 59, with which it comprises a twin prime. Sixty-one is a cuban prime of the form <math>p = (x^3 - y^3) / (x - y), x = y + 1<math>. Sixty-one is the sum of two squares, 5² + 6², and it is also a centered square number, a centered hexagonal number and a centered decagonal number.

It is also a Keith number, because it recurs in a Fibonacci-like sequence started from its base 10 digits: 6, 1, 7, 8, 15, 23, 38, 61...

In science

• The atomic number of promethium, a lanthanide

In astronomy,

Messier object M61, a magnitude 10.5 galaxy in the constellation Virgo

The New General Catalogue object (http://www.ngcic.org/) NGC 61, a double spiral galaxy in the constellation Pisces

The Saros number (http://sunearth.gsfc.nasa.gov/eclipse/SEsaros/SEsaros1-175.html) of the solar eclipse series which began on -973 May 10 and ended on 289 June 5. The duration of Saros series 61 was 1262.1 years, and it contained 71 solar eclipses.

The Saros number (http://sunearth.gsfc.nasa.gov/eclipse/LEsaros/LEsaros1-175.html) of the lunar eclipse series which began on -870 October 20 and ended on 609 March 26. The duration of Saros series 61 was 1478.4 years, and it contained 83 lunar eclipses.

In other fields

Sixty-one is:

• The number of the French department Orne.
• The code for international direct dial phone calls to Australia.
• 61*, a 2001 baseball movie directed by Billy Crystal.
• The year AD 61, 61 BC, or 1961.