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 p = (x³ - y³) / (x - y),x = y + 1. 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 NGC 61 , a double spiral galaxy in the constellation Pisces

The Saros number 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 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.