65 is the natural number following 64 and preceding 66.

In mathematics

Sixty-five is an octagonal number. It is also a Cullen number. Given 65, the Mertens function returns 0.

This number is the magic constant of 5 by 5 magic square:

This number is also the magic constant of n-Queens Problem for n = 5.

65 can be expressed as a sum of two squares in two ways, 65 = 8² + 1² = 7² + 4².

In science

• The atomic number of terbium, a lanthanide

In astronomy,

Messier object M65, a magnitude 10.5 galaxy in the constellation Leo

The New General Catalogue object (http://www.ngcic.org/) NGC 65, 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 -749 April 24 and ended on 513 May 20. The duration of Saros series 65 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 -736 August 11 and ended on 797 February 16. The duration of Saros series 65 was 1532.5 years, and it contained 86 lunar eclipses.

In other fields

Sixty-five is also:

• A common speed limit, in miles per hour, for freeways in many U.S. states.
• The designation of Interstate 65, a freeway that runs from Alabama to Indiana.
• The code for international direct dial phone calls to Singapore.
• The standard age for retirement in the UK, Germany and other countries.
• The registry of the U.S. Navy's nuclear aircraft carrier USS Enterprise (CVN-65).
• The year AD 65, 65 BC, or 1965.