Sixty-three is a natural number following 62 and preceding 64.

In mathematics

Although a number of the form 2ⁿ - 1, 63 is technically not a Mersenne number since n is not prime and 63 is certainly not prime either. It is a Woodall number and a Harshad number. It is a highly cototient number.

In science

• The atomic number of europium, a lanthanide

In astronomy,

Messier object M63, a magnitude 8.5 galaxy in the constellation Canes Venatici, also known as the Sunflower Galaxy.

The New General Catalogue object (http://www.ngcic.org/) NGC 63, a 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 -879 April 20 and ended on 401 May 29. The duration of Saros series 63 was 1280.1 years, and it contained 72 solar eclipses.

The Saros number (http://sunearth.gsfc.nasa.gov/eclipse/LEsaros/LEsaros1-175.html) of the lunar eclipse series which began on -758 October 12 and ended on 739 March 29. The duration of Saros series 63 was 1496.5 years, and it contained 84 lunar eclipses.

In other fields

Sixty-three is:

• The code for international direct dial calls to the Philippines.
• The registry of the U.S. Navy's aircraft carrier USS Kitty Hawk (CV-63).
• The number of the French department Puy-de-Dôme.
• The year AD 63, 63 BC, or 1963.