45 is the natural number following 44 and followed by 46.

In mathematics

Forty-five is a triangular number, a hexagonal and 16-gonal number, a Kaprekar number and a Harshad number.

In science

• The atomic number of rhodium

In astronomy,

Messier object M45, a magnitude 1.4 open cluster in the constellation Taurus, also known as the Pleiades

The New General Catalogue object (http://www.ngcic.org/) NGC 45, a magnitude 10.6 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 -1436 March 30 and ended on -156 May 7. The duration of Saros series 45 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 -1369 August 19 and ended on 182 March 8. The duration of Saros series 45 was 1550.6 years, and it contained 87 lunar eclipses.

In other fields

Forty-five is also:

• The total of books in the Old Testament of the Holy Bible in the Catholic version if the Book of Lamentations is considered part of the Book of Jeremiah.
• In years of marriage, the sapphire wedding anniversary.
• In reference to gramophone records, 45 refers to a type of record by its revolution speed of 45 revolutions per minute (rpm). These small discs usually contained one song on each side which resulted in calling such records singles.
  • Originally the 45 RPM record was introduced as a longer play version of the 78 RPM discs. The first 45 RPM records were monaural with recordings on both sides of the Disc. Produced in several sizes, the 7" large hole version became the most popular. As stereo recordings became popular in the 1950s, almost all of the newer 45 RPM records were recorded in Stereo.
  • See also: 78, LP and 33.
• The title of a book of essays by record producer Bill Drummond, derived both from the speed of a pop single and from his age when he finished writing it.
• The caliber of some handguns.
• a card game: Forty-five
• I-45 is the designation for a US interstate highway in Texas.
• The code for international direct dial phone calls to Denmark.
• In the Ishihara color test, 45 is on a plate of blue, green, red and brown circles which people with normal vision can discern but colorblind people can't see
• The year AD 45, 45 BC, and 1945.