43 is the natural number following 42 and preceding 44.

In mathematics

Forty-three is the 14th smallest prime number. The previous is forty-one, with which it comprises a twin prime, the next is forty-seven. 43 is a centered heptagonal number.

Let a(0) = a(1) = 1, and thenceforth a(n) = (a(0)² + a(1)² + ... + a(n-1)²) / (n-1). This sequence continues 1 1 2 3 5 10 28 154... . Amazingly, a(43) is the first term of this sequence that is not an integer.

Negative forty-three (-43) is a Heegner number.

In science

• The atomic number of technetium

In astronomy,

Messier object M43, a magnitude 7.0 H II region in the constellation of Orion, a part of the Orion Nebula, and also sometimes known as de Mairan's Nebula

The New General Catalogue object NGC 43 , a barred spiral galaxy in the constellation Andromeda

The Saros number of the solar eclipse series which began on -1512 April 29 and ended on -232 June 5. The duration of Saros series 43 was 1280.1 years, and it contained 72 solar eclipses.

The Saros number of the lunar eclipse series which began on -1481 August 27 and ended on 70 March 15. The duration of Saros series 43 was 1550.5 years, and it contained 87 lunar eclipses.

In other fields

Forty-three is also:

• The designation of Interstate 43, a freeway in Wisconsin.
• The code for direct dial international phone calls to Austria.
• Part of the title of the historical British comedy show Hancock's Forty-Three Minutes, starring Tony Hancock.
• Part of the title of a historical short story by Hershel Finman titled Forty-Three Rubles, about Rabbi Zvi Elimelech Spira .
• The year AD 43, 43 BC, and 1943.