84 (number)

84 (eighty-four) is the natural number following 83 and preceding 85.

In mathematics

84 is:

the sum of the first seven triangular numbers (making it a tetrahedral number).^[1]
the sum of a twin prime (41 + 43).
a semiperfect number, being thrice a perfect number.
a palindromic number and a repdigit in bases 11 (77₁₁), 13 (66₁₃), 20 (44₂₀), 27 (33₂₇), and 41 (22₄₁).
the lim sup of the largest finite subgroup of the mapping class group of a genus g surface divided by g.

Messier object M84, a magnitude 11.0 lenticular galaxy in the constellation Virgo
The New General Catalogue object NGC 84, a single star in the constellation Andromeda
The Saros number of the solar eclipse series which began on -181 April 14 and ended on 1099 May 22. The duration of Saros series 84 was 1280.1 years, and it contained 72 solar eclipses.^[2] Further, the number of the lunar eclipse series which began on -96 September 13 and ended on 1401 February 28. The duration of Saros series 84 was 1496.5 years, and it contained 84 lunar eclipses.^[3]

Eighty-four is also:

The year AD 84, 84 BC, or 1984.
The number of years in the Insular latercus, a cycle used in the past by Celtic peoples,^[4] equal to 3 cycles of the Julian Calendar and to 4 Metonic cycles and 1 octaeteris
The atomic number of polonium
The model number of Harpoon missile
WGS 84 - The latest revision of the World Geodetic System, a fixed global reference frame for the Earth.
The house number of 84 Avenue Foch
The number of the French department Vaucluse
The code for international direct dial phone calls to Vietnam
The town of Eighty Four, Pennsylvania
The company 84 Lumber
The ISBN Group Identifier for books published in Spain
A variation of the game 42 played with two sets of dominoes.
The film "84 Charing Cross Road" (1987) starring Anne Bancroft and Anthony Hopkins
KKNX Radio 84 in Eugene, Oregon
The B-Side to "Up All Night" (Take That song)
British Army term for the 84mm Carl Gustav recoilless rifle.
How many years it takes Uranus to orbit the sun once