90 (number)

90 (ninety) is the natural number preceded by 89 and followed by 91.

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In mathematics

90 is the sum of the squares of the integers 2 to 6. Because 90 is the sum of its unitary divisors (excluding itself), it is a unitary perfect number, and because it is equal to the sum of a subset of its divisors, it is a semiperfect number. 90 is a pronic number. But it is also a nontotient. It is a Perrin number, preceded in the sequence by 39, 51, 68.

In normal space, the interior angles of a square measure 90 degrees each. Also, in a right triangle, the angle opposing the hypotenuse measures 90 degrees, with the other two angles adding up to 90 for a total of 180 degrees.

90 is divisible by the sum of its base 10 digits, thus it is a Harshad number.

In astronomy

• Messier object M90, a magnitude 11.0 spiral galaxy in the constellation Virgo
• The New General Catalogue object NGC 90, a barred spiral galaxy in the constellation Andromeda
• The Saros number of the solar eclipse series which began on 134 BC September 28 and ended on 1345 March 4. The duration of Saros series 90 was 1478.4 years, and it contained 83 solar eclipses.
• The Saros number of the lunar eclipse series which began on 150 June 27 and ended on 1430 August 4. The duration of Saros series 90 was 1280.1 years, and it contained 72 lunar eclipses.

In other fields

Interstate 90

Ninety is:

• The atomic number of thorium, an actinide.
• The latitude of the North Pole and the South Pole.
• The designation of United States Interstate 90, a freeway that runs from Washington to Massachusetts.
• The code for international direct dial phone calls to Turkey.
• The ISBN Group Identifier for books published in the Netherlands.
• The Soviet T-90 tank is the most modern in the Russian arsenal.
• The year AD 90, 90 BC, or 1990.
• The nickname of Nintendo, a Japanese videogame company.
• The number of minutes in a Football(Soccer) match.

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