90 (number) - Definition and Overview

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

In mathematics

Ninety is the sums 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. 90 is a pronic number. And because 90 is divisible by the sum of its digits in base 10, it is a Harshad number. But it is also a nontotient.

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.

In science

• The atomic number of thorium, an actinide.

In astronomy,

Messier object M90, a magnitude 11.0 spiral galaxy in the constellation Virgo

The New General Catalogue object (http://www.ngcic.com/) NGC 90, a barred spiral galaxy in the constellation Andromeda

The Saros number (http://sunearth.gsfc.nasa.gov/eclipse/SEsaros/SEsaros1-175.html) of the solar eclipse series which began on -134 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 (http://sunearth.gsfc.nasa.gov/eclipse/LEsaros/LEsaros1-175.html) 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

Ninety is:

• The designation of American 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.