40 (Number)

40 is the natural number following 39 and preceding 41. In English, forty is the only number whose constituent letters appear in alphabetical order.
Cardinal forty
Ordinal40th (fortieth)
Numeral system
Factorization2^3 \cdot 5
Divisors 2, 4, 5, 8, 10, 20
Roman numeralXL
Binary101000
Hexadecimal28
Hebrewמ (Mem)

In mathematics

Forty is an octagonal number, and as the sum of the first four pentagonal numbers, it is a pentagonal pyramidal number. It is a Harshad number. Given 40, the Mertens function returns 0. 40 is the smallest number n with exactly 9 solutions to the equation φ(x) = n. Forty is the number of n-queens problem solutions for n = 7.

In science

In astronomy,
Messier object M40, a magnitude 9.0 double star in the constellation Ursa Major
The New General Catalogue object NGC 40, a magnitude 12.4 planetary nebula in the constellation Cepheus
The Saros number of the solar eclipse series which began on -1653 May 28 and ended on -373 July 4. The duration of Saros series 40 was 1280.1 years, and it contained 72 solar eclipses.
The Saros number of the lunar eclipse series which began on -1387 February 12 and ended on -71 April 12. The duration of Saros series 40 was 1316.2 years, and it contained 74 lunar eclipses.

In other fields

Forty is:

 

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