18 (number)

18 (eighteen) is the natural number following 17 and preceding 19.


Cardinal	18 eighteen
Ordinal	18th eighteenth
Factorization	$2 \cdot 3^2$
Divisors	1, 2, 3, 6, 9, 18
Roman numeral	XVIII
Binary	10010
Hexadecimal	12

In mathematics

Eighteen is a composite number, its proper divisors being 1, 2, 3, 6 and 9.

It is a heptagonal number, and as the sum of the first three pentagonal numbers, it is a pentagonal pyramidal number. It is also the sum of the totient function for the first seven integers.

In base 10 it is a Harshad number.

In science

The atomic number of argon

Astronomy

Messier object M18, a magnitude 8.0 open cluster in the constellation Sagittarius

The New General Catalogue object NGC 18, a double star in the constellation Pegasus

The Saros number of the solar eclipse series which began on -2416 June 2 and ended on -1118 July 21. The duration of Saros series 18 was 1298.1 years, and it contained 73 solar eclipses.

The Saros number of the lunar eclipse series which began on -2078 April 2 and ended on -780 May 21. The duration of Saros series 18 was 1298.1 years, and it contained 73 lunar eclipses.

In religion

The Hebrew word for "life" is חי (chai), which has a numerical value of 18. Consequently, the custom has arisen in Jewish circles to give donations and monetary gifts in multiples of 18 as an expression of blessing for long life

As lucky or unlucky number

In Chinese tradition, the number 18, normally 十八 (shí bā), can also be read as 幺八 (yāo bā), which sounds like 要发 (yào fā), meaning that one is going to prosper. Thus, building floors numbered "18" are often very expensive in China

Age 18

In many parts of the world, 18 is the age minimum for being allowed a driver's license and/or for purchasing alcohol or tobacco. It is also the voting age in many countries. During the Vietnam War, Americans could be drafted at an age as low as 18, but were not allowed to vote in most states until they were 21. A movement spread to lower the voting age, and in 1971, the Twenty-sixth Amendment extended a guarantee of suffrage to anyone 18 years or older in all states. The voting age was lowered from 21 to 18 in the U.K. and many other nations around that time.

Eighteen is also the age in most countries at which a person may appear in a pornographic video, if pornography is legal at all, or have sex with a person in a position of authority over them (a teacher, athletic coach, or employer, for instance, or in general anyone else over the age of 18).

In addition, eighteen year-olds in America have the right to bear arms, own property, marry without parental consent, get an abortion, donate their body to science, obtain a passport, and serve on a jury. With these new privileges come responsibility: eighteen year-olds are tried as adults in court.

In the U.K. 18 is the age of majority.

In music

String Quartets, Opus 18 by Ludwig van Beethoven is his first set of (six) string quartets
18, the title of an album by Moby
Alice Cooper recorded the song "I'm Eighteen"
Red Hot Chili Peppers recorded the song "She's Only 18"
Skid Row recorded the song "18 and Life"

In other fields

Eighteen is also:

Bobby Labonte's car number in the NASCAR Winston Cup Series when he won the 2000 Championship
The number of holes in a game of golf
The number of chapters into which James Joyce's epic novel Ulysses is divided
A slang term used in Soccer for the Penalty Area
The number of wheels on the most common type of tractor-trailer truck, which are hence often called 18-wheelers
The customary percentage to tip a waitress in a restaurant by groups of four or more
The number of the French department Cher and the Turkish province Bitlis
DVD-18 is double-sided, double-layered DVD format
A Canon error message, see E18 error
In neo-Nazi circles, a code word for Adolf Hitler. The number comes from the position of the letters in the alphabet: A = 1, H = 8. See also 88

Historical years

A.D. 18, 18 B.C., 1918, 2018, etc.

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