# 21 number

21 (twenty-one) is the natural number following 20 and preceding 22.

 Template:Numbers 20s Cardinal twenty-one Ordinal twenty-first Factorization $\displaystyle 3 \cdot 7$ Roman numeral XXI Binary 10101 Hexadecimal 15

## In mathematics

Twenty-one is a Fibonacci number, a Harshad number, a Motzkin number, a triangular number and an octagonal number, as well as a composite number, its proper divisors being 1, 3 and 7.

21 appears in the Padovan sequence, preceded by the terms 9, 12, 16 (it is the sum of the first two of these).

## In science

In astronomy,

Messier object M21, a magnitude 7.0 open cluster in the constellation Sagittarius
The New General Catalogue object NGC 21, a barred spiral galaxy in the constellation Andromeda
The Saros number of the solar eclipse series which began on -2275 May 5 and ended on -995 June 11. The duration of Saros series 21 was 1280.1 years, and it contained 72 solar eclipses.
The Saros number of the lunar eclipse series which began on -1973 February 11 and ended on -639 April 22. The duration of Saros series 21 was 1334.2 years, and it contained 75 lunar eclipses.

## In human affairs: age 21

21 had been the age of majority in England and Wales before that age was lowered to 18 with the coming into force, on 1 January 1970, of the Family Law Reform Act 1969.

In the United States, when one reaches 21, one gains the right to drink alcohol. It is also the age at which it is legal to gamble and have sex with a prostitute in Nevada. In most states, a person who is 21 or over may also rent a car. 21 was the age for voting in the majority of states until the introduction of the Twenty-sixth Amendment.

## In other fields

The current century -- spanning the years from 2001 to 2100 -- is referred to as the 21st century. This inspired many forward-looking 20th century events, company, and product names, like the Century 21 Exposition (another name for the 1962 Seattle World's Fair), Century 21 Real Estate, and Century 21 Television (producers of Sylvia and Gerry Anderson Supermarionation shows like Thunderbirds and Captain Scarlet).

The Twenty-first Amendment repealed the Eighteenth Amendment, thereby ending Prohibition.

Pier21 was, from 1928 to 1971, the place where immigrants entered Canada. It was called the "Gateway to Canada".

Twenty-one is: