21 (number)
Encyclopedia : 2 : 21 : 21N : 21 (number)
21 (twenty-one) is the natural number following 20 and preceding 22.
| Cardinal | 21 twenty-one |
| Ordinal | 21st twenty-first |
| Factorization | [3 \cdot 7] |
| Roman numeral | XXI |
| Binary | 10101 |
| Hexadecimal | 15 |
Contents
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).
Adding up the sums of divisors for the integers 1 through 5 yields 21.
21 is a repdigit in base 4 (111).
In science
- The atomic number of scandium
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.
Age 21
In the United States (excluding insular areas), 21 is the minimum legal age at which one can purchase and drink alcoholic beverages. It is also the age at which it is legal to gamble and work as a prostitute in Nevada (except in Storey County and Lyon County, where the legal age is 18). In most states, a person who is 21 or over may also rent a car. Twenty-one was the minimum age of suffrage in the majority of states until the introduction of the Twenty-sixth Amendment.Twenty-one was the age of majority in England and Wales before that age was lowered to 18 with the enactment of the Family Law Reform Act on 1 January 1970.
In sports
Players with jersey number twenty-one include:- Mike Eruzione of the 1980 Men's Gold Olympic Hockey Team
- Roberto Clemente, Hall-of-Famer of the Pittsburgh Pirates
- Peter Forsberg, hockey player for the Quebec Nordiques, Colorado Avalanche and now Philadelphia Flyers
In other fields
21 is:- The current century—spanning the years from 2001 to 2100—referred to as the 21st century. The approach of this century inspired many forward-looking 20th century events and company and product names, such as 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."
- Forever 21 (also operating as XXI Forever) is a U.S.-based chain of clothing stores that was founded in 1984.
- The number of spots on a standard cubical dice (1+2+3+4+5+6)
- The number of firings in a 21-gun salute honoring Royalty or leaders of countries
- A card game, also called vingt-et-un (French for "twenty-one"), pontoon, or blackjack
- A drinking game in which the goal is to get the group to 21 without saying it oneself
- The title of a TV quiz show that ran from 1956 to 1958, most remembered for the scandal that the wins were fixed (see Quiz show scandals). It was remade in 2000
- The title of at least three different movies:
- A 1918 comedy starring Bryant Washburn and Gertrude Selby
- A 1923 drama starring Richard Barthelmess and Dorothy Mackaill
- A 1991 drama starring Patsy Kensit
Historical years
21 A.D., 21 B.C., 1921, 2021, etc.
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