Number 21

A page of the Numeropedia - the Special Encyclopedia of Numbers

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[Calendar]  March 20 or 21 is the Spring Equinox day: Spring begins; day and night are equal lengths.

June 20 or 21 is the Summer Solstice day: Summer begins; the Sun is farthest north of the Equator

December 21 or 22 is Winter Solstice day: winter begins; the Sun is farthest south of the Equator.

 

[Chemistry]  The atomic number of Scandium (Sc).

 

[Game]  The total number of dots on a regular dice.

 

[History]  Illinois became the 21st state of the U.S. on 3 December 1818.

 

[History]  The 21st U.S. President: Chester Alan Arthur (Republican, Vermont, 1881-1885). There was no Vice President.

 

[History]  The 21-gun salute is for U.S. Presidents, chiefs of government or States or reigning royal family.

The 19-gun salute is for U.S. Vice President, Prime ministers, ambassadors and cabinet, department chiefs. Other odd-number gun salutes (17, 15, 13, 11, 7 and 5) are for lower-level chiefs.

 

[History]  The Royal Salute is normally a 21-gun salute. On some occasions or anniversaries, if fired from a Royal Residence, it is a 41-gun salute and if fired from Tower of London, it is a 62-gun salute.

 

[Language]  The number of consonants in English language: B, C, D, F, G, H, J, K, L, M, N, P, Q, R, S, T, V, W, X, Y and Z.

 

[Library]  The number of letters representing major divisions/branches, used as the first letter in a call number for book classification in the U.S. Library of Congress System: all letters A-Z, except 5 letters: I, O, W, X and Y.

 

[Math.]  A triangular number (sum of all integers from 1 to 6): 21 = 1+2+3+4+5+6. The pair 15 and 21 is the smallest pair of triangular numbers whose sum (36) and difference (6) are also triangular.

 

[Math.]  Reversible numbers and their squares: 122 = 144 and 212 = 441. Also: 1022 = 10404, 1202 = 14400, 2012 = 40401 and 2102 = 44100.

 

[Math.]  The 8th Fibonacci number. There are only 4 triangular numbers that are also Fibonacci numbers: 1, 3, 21 and 55.

 

[Math.]  510,510 is the product of first 7 prime numbers, of 2 consecutive numbers and of 4 consecutive Fibonacci numbers: 510,510 = 2×3×5×7×11×13×17 = 714×715 = 13×21×34×55.

 

[Math.]  The number of 2-digit prime numbers (from 11 to 97).

 

[Math.]

12×21 = 252 = 21×12             

12×42 = 504 = 21×24          24×21 =    504 = 42×12

12×63 = 756 = 21×36          24×63 = 1,512 = 42×36    

12×84 = 1,008 = 21×48       24×42 = 1,008 = 42×24

36×84 = 3,024 = 63×48       24×84 = 2,016 = 42×48.

 

[Math.]  The 2nd square number that is the sum of squares of 2 consecutive numbers: 841 = 292 = 202+212. Each root number in the series is 6 times the previous root number minus the earlier root number: 1, 52, 292, 1692, 9852

Or in other words:  (20, 21, 29) is the 2nd Pythagorean triple (lengths of 3 sides of a right triangle), where 2 sides are consecutive integers.

 

[Math.]  2520 = 120×021 = 210×012.

 

[Math.]  The number of quadratic fields of the form Q(d1/2) that is Euclidean: d = –11, –7, –3, –2, –1, 2, 3, 5, 6, 7, 11, 13, 17, 19, 21, 29, 33, 37, 41, 57 and 73.

 

[Measure]  In the pre-decimalization British money system, a guinea is equal to 21 shillings (1 pound and 1 shilling), i.e., 252 pennies (or pence).

 

[Movie]  Names of TV shows/series or movies: “21 Grams”, “Twenty-One” and "21".


[Science]  Prefix: Zetta- = 1021.

 

 [Sciene]

American system

British system

Number

*****

undecillion

1066

undecillion

sextillion

1036

sextillion

*****

1021

  

[Science]  The number of names of tropical cyclones in the Atlantic basin in the list for a year. There are six such lists used in rotation. The names are in alphabetical order, excluding the five letters: Q, U, X, Y and Z. If there are more than 21, the addition names will be from Greek alphabet: Alpha, Beta, Gamma…

 

[Sport]  The distance (feet) from the service line to the net in tennis.

 

[Sport]  The score (or points) to win a ping-pong (table tennis) game or volleyball game, while the opposite player wins only 19 or less points.

 

[Tech.]  The airspace over the United States is divided into 21 Air Route Traffic Control Centers.

 

[Money]  The diameter (mm) of a U.S. nickel has been 21.2mm, since 1883. Earlier it was 20.5mm.

 

[Sport]  The diameter (inches) of a bowling ball: 8.5 inches, (or 21.6 cm)

 

21 & 22 and 23 & 24

[History]  The 22nd and 24th U.S. President: Stephen Grover Cleveland (Democrat, New Jersey, 1885-1889 and 1893-1897).

His Vice President of the 22nd Presidency, Thomas Andrews Hendricks (Indiana, 1885), was the 21st U.S. Vice President.  There was no Vice President in the period 1885-1889.  His Vice President of the 24th Presidency, Adlai Ewing Stevenson (Illinois, 1893-1897), was the 23rd U.S. Vice President.  

The 21st U.S. President: Chester Alan Arthur (Republican, Vermont, 1881-1885). There was no Vice President.