28 (number)
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28 (twenty-eight) is the natural number following 27 and preceding 29.
| Cardinal | twenty-eight |
| Ordinal | twenty-eighth |
| Factorization | <math>2^2 \cdot 7<math> |
| Roman numeral | XXVIII |
| Binary | 11100 |
| Hexadecimal | 1C |
In mathematics
It is a composite number, its proper divisors being 1, 2, 4, 7 and 14, making it a perfect number and a harmonic divisor number.
Twenty-eight is the second perfect number. As a perfect number, it is related to the Mersenne prime 7, since 22(23 - 1) = 28. The next perfect number is 496.
Twenty-eight is a triangular number, a hexagonal number and a centered nonagonal number. Twenty-eight is the sum of the first five prime numbers.
It is also a Keith number, because it recurs in a Fibonacci-like sequence started from its base 10 digits: 2, 8, 10, 18, 28...
Twenty-eight is the ninth and last number in early Indian Magic square of order 3.
In science
- The atomic number of nickel
- The fourth Magic number (physics)
In astronomy,
- Messier object M28, a magnitude 8.5 globular cluster in the constellation Sagittarius
- The New General Catalogue object (http://www.ngcic.org/) NGC 28, an elliptical galaxy in the constellation Phoenix
- The Saros number (http://sunearth.gsfc.nasa.gov/eclipse/SEsaros/SEsaros1-175.html) of the solar eclipse series which began on -1910 March 22 and ended on -630 April 28. The duration of Saros series 28 was 1280.1 years, and it contained 72 solar eclipses.
- The Saros number (http://sunearth.gsfc.nasa.gov/eclipse/LEsaros/LEsaros1-175.html) of the lunar eclipse series which began on -1915 June 27 and ended on -581 September 6. The duration of Saros series 28 was 1334.2 years, and it contained 75 lunar eclipses.
In other fields
Twenty-eight is:
- The number of days in the shortest month of the Gregorian calendar, February. (Except in leap years, when there are twenty-nine.)
- The common name for the parrot Barnardius zonarius semitorquatus, widely distributed in Western Australia and South Australia. Its call sounds like "wenniate".
- The number of letters in the Danish and Swedish alphabets (not counting W), amd also in the Arabic and Esperanto alphabets.
- In neo-Nazi circles, twenty-eight indicates Blood and Honour (28 = BH - B - second letter of the alphabet and H - the eight letter).
- Part of the title of a romantic comedy movie 28 Days, and an unrelated science fiction movie 28 Days Later.
- Part of the name of a well known indie rock band from Melbourne, Australia called 28 Days - the name is unrelated to either of the movies above.
- Historical years: 28 A.D., 28 B.C., or 1928
- The number of Chinese constellations, "Xiu" or "mansions" (a literal translation), equivalent to the 12 western zodiac constellations.
- The number of normal human teeth, not including the third molars (wisdom teeth).
- The number of dominoes in standard domino sets.
- The number of Scudetto won by Turin-based football club, Juventus.
da:28 (tal)
es:Veintiocho fr:28 (nombre) ko:28 ia:28 it:Ventotto he:28 (מספר) nl:28 (getal) ja:28 no:28 (tall) pt:Vinte e oito sv:28 (tal) zh:28