# 28 (number)

28 (twenty-eight) is the natural number following 27 and preceding 29.

 << 20 21 22 23 24 25 26 27 28 29 >> Cardinal twenty-eight Ordinal twenty-eighth Factorization [itex]2^2 \cdot 7[itex] 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

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:

da:28 (tal)

• Art and Cultures
• Countries of the World (http://www.academickids.com/encyclopedia/index.php/Countries)
• Space and Astronomy