**25** (*twenty-five*) is the natural number following 24 and preceding 26. ## In mathematics
It is a square number, being 5^{2} = 5 × 5, and it is also the sum of two squares, 3^{2} + 4^{2}, often used in demonstrations of the Pythagorean theorem. It is the smallest square that's also a sum of two squares. 25 is a centered octagonal number and an automorphic number. It is also a Cullen number. 25 is the smallest pseudoprime satisfying the congruence 7^{n} = 7 mod n. It is also a Somer pseudoprime based on 6. Within base 10 one can readily test for divisibility by 25 by seeing if the last two digits of the number match 25, 50, 75 or 00.
## In science **In astronomy**, - Messier object M25, a magnitude 4.9 open cluster in the constellation Sagittarius
- The New General Catalogue object (
*http://www.ngcic.org/*) NGC 25, a magnitude 11.9 barred spiral 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 -2051 April 19 and ended on -771 May 26. The duration of Saros series 25 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 -2038 August 6 and ended on -469 March 5. The duration of Saros series 25 was 1568.6 years, and it contained 88 lunar eclipses. ## In other fields **Twenty-five** is: |