13 (Thirteen) is the natural number following 12 and preceding 14.

In mathematics

Thirteen is the 6th smallest prime number, the next is seventeen. 13 is a Wilson prime. 13 is the fifth Mersenne prime exponent, yielding 8191. 13 is also the third lucky prime.

13 is a star number and the seventh Fibonacci number. There are 13 Archimedean solids.

13 goes into 999,999 exactly 76,923 times, so fractions with 13 in the denominator have six digit repeating sequences in their decimal expansions.

At 13, the Mertens function sets a new low of -3, subseded later at 31 with a value of -4.

In science

• The atomic number of aluminium
• The number of dimensions in some theories of relativity

In astronomy,

Messier object M13, a magnitude 7.0 globular cluster in the constellation Hercules, also known as the Hercules cluster. It is one of the the brightest and best-known of the globular clusters.

The New General Catalogue object (http://www.ngcic.org/) NGC 13, a spiral galaxy in the constellation Andromeda

The Saros number (http://sunearth.gsfc.nasa.gov/eclipse/SEsaros/SEsaros1-175.html) of the solar eclipse series which began on -2561 September 12 and ended on -1046 March 9. The duration of Saros series 13 was 1514.5 years, and it contained 85 solar eclipses.

The Saros number (http://sunearth.gsfc.nasa.gov/eclipse/LEsaros/LEsaros1-175.html) of the lunar eclipse series which began on -2313 May 20 and ended on -1015 July 6. The duration of Saros series 13 was 1298.1 years, and it contained 73 lunar eclipses.

The 13th moon of Jupiter is Elara.

As lucky, unlucky, or significant number

Thirteen is regarded as an unlucky number in many cultures. One (probably unverifiable) theory holds that 13 is a reflection of the human fear of the unknown, since it is the first number that cannot be enumerated by using our 10 fingers and 2 feet. Unreasoned fear of the number 13 is termed triskaidekaphobia. Due to this fear, some tall buildings have resorted to skipping the "thirteenth floor", simply by numbering it "14" (though it's really still the thirteenth floor) or by designating the floor "12a" or similar instead. The thirteenth of a month is likewise ominous, particularly when it falls on a Friday (see Friday the 13th). The 13th falls on a Friday more often than any other day. In fact it occurs more often than any other day/number combination except those that coincide with it (Thursday the 12th, Saturday the 14th, etc). On months with a Friday the 13th, the first day of the month begins on a Sunday. The calendar for such a month has a full first line, and thus a more square, completed look.

In the Christian religion, the number is considered significant since there were thirteen present at the Last Supper. Also, if both Judas and Matthias are counted, there were thirteen apostles. Thirteen was also once associated with the Epiphany by Christians, the child Jesus having received the Magi on his thirteenth day of life.

Modern day witches have reclaimed the number 13 as a lucky and significant number. This may be similar to witches reclaiming other previously negative terms and ideas associated with witchcraft, such as black cats, brooms, and even the term witch itself. In modern day Wicca, thirteen is considered the maximum size of a coven, and in some traditions the ideal number of members. In Gardnerian Wicca, the ideal coven has thirteen members.

Amongst some Chinese people, it is regarded as a lucky number because it is synonymous with "實生", which means "must be alive". But this belief is not universal, or even known to all Chinese. Also, 13 is regarded as unlucky by those Chinese under the influence of foreign cultures.

In other fields

Thirteen is:

• The number of loaves in a "baker's dozen".
• The ASCII and Unicode value for carriage return.
• "Unlucky for some", according to bingo callers.
• The number of players in a rugby league team.
• The age of maturity in Jewish tradition. A boy gets his bar mitzvah on the Shabbat after his thirteenth birthday.
• The number of steps on the pyramid on the back of the American $1 Bill
• The number of original colonies the United States was founded from. The original flag had thirteen stars, one for each state. New stars have since been added whenever a new state joins the union, but the idea of adding stripes for new states was soon dropped, so the American flag to this day has thirteen horizontal stripes, six white ones and seven red ones.
• The number of guns in a gun salute to U.S. Army, Air Force and Marine Corps Major Generals, and Navy and Coast Guard Rear Admirals Upper Half.
• The number of Plutonium slugs in Fat Man, the atomic bomb dropped on Nagasaki.
• Thirteen, a 2003 movie by Catherine Hardwicke.
• Thirteen Days, a 2000 movie about the Cuban Missile Crisis
• The Thirteenth Floor, a 1999 movie.
• In the title of the movies:
  • 13 Going on 30, starring Jennifer Garner (Age 13 of the protagonist);
  • Apollo 13, starring Tom Hanks (13th Apollo mission);
  • Thir13en Ghosts, horror movie starring Tony Shalhoub;
  • 13 Conversations About One Thing, starring Matthew McConaughey;
  • Assault on Precinct 13 (1976 movie), action movie starring Austin Stoker;
  • Assault on Precinct 13 (2005 movie), loose remake of the 1976 movie, starring Ethan Hawke and Laurence Fishburne
  • Dementia 13, horror movie directed by Francis Ford Coppola;
  • Ward 13, animated short movie; and
  • Thir13en Ghosts, horror movie starring Charles Herbert.
• Strauss and Howe named Generation X the "Thirteenth Generation".
• The number on a Hell's Angel's patch or Mexican street gang Sureños. It refers to the thirteenth letter of the alphabet, M, for Mexico.
• In rugby union one of the centres, most often but not always the outside centre, wears the 13 shirt.
• Historical years: A.D. 13, 13 B.C., or 1913.
• In the episode of King of the Hill when Connie has her first period and she goes with Hank to the Megalomart to buy tampons, the feminine hygiene items are in Aisle 13. This was changed to Aisle 8A when the episode reran.
• Israel Naval covert unit - Shayetet 13