# 13 number

"Thirteen" redirects here. For the movie of the same name, see Thirteen (film), for the album by Blur, see 13 (album)

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

Template:Numbers 10s
Cardinal 13
thirteen
Ordinal 13th
thirteenth
Numeral system tredecimal
Factorization prime
Divisors 1, 13
Roman numeral XIII
Binary 1101

## In mathematics

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

13 is the second star number and the seventh Fibonacci number. As it is an odd-indexed Fibonacci number, it is a Markov number, appearing in solutions to the Markov Diophantine equation: (1, 5, 13), (1, 13, 34), (5, 13, 194), ...

There are 13 Archimedean solids.

13 goes into 999,999 exactly 76,923 times, so vulgar 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.

13 is the only positive integer that is the fourth root of the sum of the squares of two successive positive integers (119 and 120).

## In science

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 brightest and best-known of the globular clusters.
The New General Catalogue object NGC 13, a spiral galaxy in the constellation Andromeda
The Saros number 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 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.
The known universe is currently thought to be about 13.7 billion years old, with an error of about one percent.
There are 13 zodiac constellations, which consist of the 12 signs in the astrological zodiac and Ophiuchus.

## 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), or in the Spanish-speaking world, a Tuesday. 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). Months with a Friday the 13th always begin on a Sunday.

A small minority of left handers (see "left handed") consider 13 to be their "lucky" number. This is possibly derived from unlucky being an antonym for lucky (7 is a popular lucky number in Western culture). Many left handers are taught to do the opposite when learning new skills from a right handed instructor. Thus they consider 13 to be their "lucky" number as it is the opposite of 13 being the "unlucky" number.

The number is also considered unlucky by some as it is the sum of 1-4-5-3, the year of the fall of Constantinople to the Turks, marking the end of the Byzantine Empire.

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.

In the Jewish tradition, 13 signifies the age of maturity, a 13 year old Jew is said to be Bar mitzvah. 13 appears in other places in the Hebrew Bible though not as often as other famous biblical numbers such as 7 and 12. In fact, one could say that for Jews, 13 is more of a lucky number than an unlucky one.

In the Sikh religion, the number 13 is considered a special number since 13 is tera in punjabi which also means "yours" (as in, "I am yours, O Lord"). When Guru Nanak Dev Ji had a job of counting stocks of items, he counted from 1 to 13 (in Punjabi) as normal but after 13 he would just repeat "tera" for all the items since all the items were in fact God's since God created them. When somebody heard this while passing and Guru Nanak Dev Ji was confronted about this his records were checked, it was a miracle since they were all perfect, yet he never seemed to count the items properly.

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.

Amongst some Chinese people, it is regarded as a lucky number because it sounds similar to "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 The Hitchhiker's Guide to the Galaxy, "What is six times nine" is posed as the question to the answer to life, the universe and everything (42). This is true in base 13, although this was merely coincidence according to author Douglas Adams.

Thirteen is:

## In other fields

• The original number of members of The Thirteen Club.
• The number of loaves in a "baker's dozen".
• The number of Popes named Innocent, for which a honor society containing 13 members at the University of Nebraska is named ("The Society of Innocents")
• 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 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.
• A U.S. 1 Dollar note has:
• 13 levels of the truncated pyramid,
• 13 stripes on the flag,
• 13 letters in "E Pluribus Unum" *edit* E Pluribus Unum does not appear on the dollar bill. it only appears on coins. the words "Annuit Coeptis" do appear on the dollar bill, and there are 13 letters in that phrase,
• 13 stars above the Eagle,
• 13 leaves on the olive branch,
• 13 olives on the olive branch,
• 13 arrows held by the Eagle, and
• 13 bars on the shield.
• A U.S. 5 Dollar note has:
• 26 states along the top of the Lincoln Memorial. 26 divided by 2 is 13
• 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.
• The number on a Hell's Angel's patch or tattoo that refers to the thirteenth letter of the alphabet, M, for "marijuana".
• 13, or "X3", is the number of the 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.
• Israel Naval covert unit - Shayetet 13
• Former professional wrestler Tazz used the number 13 in his ring gear, merchandise, and Titantron video, signifying the 13 years he wrestled in the independent circuit and ECW before being signed by WWE.
• A card game very similar to Tien len.