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# 13 (number)

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

Cardinal 13
thirteen
Ordinal 13th
thirteenth
Numeral system tredecimal
Factorization prime
Divisors 1, 13
Roman numeral XIII
Binary 1101
 Contents

## 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.

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.

## 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. In cultures that consider Sunday to be the first day of the week, 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.

In the Jewish tradition, 13 signifies the age of maturity, a 13 year old Jew is said to be Bar mitzvah. It is also the number of the tribes (of them 11 are the sons of Jacob and two more are named after the two sons of Joseph) and 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.

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 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 other fields

Thirteen is:

03-10-2013 05:06:04