Evolution of the glyph

In the beginning, various Indians wrote 9 in a way that pretty much looks like our modern closing question mark, but dotless. The Kshtrapa, Andhra and Gupta started curving the bottom vertical line coming up with a 3-look-alike. The Nagari continued the bottom stroke to make a circle to enclose the 3-look-alike, in much the same way that the @ character encircles a lowercase a. (The Nagari glyph might remind some of the Baskin Robbins 31 flavors logo). As time went on, the enclosing circle became bigger and its line continued beyond the circle downwards, and the 3-look-alike became smaller. Soon, all that was left of the 3-look-alike was a squiggle. The Arabs simply connected that squiggle to the downward stroke at the middle, and all the Europeans did to the glyph was purely cosmetic.

In mathematics

In base-10 a number is evenly divisible by nine if and only if the iterative sum of its digits reduces to 9. This is equivalent to saying a number is divisible by 9 if and only if its decimal digit total is divisible by 9. The only other number with this property is three. In base-N, the divisors of N − 1 have this property. Another consequence of 9 being 10 − 1, is that it is also a Kaprekar number. 9 is a Motzkin number.

In probability, the nine is a logarithmic measure of probability of an event, defined as the negative of the base-10 logarithm of the probability of the event's complement. For example, an event that is 99% likely to occur has an unlikelihood of 1% or 0.01, which amounts to −log₁₀ 0.01 = 2 nines of probability. Zero probability gives zero nines (−log₁₀ 1 = 0). The purity of chemicals, the effectivity of processes, the availability of systems etc. can similarly be expressed in nines. For example, "five nines" (99.999%) availability implies a total downtime of no more than five minutes per year. This measure can be confusing, a fact which is discussed in The Myth of the Nines.

Six recurring nines appear in the decimal places 762 through 767 of pi. This is known as the Feynman point (see also MathWorld).

In numeral systems

In science

• Nine is the atomic number of fluorine

Messier object M9, a magnitude 9.0 globular cluster in the constellation Ophiuchus.

The New General Catalogue object NGC 9, a spiral galaxy in the constellation Pegasus

The Saros number of the solar eclipse series which began on -2568 February 6 and ended on -1252 April 4. The duration of Saros series 9 was 1316.2 years, and it contained 74 solar eclipses.

The Saros number of the lunar eclipse series which began on -2501 June 26 and ended on -1149 September 16. The duration of Saros series 9 was 1352.2 years, and it contained 76 lunar eclipses.

There are nine planets in our solar system.

In music

Bands with the number nine in their name include Stroke 9, Nine Days and Nine Inch Nails.

In pop music, the song "Love Potion #9" was written by Jerry Leiber and Mike Stoller (who also wrote "Riot in Cell Block #9"), and became a hit for The Searchers in 1965. The Beatles released a song called "Revolution 9" which appears on The White Album (more properly known as The Beatles); its principle vocal feature is a voice repeating, "Number nine...number nine...number nine..." John Lennon, the primary composer of "Revolution 9", also released a solo recording entitled "#9 Dream", leading his biographer to speculate that he had an obsession with the number.

Both Ludwig van Beethoven and Anton Bruckner wrote nine symphonies. After Beethoven died leaving his Tenth Symphony unfinished, composers were superstitious about writing Ninth Symphonies for the rest of the nineteenth century. Gustav Mahler tried to "cheat death" by writing his Ninth Symphony but calling it Das Lied von der Erde instead. Afterwards he wrote a work called Ninth Symphony, but thinking of it as his Tenth. He died with his Tenth (or Eleventh in his mind) unfinished. Perhaps Antonín Dvořák was also superstitious about the number nine, because he wrote no symphonies after his New World Symphony, which is nowadays considered his Ninth, but which he thought was his Eighth because he considered the score of his early C minor Symphony lost forever. He lived for seven more years. In the twentieth century, a handful of composers, such as Dmitri Shostakovich, have written a Ninth Symphony and lived to write more.

In other fields

• Nine is the highest single-digit number in the decimal system. In the NATO phonetic alphabet, the digit 9 is called "niner".
• Nine is the number of musicians in a nonet.
• Nine babies born into a single birth are called nonuplets, although not one baby born into a set of nonuplets has ever survived infancy.
• A polygon with nine sides is called an enneagon (technically) or nonagon (in common usage). A group of nine of anything is called an ennead.
• A novena lasts for nine days.
• Nine judges sit on the United States Supreme Court.
• A batting lineup in baseball consists of nine players.
• In Greek mythology there are nine Muses.
• In Egyptian mythology, there are nine major gods and goddesses (see also Ennead).
• There are nine basic personality types represented on the enneagram.
• In Astrology, Sagittarius is the 9th astrological sign of the Zodiac.
• In chess, the maximum number of queens one side could possibly have after queening all pawns.
• Joel Garreau identified nine different "nations" of North America.
• Nine months is the gestation period for humans.
• A standard work day of 9 to 5 begins at 9 a.m.
• Historical years: 9 A.D., 9 B.C., or 1909
• Stanines are measured on a scale of 1 to 9.
• The name of the area called Kowloon in Hong Kong literally means: nine dragons.
• Someone dressed "to the nines" is dressed up as much as they can be.
• Nine is considered an unlucky number in Japanese culture.
• Nine (九 pinyin jiǔ) is considered a good number in Chinese culture because it sounds the same as the word "longlasting" (久 pinyin jiǔ).
• Nine Lives Cat Food got its name from the legend that a cat has nine lives.

• Nine rank system
• The Nine Chapters on the Mathematical Art
• Nine Worthies
• Nine ball is the standard professional billiards variant played in the United States
• ISO 9 is the ISO's standard for the transliteration of Cyrillic characters into Latin characters
• The album Nine by Fairport Convention
• Nine is a musical, based on the film 81/2.
• In rugby union the scrum-half wears the 9 shirt.
• In baseball, nine represents the right fielder's position.
• The number of minutes some alarm clocks give you after you press the snooze button.
• The number of Valkyries in Richard Wagner's Die Walküre.
• 9 was a 1995 computer game.

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