7 (number) - Definition and Overview

7 (seven) is the natural number following 6 and preceding 8. Prefixes for 7 include hepta- (Greek) and septua- Latin.

The SI prefix for 1000⁷ is zetta (Z), and for its reciprocal zepto (z).

Evolution of the glyph

In the beginning, various Indians wrote 7 more or less in one stroke as a curve that looks like an uppercase J upside down. The western Ghubar Arabs' main contribution was to make the longer line diagonal rather than straight, though they showed some tendencies to making the character more rectilinear. The eastern Arabs developed the character from a 6-look-alike into an uppercase V-look-alike. Both modern Arab forms influenced the European form, a two-stroke character consisting of a horizontal upper line joined at its right to a line going down to the bottom left corner, a line that is slightly curved in some font variants. As is the case with the European glyph, the Cham and Khmer glyph for 7 also evolved to look like their glyph for 1, though in a different way, and so they were also concerned with making their 7 more different. For the Khmer this often involved adding a horizontal line above the glyph. This is analogous to the horizontal stroke through the middle that is sometimes used in handwriting in the Western world but which is almost never used in computer fonts.

Digital7sL.png
Image:Digital7sL.png

On the displays of pocket calculators and digital watches, 7 is the only number to show variation in its glyph. Most calculators use three line segments, but on Sharp and a few other brands of calculators, 7 is written with four line segments.

In mathematics

Seven is the fourth-smallest prime number; the next is 11. Seven is a Mersenne prime, since 2³ - 1 = 7. Seven is also the fourth Mersenne prime exponent, a Newman-Shanks-Williams prime, and a Woodall prime. 7 is also the second lucky prime.

999,999 divided by 7 is exactly 142,857, so fractions with 7 in the denominator have six-digit repeating sequences in their decimal expansions. 1/7 = 0.142857142....

A seven-sided polygon is a heptagon. The regular n-gons for n ≤ 6 can be constructed by ruler and compass alone, but the regular heptagon cannot. Figurate numbers representing heptagons (including seven) are called heptagonal numbers. Seven is also a centered hexagonal number.

There are seven frieze groups, the groups consisting of symmetries of the plane whose group of translations is isomorphic to the group of integers.

In numeral systems

In science

• The atomic number of nitrogen.
• The number of periods, or horizontal rows of elements, on the periodic table.
• The number of carbon atoms in heptane, a hydrocarbon.
• The neutral pH - the pH level of pure water

In astronomy,

Messier object M7, a magnitude 3.5 open cluster in the constellation Scorpius.

The New General Catalogue object (http://www.ngcic.org/) NGC7, a 14th magnitude spiral galaxy in the constellation Sculptor

The Saros number (http://sunearth.gsfc.nasa.gov/eclipse/SEsaros/SEsaros1-175.html) of the solar eclipse series which began on -2590 April 8 and ended on -1310 May 16. The duration of Saros series 7 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 -2595 July 15 and ended on -1008 February 22. The duration of Saros series 7 was 1586.6 years, and it contained 89 lunar eclipses.

In other fields

Seven is the smallest positive integer requiring two syllables in English.

There are seven musicians in a septet and seven babies born in a set of septuplets. The most famous set of septuplets are the McCaughey Septuplets, who were born in 1997.

The British fifty-pence and twenty-pence coins are heptagons, with the sides curved to give them a constant diameter.

Cities constructed amidst seven hills include:

• Rome, see Seven hills of Rome
• Cincinnati
• Pretoria
• Lynchburg, Virginia
• Somerville, Massachusetts
• Bath, England

Seven is also:

• a highly symbolic number in the Hebrew Bible, being, for example, the day on which God rested in Genesis
• the number of ayat in surat al-Fatiha
• the number of heavens in Islamic tradition
• the number of spheres in the Ptolemaic system
• in Astrology, Libra is the 7th astrological sign of the Zodiac.
• Boomer Esiason's jersey number.
• the traditional number of Wonders of the Ancient World.
• viewed as a lucky number in many Western cultures, and in Japanese culture.
• the number of days in a week.
• referred to in bingo as 'Lucky Seven'.
• the number of the Deadly Sins: lust, avarice, envy, pride, sloth, gluttony and anger.
• considered the usual number for a witches' coven.
• the number of palms in an Egyptian Sacred Cubit.
• the number of the ages of man into which William Shakespeare divided a lifetime.
• the figurative number of seas.
• the number of colors of the rainbow.
• the number of basic principles of the bushido.
• the number of heads of the beast of Revelations, and of some other monsters, like the hydra.
• the number of points on a sheriff's star.
• the code for international direct dial phone calls to Russia and Kazakhstan.
• the title of:
  • a 2003 album by Enrique Iglesias, see 7 (Enrique Iglesias album).
  • the film Seven (or Se7en), directed in 1995 by David Fincher and starring Morgan Freeman, Brad Pitt and Kevin Spacey.
• the name of one of the monsters in the cartoon television show Seven Little Monsters.
• the name George Costanza desired to give his first-born (with Susan Ross) due to a promise he had made to the widow of Micky Mantle, whose number was 7.
• the number of digits in an American telephone number, excluding the area code.
• the number of "Zoomers" on the PBS children's show Zoom.
• the number of sacraments in the Roman Catholic faith.
• the number of individual segments in one digit on a digital clock or calculator screen.
• part of the names of:
  • Seven Pillars of Wisdom, a book written by T. E. Lawrence.
  • The Secret Seven, name of children adventure book by Enid Blyton.
  • the days in the movie The Ring</i> until you die. "You will die in 7 days."
  • the 1960 movie with Steve McQueen The Magnificent Seven.
  • the film Seven Years in Tibet.
  • the film The Seven Samurai.
  • the video game Final Fantasy VII
  • Several groupings called the Seven Sisters.
  • Seven Sisters, a 1998 song album by pop singer Meja.
  • the crewmember, Seven of Nine, in the Star Trek universe.
  • the World Sevens Series in Rugby Union, including the Hong Kong Sevens.
  • the sport itself.
  • the Seven Network of Australia.
  • The Seven Habits of Highly Effective People, a book by Stephen Covey.
  • A character added to the cast of Married with Children who was quickly removed.
• In rugby union, the openside flanker.
• In baseball, seven represents the left fielder's position.
• In all Microsoft Office programs, the F7 key calls up the spelling and grammar checker
• Historical years: 7 A.D., 7 B.C., or 1907
• In music, the roman numeral vii is the subtonic (bVII) or leading tone (VII) scale degree, chord, or diatonic function, when distinguished V = major and v = minor.
• In most Sonic the Hedgehog video games, the number of Chaos Emeralds.