The seven ages of man, the seven seas,
the seven deadly sins (p 1592), the seven league boot, seventh heaven, the seven wonders of
the world, the seven pillars of wisdom, Snow White and the seven dwarves, 7-up.... There are
an awful lot of sevens around. Numerologists see seven as the sum of the spiritual three and
the material four, signifying creation. The Old Testament tells us that the world was
created in seven days. The seventh generation after Adam produced Lamech, who lived for 777
years and was to be avenged 77 fold. Psalm 90 sets the human life span at seven decades.
Solomon's temple has seven steps, and Noah's dove returns after seven days.
In the
Hippocratic tradition the number seven governs the ailments of the body. In Germany it was
believed that people would not catch swine fever if they spent seven days drinking and
bathing in water containing asphodel. A Jewish cure for fever involved taking "seven
prickles from seven palmtrees, seven chips from seven beams, seven nails from seven doors,"
and so on up to "seven hairs from the beard of an old dog."(10) And in this issue of
the BMJ a long but neglected tradition alleging seven year cycles in human
development is vindicated in Jos Verhulst and Patrick Onghena's finding of a circaseptennial
(roughly seven year) rhythm in the growth of male human ears (p 1597).(2)
In
Iranian folklore the cat has seven, not nine, lives and carries her kittens seven times to
seven different places.(1)"To do the work of seven mullahs" means to accomplish
absolutely nothing. In ancient India the most important numbers were three and seven. The
ancient Indian text Rigveda talks of seven stars and seven streams of soma (the drink
of the gods).(1) Indra, the rain god, is "the seven-slayer," the ocean has seven
depths, and there are seven concentric continents. The sacred cow has 21 names-three times
seven.
In ancient Greece, seven was associated with Athena, the virgin goddess, because
it is neither producing nor produced-that is, it has no multiple in the first decade (1-10)
and also no divisor (other than 1). The first property has little mathematical importance,
but the second tells us that seven is a prime number-having no divisors other than itself
and one. It is the second Mersenne prime-one less than a power of two.(3)
Seven
numbers suffice to colour every map on a torus-a surface shaped like a doughnut with a
central hole-so that no two adjacent regions have the same colour. The smallest regular
polygon that cannot be constructed using ruler and compasses is the heptagon, with seven
sides. All sufficiently large whole numbers are the sum of seven perfect cubes. And it you
want to get more esoteric, the unit sphere in a multidimensional space has the greatest
surface area when the space has dimension seven.
Seven is an astronomical number. The
week has seven days because successive phases of the moon occur roughly seven days apart.
Other astronomical sevens include the "seven sisters," the stars of the constellation of
the Pleiades, and-in ancient times only-the seven planets: Sun, Moon, Mercury, Venus,
Mars, Jupiter, Saturn. In Douglas Adams's The Hitch Hiker's Guide to the Galaxy the
answer to the "Great Question of Life, the Universe, and Everything," is 42 (six times
seven).(4) In November 1996 the Independent reported the latest measurements of
Hubble's constant-a fundamental physical quantity that determines the rate of growth of our
expanding universe.(5) "Dr Richard Saunders," the newspaper wrote, "sounded a
trifle abashed by the result. 'We have taken two measurements for the constant, and the
average of them is, well, it's 42.""
Ian Stewart
Professor
Mathematics Institute,
University of Warwick,
Coventry CV4 7AL
References:
1 Schimmel A. The mystery of numbers. Oxford:Oxford University Press,1993.
2 Verhulst J, Onghena P. Circaseptennial rhythm in ear growth. BMJ
1993;313:1597-8.
3 Wells D. The Penguin dictionary of curious and
interesting numbers. Harmondsworth, Penguin,1986.
4 Adams D. The hitch hiker's
guide to the galaxy. London: Pan Books, 1979.
5 Arthur C. Yes, the answer to the
Universe really is 42. The Independent, 1996.Nov 8:1.
