126 5. COUNTINGrequired for the Moon to go halfway from full to new or vice versa. There is
another, more plausible, astronomical connection, however, since there are exactly
seven heavenly bodies visible to the unaided eye that move around among the fixed
stars: the Sun, the Moon, and the five planets Mercury, Venus, Mars, Jupiter, and
Saturn. These planets were also gods to many ancient peoples and gave their names
to the weekdays in some of the Romance languages, such as French and Italian. In
English the Norse gods serve the same purpose, with some identifications, such as
Tiw (Tiu) with Mars, Odin (Wotan) with Mercury, Thor with Jupiter, and Frigga
with Venus. Saturn was not translated; perhaps the Norse simply didn't have a
god with his lugubrious reputation. This dual origin of the week, from Jewish law
and from astrology, seems to have spread very widely throughout the world. It
certainly reached India by the fifth century, and apparently even went as far as
Japan. References to a seven-day week have been found in Japanese literature of
1000 years ago. When referring to the Gregorian calendar, the Japanese also give
the days of the week the names of the planets, in exactly the same order as in
the French and Italian calendars, that is, Moon, Mars, Mercury, Jupiter, Venus,
Saturn, and Sun.
Colson (1926) gives a thorough discussion of what he calls the "planetary week"
and explains the more plausible of two analyses for the particular order derived
from a history of Rome written by Dion Cassius in the early third century. The
natural ordering of the nonfixed heavenly bodies, from a geocentric point of view,
is determined by the rapidity with which they move among the stars. The Moon is
by far the fastest, moving about 13° per day, whereas the Sun moves only 1° per
day. Mercury and Venus sometimes loop around the Sun, and hence move faster
than the Sun. When these bodies are arranged from slowest to fastest in their
movement across the sky, as seen from the earth, the order is: Saturn, Jupiter,
Mars, Sun, Venus, Mercury, Moon. Taking every third one in cyclic order, starting
with the Sun, we get the arrangement Sun, Moon, Mars, Mercury, Jupiter, Venus,
Saturn, which is the cyclic order of the days of the week. Dion Cassius explains
this order as follows: The planets take turns keeping one-hour tours of guard duty,
so to speak. In any such cyclic arrangement, the planet that was on duty during
the fourth hour of each day will be on duty during the first hour of the next day.
The days are named after the planet that is on watch at sunrise. Since there are
24 hours in the day and seven planets, you can see that number of the first hour
of successive days will be 1, 25, 49, 73, 97, 121, and 145. Up to multiples of seven,
these numbers are equal to 1, 4, 7, 3, 6, 2, 5 respectively, and that is the cyclic
order of our weekdays.
In his Aryabhatiya the Hindu writer Arybhata I (476-550) uses the planetary
names for the days of the week and explains the correlation in a manner consistent
with the hypothesis of Dion Cassius. He writes:
[C]ounting successively the fourth in the order of their swiftness they
become the Lords of the days from sunrise. [Clark, 1930, p. 56]
The hypothesis of Dion Cassius is plausible and has been widely accepted for
centuries. More than 600 years ago the poet Geoffrey Chaucer wrote a treatise on
the astrolabe (Chaucer, 1391) in which he said
The firste houre inequal of every Saturday is to Saturne, and the
seconde to Jupiter, the thirde to Mars, the fourthe to the sonne,