The Days and Length of the Week

The week in some cultures did not always have seven named days in it. In Java, there was a five-day week, in mainland Asia, a nine-day week. A 3rd-century Hindu book shows both seven- and nine-day weeks. Parker (1974) writes of an early Egyptian 10-day week. In the Roman republic, there was an eight-day week (nine by their inclusive counting technique), the last day of which was a market day. See ยง15.4.4 for further discussion of the spread of the seven and nine-day weeks.

Although the days of the planetary week, in use in much of the world, are not older than the 1st century b.c., they have their roots in the seven planets of antiquity. By the time that Christianity became the state religion of the Roman Empire, the planetary week was so firmly entrenched that it defied all attempts to change it. The scheme is based on geocentric cosmology that places the spheres of the planets in the following order from Earth: Moon, Mercury, Venus, Sun, Mars, Jupiter, and, in the "seventh heaven," Saturn. Each hour was associated with and held to be ruled by a planetary god. Proceeding in descending order through the heavens, in endlessly repeated cycles, Saturn would rule the first hour, Jupiter the second, Saturn the eighth hour, and so on. A day belonged to the god that ruled the first hour of that day. The ruler of the 25th hour became the ruler of the first hour of the next day. Thus, beginning with Saturn, we have first Saturday, next Sunday, and so on. Figure 4.11 illustrates the progression.

The planetary seven-day week became widespread. It was in China by the 3rd century a.d., in Ireland with Christian-

13 This is because the Sun moves on average 1/365 of its annual motion each day, so this is the rate by which any mean solar time interval differs from its sidereal counterpart. More precisely, in a full tropical year, the number of sidereal revolutions of the Earth is 366.25639 (i.e., 365.25636/0.9972696). The quantity 0.9972696 is the ratio of an interval of mean solar time to the same interval in analogous units (seconds, for example) of sidereal time.

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