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Last Updated: Sunday, February 18, 2001

If you read why a week has 7 days, you now know that most ancient societies after a while named days after gods they could see in the sky, which were the sun, moon, and the five planets they could see with their eyes. Planets were moving unlike the stars which confirmed they were special. These 7 special objects were even used to denote the hours in a day, so if there was not 7 days in a week then each god would not start at the beginning of at least one day. That ought to upset that god.
You noticed that some societies did have ten days (Egyptians) which fit very well into their 30 day months. But they changed over time feeling the 7 planets must be important to mirror in time. The Romans had 8 days in a week until the Emperor Constantine changed it to conform to the usage made in all the provinces of the empire, and as a political good will gesture towards his North African legions whom he needed for another campaign in east Gaul near the Rhine.
So we are left with 7 days not dividing into the 365 days that occurs in most years. The Egyptians knew this and with their 12 months of 30 days had 5 days left over. They explained this with the story of Thoth's gambling with the moon for the extra five days so that Nut could give birth and that has been described elsewhere

The whole problem arises because the time for the earth to rotate in one day does not divide without remainder into the time it takes the earth to revolve about the sun. It is nearly 365 days but not quite. As you can tell 5 day weeks would be great, dividing exactly 73 times into 365, so we could have a 73 weeks a year every year with an extra day to goof off in every 4 (except for the start year, the zero year, for 3 consecutive centuries and then will come a leap century, in the Gregorian calendar anyway). Even 7 does not do badly as it give 52 weeks each year with one day left over (2 in leap years). However no one has successfully suggested making that extra day into a goof off day.

But to complicate it further, people who lived on the land would see the moon and would see that it regularly cycled in 28 (a little over 29 in fact) days (full moon to full moon for example). Now actually 28 days is not bad as 7 will divide into with no remainder. But 28 will divide into 365 and leave a big remainder. Actually it leaves 7 days remaining. That extra week could be used as a goof off time but no one has succeeded at doing that either. Instead you get 13 moon cycles, and one week (of seven days) in a solar year. The extra week is tagged on to the 13th month and this still doesn't solve the problem of the extra day every four years. Some cultures tried to have some months with 30 days and some with 29. That obviously doesn't make it simplier to do the math with.

One of the attractions to using the moon to mark the passage of time is the need to have some way of knowing when it is time to plant, and harvest crops, and moon cycles are very excellent way of knowing how of determining the part of the plant cycle that is about to occur.Since planting and harvesting grain if not done meant people would starve, a lot of attention was paid to these indicators in the sky of what was the correct time to do these vital activities.

How many seasons? Well the Celts had two, a growing season, and a not growing season. Further south the Greeks had three seasons, a wet season (Spring) rejuvenating from the dead season (Winter), and a dry season (Summer). Again these cycles were the cycles they lived in. Why do we have four? A guess would be that if you are tied to a northern climate and are growing food, you need to know when to plant (Spring), when to harrow and pray for occassional rain (Summer), a time to harvest and preserve your crops (Autumn) and then there is the dead season (Winter).

How about the hours in a day? Well 24 is not a selection most people would make, I think. Two is the easiest to understand, night and day. But days are long, and so are nights. Equally long if you get near the equator, all year round. But the Norse used it until Christianized.
Three could seem reasonable, as there is dark, then light growing and light diminishing. The Yorubians had this division.
But if you include the moon then there is two parts of the dark, before the moon rises, and after it rises, so you would have four parts of the day easily marked (although a new moon is not much light). The Mongols thought this very reasonable.
If you tied it in with meals, and rising and going to bed you would have 5 with rise time, first eat, (work), second eat (work), third eat (work), then sleep, and then sleep to rise is the fifth time.
The Christians got eight divisions by having 7 prayer times (Nocturne, Martin Lauds, Prime, Third, Sixth and Ninth, then Vespers.
The Sumerians broke the day into 10 divisions, perhaps for coffee breaks and while taking account also of the rising and falling moon.
The Phoenicians had eight, which became the basis of our bells system used aboard navy vessels in the West at one time. Basically there are 3 hours between bells, which is the time a person stayed alert watching for hazards at sea.
And then there are 12 hours, the Babylonians took the Sumerian system and added dusk and dawn to give 12. Now twelve is convenient because you can fold your fingers and thumb in 6 ways on one hand, that is twelve for the two hands together. The positions are open, one finger folded, two fingers folded, three, four, and then all fingers and the thumb. So the day was divided into 12 and the night was divided into 12 and you got the 24. But the reason for 12 exactly may have the same as the Egyptians, tied into how many stars rose during the day or night. See the Egyptian reference below.
The Egyptians likely had twelve months for the same reason the Chinese had 12 months, their zodiac. The Chinese had 12 hours a day which corresponded to their zodiac of 12 animals. So 12 and 24 became a standard for many cultures. And it all happened over the time of about 4000 years.

There is a clear short explanation of Calendars here
And this site gives an explanation for 24 hours, explaining it was the Egyptians who created it, and the 365 days in the year.

And then there is the millenium count in the Christian calendar. A monk in the third century A.D. made a big mistake. Now monks often take vows of chastity, so they don't help raise children, so they might not know that the first year of a child's life is not year one, it is the zero year. Now beside his vow the monk had another reason for blowing it and that is he was not using our numbering system but the clumsy Roman shorthand for number words, which did not have a zero. If you actually want to learn something of Roman arithmetic go to Dr Math's Roman Arithmetic
However just because some monk made a mistake 1700 years ago does not mean we have to keep making the same mistake.




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