by the twenty-first century BC had developed a slightly different system founded on a calendar year of 360 days. This came from rounding off the lunar month to 30 days, which fitted neatly into the Sumerians’ mathematic and astronomic system. This system is based on the numbers 6 and 60, which equal 360 when multiplied--the number we still use to divide the sky and every circular plane. No one knows why the Sumerians and later the Babylonians chose these numbers, though four thousand years later they remain the numeric basis for everything from determining one’s position at sea to the location in the sky of a distant galaxy vis-a-vis the earth.
The Babylonians inherited and refined the older Sumerian numerology to divide the day up into 24 hours, which is divisible by six and also divides evenly into 360. Again, the reason for using 24 has been obscured by time, though it’s likely that it had something to do with the zodiac, which the astrology-crazed Babylonians used with great fervour to guide their lives. Possibly they divided first the day and then the night into 12 hours each to correspond to the signs of the zodiac, and then added them together in order to reach the 24-hour day we still follow.
In the fifth century BC the Greek historian Herodotus told a story that points up the complications with these less-than-perfect luni-solar calendars. In The Histories Herodotus tells how the Greek law-giver Solon once answered a question put to him by the rich and haughty Croesus of Sardis: Who was the happiest man he had ever seen? In answering, Solon refused to name Croesus, explaining that fate could still render him unhappy. He used the Greek calendar to emphasize his point. ‘Take seventy years as the span of a man’s life,’ says Solon. ‘Those seventy years contain 25,200 days, without counting intercalary months. Add a month every other year, to make the seasons come round with proper regularity, and you will have thirty-five additional months, which will make 1,050 additional days. Thus the total of days for your seventy years is 26,250, and not a single one of them is like the next in what it brings. You can see from that, Croesus, what a chancy thing life is. You are very rich, and you rule a numerous people; but the question you asked me I will not answer, until I know that you have died happily.’
Egypt was the first ancient civilization to correct the error of the moon and embrace the sun. Remarkably, they did it quite early--almost six thousand years ago, when people living along the Nile figured out the solar year was very close to 365 days. This led to a calendar with 12 months of 30 days each and an additional 5 days that Egyptian mythology says were added to the year by the god Thoth. These became the birthdays of Osiris, Isis, Horus, Nephthys and Set.
How these Neolithic Egyptians figured out so close an approximation to the true year remains a mystery. Egyptian science was advanced very early, but they were never renowned for their astronomy, like the Babylonians, or for a keen interest in mathematics, like the Greeks.
The most plausible explanation is the Nile. Herodotus called Egypt ‘the gift of the Nile’, and anyone who has visited understands instantly the division between the green along the river and the brown of the desert, between life and death. The Nile was responsible for the crops, the commerce, and the continuity of Egypt. The ancient Egyptians called it simply ‘the sea’. Flooding from late June till late October, each year the Nile brought down rich silt for crops to be grown from October to February, and harvested from February until the end of June. These were the three seasons of life in Egypt: flooding, growth and harvest. The regularity of this cycle and the availability of the great river as a natural timepiece provided an easy and dramatic alternative to the moon.
North-east Africa was not always dependent on the Nile. Until the final retreat of the glaciers