A Matter of Time... Part the Second
The ancient Babylonians started with the lunar calendar and stayed with it. Their stubborness in usung moon cycles for their calendar making had consequences. While searching for a way to measure the cycle of the seasons in multiples of moon cycles, they discovered the so-called Metonic cycle of nineteen years. They found that if they used a nineteen-year cycle, assigned to seven of the years thirteen months, and assigned to the other twelve years only twelve months, they could continue to use the conveniently visible phases of the moon as the basis of their calendar. This intercalculation (insertion of extra months) avoided the inconveniences of a 'wandering' year, one in which the seasons gradually wandered through the lunar months to the point of being hopelessly out of synchronization with the seasons. Obviously, this Metonic calendar was too complicated for everyday use.
The Egyptians managed to escape the temptation to measure the passage of months by the moon. So far as we know, they were the first to discover the length of the solar year and to define it in a useful, practical fashion. The trouble is that while we know the what, we remain unenlightened about the why, the how, and even the when. Why was it the Egyptians? They had no astronomical instruments not already well known to the ancient world. They showed no special genius for mathematics. Their astronomy remained crude when compared to that of the Greeks and others in the Mediterranean and was dominated by religious ritual. But it seems that by about 2500 BC they had knowledge of a method to predict when the rising or the setting sun would gild the tip of any particular obelisk, which helped them add a glow to their ceremonies and anniversaries.
Somehow, the Egyptians invented a calendar that served everyday needs throughout their land. As early as 3200 BC, the whole Nile Valley was united with the Nile Delta into a single kingdom which lasted for three thousand years, until the time of Cleopatra. Political unity was reinforced by nature, for like the heavens, the Nile displayed a regular rhythm. It made possible the crops, the commerce, and the architecture of Egypt. This vast river was a freightway for the materials of huge temples and pyramids. A granite obelisk of three thousand tons could be quarried at Aswan and then floated two hundred miles down the river to Thebes. The Nile fed the cities along its banks.
The rhythm of the Nile was the rhythm of Egyptian life. The annual rising of its waters set the calendar of sowing and reaping its three seasons: inundation, growth, and harvest. The flooding of the Nile from theend of June till late October brought down rich silt, in which crops were planted and grew from late October to late February, to be harvested from late February till the end of June. Thus, the primitive Egyptian calendar was the "Nilometer" -- a simple verticle scale on which the flood level was yearly marked. It took only a few years reckoning of the Nile year showed that it did not keep in step with the phases of the moon. But early Egyptians found that twelve months of thirty days each could provide a useful calendar of the seasons if another five days were added to the end, to make a year of 365 days. This was the 'civil' year or the 'Nile year' that the Egyptians began to use as early as 4241 BC.
(More to come...)