ey to predicting the great annual event, the inundation of the Nile which coincided with the pre-dawn appearance on the eastern horizon of Sothis (Sirius), the brightest star in the sky. To the ancient Egyptians the event was so important that they dubbed the rising of Sirius the 'Opener of the Year', and the calendar was arranged around this event.
             Many ancient calendars had twelve months based on the average period of around thirty days between new moons, making a year that fell short of the solar year and had to be extended: the original Egyptian calendar had twelve months and, being linked to the Moon's thirty-day cycle, produced a three hundred and sixty day year. Later it had additional five days added at the end of the year to keep the lunar month and the solar-based seasons co-ordinated and thus the calendar in step with the 'Opener of the Year'.
             We thank the Egyptians for the twenty-four hour day, running from sunrise to sunrise. A twelve-hour cycle fitted the movement of the stars across the sky, with an hour marking out the time between each star, or group of stars, rising on the eastern horizon. In this way, the twelve hour night evolved and, probably for the sake of symmetry, a twelve hour day. The Egyptians measured the passing da
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