Earth and the Moon

Clocks and Calendars

Who established the length of the calendar solar day?

The ancient Egyptians originally based the length of the solar day on nightly observations of a series of thirty-six stars (called decan stars), that rose and set in the sky at forty-to sixty-minute intervals. For ten days, one particular star would be the first decan to appear in the sky, rising a little later each night until a different decan star would be the first to rise. Thus, the first “hours” were marked nightly by the appearance of each new decan in the sky. Depending on the season, between twelve and eighteen decans were visible throughout the course of a night. Eventually, the official designation of the hours came at midsummer, when twelve decans (including Sirius, the Dog Star) were visible. This event coincided with the annual flooding of the Nile River—a crucial event in the ancient Egyptian civilization. Thus, the night was eventually divided into twelve equal parts. The twelve daylight hours were marked by a sundial-like device—a notched, flat stick attached to a crossbar. The crossbar would cast a shadow on successive notches as the day progressed. Eventually, the combination of the twelve hours of the day and twelve hours of the night resulted in the twenty-four-hour day we use today.


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