The months of the Egyptian civil calendar were further divided into three sections called "decades", each of ten days. The Egyptians noted that the helical rising of certain stars, such as Sirius and Orion, matched the first day of the 36 successive decades and called these stars decans. During any one night, a sequence of twelve decans would be seen to rise and were used to count the hours. (This division of the night sky, later adjusted to account for the epagomenal days, had close parallels to the Babylonian zodiac. The signs of the zodiac each accounting for 3 of the decans. This astrological device was exported to India and then to Medieval Europe via Islam.)

Early man divided the day into temporal hours whose length depended upon the time of year. A summer hour, with the longer period of daylight, would be longer than that of a winter day. It was the Egyptians who first divided the day (and night) into 24 temporal hours.

The Egyptians measured time during the day using shadow clocks, precursors to the more recognizable sun dials seen today. Records suggest that early shadow clocks were based on the shadow from a bar crossing four marks, representing hourly periods starting two hours into the day. A midday, when the sun was at its highest the shadow clock would be reversed and hours counted down to dusk. An improved version using a rod (or gnomon) and which indicates the time according to the length and position of the shadow has survived from the second millennia BCE.

Problems with observing the sun and stars may have been the reason the Egyptians invented the water clock, or "clepsydra" (meaning water thief in Greek). The earliest remaining example survives from the Temple of Karnak is dated to the fifteenth century BCE. Water drips through a small hole in one container to a lower one. Marks on either container can be used to give a record of hours passed. Some Egyptian clepsydras have several sets of marks to be used at different times of the year, to maintain consistency with the seasonal temporal hours. The design of the clepsydra was later adapted and improved by the Greeks.

As a result of the campaigns of Alexander the Great, a great wealth of knowledge of astronomy was exported from Babylon into India, Persia, the Mediterranean and Egypt. The great city of Alexander with its impressive Library, both founded by the Greek-Macedonian family of Ptolemy, served as an academic center.

Temporal hours were of little use to astronomers, and around 127 CE Hipparchus of Niceae, working in the great city of Alexandria, proposed dividing the day into 24 equinoctial hours. These equinoctial hours, so called because they are based on the equal length of day and night at the equinox, split the day into equal periods. (Despite his conceptual advance, ordinary people continued to use temporal hours for well over a thousand years: the conversion to equinoctial hours in Europe was made when mechanical, weight driven clocks were developed in the fourteenth century.)

The division of time was further refined by another Alexandrian based philosopher, Claudius Ptolemeus, who divided the equinoctial hour into 60 minutes, inspired by the scale of measurement used in ancient Babylon.

Claudius Ptolemeus also compiled a great catalogue of over one thousand stars, in 48 constellations and recorded his concept that the universe revolved around the Earth. Following the collapse of the Roman Empire it was translated into Arabic (in 827 CE) and later into Latin (in the twelfth century CE). These star tables provided the astronomical data used by Gregory XIII for his reform of the Julian calendar in 1582.

Sources:

Mapping Time: The Calendar and its History by E. G. Richards, Pub. by Oxford University Press, 1998, ISBN 0-19-286205-7, 438 pages.

General History of Africa II: Ancient Civilizations of Africa, Pub. by James Curry Ltd., University of California Press, and the United Nations Educational, Scientific and Cultural Organization (UNESCO), 1990, ISBN 0-520-06697-9, 418 pages.

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