The Egyptians had ten hours of daylight from sunrise to sunset (exemplified by a sundial described in
1300 B.C.E.), two hours of twilight and twelve hours of night.
The calendar year was divided into 36 decans, each ten days long, plus five extra days, for a 365-day
year. Each decan corresponded to a third of a zodiacal sign and was represented by a decanal constellation.
In the summer sky the night corresponded to about twelve decans, although half a day would correspond
to eighteen decans. This led to the division of the night into twelve hours.
The first hours were seasonal in that their length varied with the season. (Note that this system
was also used in oriental clocks.) Later, Hellenistic astronomers introduced equinoctal hours of
The Minute and Second
The Babylonians (about 300-100 B.C.E.) did their astronomical calculations in the sexagesimal (base-60)
system. This was extremely convenient for simplifying division, since 60 is divisible by 2, 3, 4, 5, 6,
and 10. The first fractional sexagesimal place we now call a minute, the second place, a second.
For details of this interesting history see the works of Otto Neugebauer:
The Exact Sciences in Antiquity, 2nd Edition, O. Neugebauer, Dover Publications, New York, 1969
A History of Ancient Mathematical Astronomy, O. Neugebauer, Springer-Verlag, Berlin, 1975.
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