UNESCO Ancient Civilizations of Africa (Editor G. Mokhtar)

The legacy of Pharaonic Egypt
Papyrus. Unfortunately the earlier sources are not extant and the Egyptian
contribution to astronomy must therefore be deduced from practical
applications made on the basis of observations. This contribution is, however,
far from insignificant.
As we have seen (see Introduction), the Egyptian calendar year was
divided into three seasons of four months, each having thirty days; to these
360 days, five were added at the end of the year. The 365-day calendar
year, the most accurate known in antiquity, is at the origin of our own
calendar year in as much as it served as the basis of the Julian reform
( — 47) and of the Gregorian reform of 1582. Side by side with this civil
calendar, the Egyptians also used a religious, lunar calendar and were able
to predict the moon's phases with adequate accuracy.
Ever since the Napoleonic expedition to Egypt, Europeans have been
struck by the accuracy of the alignment of structures built at the time
of the Pharaohs, particularly the pyramids, the four façades of which face
the four cardinal points. The Great Pyramids deviate from true North by
less than one degree. Such accuracy could have been achieved only by
astronomical observation either of the direction of the Pole Star at the
time; or the culmination of a fixed star; or the bisectrix of the angle
formed by the direction of a star at twelve-hour intervals, the bisectrix of
the angle of the rising and setting of a fixed star; or the observation of
the maximum deviations of a fixed star (which would have been 7 from
Ursa Major, according to Z. Zorba). In all these cases, precise astronomical
observation is required to calculate the alignment. The Egyptians were
perfectly capable of such observations because they possessed a corps of
astronomers working under the authority of the vizier whose job it was
to observe the night sky, to note the rising of the stars, especially of
Sirius (Sóthis), and, above all, to determine the passage of the hours of
darkness. These, for the Egyptians, varied in length according to the
seasons: night, which was supposed to contain twelve hours, always
commenced at sunset and ended at sunrise. Tables have come down to us
which indicate that each night hour was marked, month by month, at
ten-day intervals, by the appearance of a constellation or a star of the first
magnitude. The tables distinguished thirty-six such constellations or stars
which constituted decans, each one of which inaugurated a ten-day period.
This system dates back at least to the third dynasty (c. —2600). Apart
from the tables, the priest-astronomer possessed simple observation instruments:
a sighting-rod and a square to which a plumb-line was attached
and which required a team of two observers. Despite the rudimentary
nature of this technique, the observations were precise, as evidenced by
the accuracy of the orientations of the pyramids. Certain tombs have
paintings representing the sky. The stars are represented in picture form
which has made it possible to identify some of the constellations recognized
by the Egyptians. Ursa Major is called the Ox Leg; the stars surrounding
Arcturus are represented by a crocodile and hippopotamus coupled
169