t the astronomer
priests were oblivious to the true state of things. Upon them devolved
the duty of predicting the time of the Nile flood; a duty they were
enabled to perform without difficulty through observation of the rising
of the solstitial sun and its Sothic messenger. To these observers it
must finally have been apparent that the shifting of the seasons was
at the rate of one day in four years; this known, it required no great
mathematical skill to compute that this shifting would finally effect a
complete circuit of the calendar, so that after (4 X 365 =) 1460
years the first day of the calendar year would again coincide with the
heliacal rising of Sothis and with the coming of the Nile flood. In
other words, 1461 vague years or Egyptian calendar years Of 365 days
each correspond to 1460 actual solar years of 365 1/4 days each. This
period, measured thus by the heliacal rising of Sothis, is spoken of as
the Sothic cycle.

To us who are trained from childhood to understand that the year
consists of (approximately) 365 1/4 days, and to know that the calendar
may be regulated approximately by the introduction of an extra day every
fourth year, this recognition of the Sothic cycle seems simple enough.
Yet if the average man of us will reflect how little he knows, of his
own knowledge, of the exact length of the year, it will soon become
evident that the appreciation of the faults of the calendar and the
knowledge of its periodical adjustment constituted a relatively
high development of scientific knowledge on the part of the Egyptian
astronomer. It may be added that various efforts to reform the calendar
were made by the ancient Egyptians, but that they cannot be credited
with a satisfactory solution of the problem; for, of course, the
Alexandrian scientists of the Ptolemaic period (whose work we shall have
occasion to review presently) were not Egyptians in any proper sense of
the word, but Greeks.

Since so much of the time of the astronomer priests was devoted to
observation of the heavenly bodies, it is not surprising that they
should have mapped out the apparent course of the moon and the visible
planets in their nightly tour of the heavens, and that they should have
divided the stars of the firmament into more or less arbitrary groups
or constellations. That they did so is evidenced by various sculptured
representations of constellations corresponding to signs of the
zodiac which still ornament the ceilings