would have been so gradual, covering the period of two life-times
or of four or five average generations, that it might well escape
general observation.

Each succeeding generation of Egyptians, then, may not improbably
have associated the names of the seasons with the contemporary
climatic conditions, troubling themselves little with the thought
that in an earlier age the climatic conditions for each period of
the calendar were quite different. We cannot well suppose,
however, that 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