A HISTORY OF SCIENCE 



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} 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! days, and 

 to know that the calendar may be regulated approxi- 

 mately 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 Ptole- 

 maic period (whose work we shall have occasion to re- 

 view 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 



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