22 A Short History of Astronomy [Ca. 11. 



resulted, of which Aristophanes makes the Moon complain 

 in his play The Clouds, acted in 423 B.C. : 



44 Yet you will not mark your days 



As she bids you, but confuse them, jumbling them all sorts of ways. 

 And, she says, the Gods in chorus shower reproaches on her head, 

 When, in bitter disappointment, they go supperless to bed, 

 Not obtaining festal banquets, duly on the festal day." 



20. A little later, the astronomer Meton (born about 

 460 B.C.) made the discovery that the length of 19 years 

 is very nearly equal to that of 235 lunar months (the 

 difference being in fact less than a day), and he devised 

 accordingly an arrangement of 12 years of 12 months and 

 7 of 13 months, 125 of the months in the whole cycle 

 being "full" and the others "empty." Nearly a century 

 later Callippus made a slight improvement, by substituting 



, I in every fourth period of 19 years a "full" month for one of 

 n I the " empty " ones. Whether Meton's cycle, as it is called, 

 was introduced for the civil calendar or not is uncertain, 

 but if not it was used as a standard by reference to which 

 the actual calendar was from time to time adjusted. The 

 use of this cycle seems to have soon spread to other parts 

 of Greece, and it is the basis of the present ecclesiastical 

 rule for fixing Easter. The difficulty of ensuring satisfactory 

 correspondence between the civil calendar and the actual 

 motions of the sun and moon led to the practice of publish- 

 ing from time to time tables (TrapaTn/y/xara) not unlike 

 our modern almanacks, giving for a series of years the 

 dates of the phases of the moon, and the rising and setting 

 of some of the fixed stars, together with predictions of the 

 weather. Owing to the same cause the early writers on 

 agriculture (e.g. Hesiod) fixed the dates for agricultural 

 operations, not by the calendar, but by the times of the 

 rising and setting of constellations, i.e. the times when 

 they first became visible before sunrise or were last visible 

 immediately after sunset a practice which was continued 

 long after the establishment of a fairly satisfactory calendar, 

 and was apparently by no means extinct in the time of 

 Galen (2nd century A.D.). 



21. The Roman calendar was in early times even more 

 confused than the Greek. There appears to have been 



