GENERATION OF ANIMALS, IV. x. 



year and the times which are measured by these ; 

 also the moon's "' periods ' which are : full moon and 

 waning moon, and the bisections of the intervening 

 times," since these are the points at which it stands 

 in a definite " aspect " with the sun, the month 

 being a joint period ** of both moon and sun/ The 

 moon is a "principle" on account of its associa- 

 tion ^\-ith the sun and its participation in the sun's 

 light, being as it were a second and lesser sun,** and 

 therefore is a contributory factor in all processes of 



moon to go through all its phases once, is, literally and 

 properly speaking, not a private period of the moon's, but, as 

 Aristotle says, a joint period of the moon and sun, since it is 

 the moon's position relative to the sun which determines how 

 much of the moon's disk is illuminated. If the moon were 

 self-luminous, there would l^e no phases, and therefore there 

 could be no " phase-p)eriod." This is made even more clear 

 if we consider that the moon does in fact possess a " period " 

 proper to itself, pertaining to the moon's own actual motion, 

 and not to the mere illumination of its surface by another 

 body, and it is a period which differs in length from the 

 lunation or " phase-period " — a fact which was probably 

 better known to Aristotle than to some moderns. This is 

 the period known in astronomy as the " sidereal period," 

 i.e., the time taken by the moon to return again to its same 

 apparent position among the stars — not to return into con- 

 junction with the sun. The duration of this period is roughly 

 27 days 8 hours, as against an average of 29 days 13 hours 

 for the " phase-period." Aristotle is therefore quite correct 

 in stating that the " month," by which, as the context clearly 

 shows, he means the " phase-period," is a joint period of the 

 sun and, the moon. (I should, perhaps, apologize to astro- 

 nomers for the un-astronomical term " phase-period," which 

 I have used instead of " synodic period " in order to 

 emphasize the point that phases are an incidental pheno- 

 menon, and not an essential concomitant of a synodic j)eriod.) 

 "* This statement reappears in Theophr. De vent. 17 ij ae 

 A'^vTj . . . otoi' dadeyijs rjXios iori, and cf. id. De signis temp. 5, 

 where the moon is,described as " the sun of the night." 



4-79 



