17] The Measurement of Time: Eclipses 19 



the 6th day. In this way the first hours of successive 

 days fell respectively to Saturn, the Sun, the Moon, Mars, 

 Mercury, Jupiter, and Venus. The first three are easily 

 recognised in our Saturday, Sunday, and Monday ; in the 

 other days the names of the Roman gods have been 

 replaced by their supposed Teutonic equivalents Mercury 

 by Wodan, Mars by Thues, Jupiter by Thor, Venus by 

 Freia.* 



^vi;. Eclipses of the sun and moon must from very early 

 times have excited great interest, mingled with superstitious 

 terror, and the hope of acquiring some knowledge of them 

 was probably an important stimulus to early astronomical 

 work. That eclipses of the sun only take place at new 

 moon, and those of the moon only at full moon, must have 

 been noticed after very little observation ; that eclipses of 

 the sun are caused by the passage of the moon in front 

 of it must have been only a little less obvious ; but the 

 discovery that eclipses of the moon are caused by the 

 earth's shadow was probably made much later. In fact 

 even in the time of Anaxagoras (5th century B.C.) the idea 

 was so unfamiliar to the Athenian public as to be regarded 

 as blasphemous. 



One of the most remarkable of the Chaldaean con- 

 tributions to astronomy was the discovery (made at any 

 rate several centuries B.C.) of the recurrence of eclipses 

 after a period, known as the saros, consisting of 6,585 days 

 (or eighteen of our years and ten or eleven days, according 

 as five or four leap-years are included). It is probable 

 that the discovery was made, not by calculations based on 

 knowledge of the motions of the sun and moon, but by 

 mere study of the dates on which eclipses were recorded 

 to have taken place. As, however, an eclipse of the sun 

 (unlike an eclipse of the moon) is only visible over a small 

 part of the surface of the earth, and eclipses of the sun 

 occurring at intervals of eighteen years are not generally 

 visible at the same place, it is not at all easy to see how 

 the Chaldaeans could have established their cycle for this 

 case, nor is it in fact clear that the saros was supposed to 

 apply to solar as well as to lunar eclipses. The saros may 



* Compare the French : Mardi, Mercredi, Jeudi, Vendredi ; or 

 better still the Italian : Martedi, Mercoledi, Giovedi, Venerdi. 



