28 A SHORT HISTORY OF SCIENCE 



required for traversing the whole path i.e. the day was 

 then divided into 12 double hours, in one of which the sun's disk 

 advanced by its own diameter multiplied by 60. Their use of 

 the number 60 as a base led also to the further subdivision of the 

 hour into 60 minutes of 60 seconds each. The year was reckoned 

 as 365 days, and even the unequal rate of the sun's motion at 

 different periods was recognized. 



Particularly noteworthy in connection with Chaldean astronomy 

 is the discovery of a period of 6585 days, --a little more than 18 

 years, for the recurrence of eclipses. This would appear to have 

 been based on a long series of observations, but to have taken no 

 account of the region of visibility of eclipses of the Sun. The 

 periods of the planets in their orbits were approximately deter- 

 mined, but there is no evidence of a systematic geometrical 

 theory of celestial motions. 



As to accuracy of direct observation it is said that in later Baby- 

 lonian times angles were measured to within 6 minutes and time to 

 less than a minute. Quantities obtained indirectly by observa- 

 tions extended over long periods, as the length of the lunar month, 

 were naturally determined with correspondingly greater precision. 



A list of eclipses of the moon from 747 B.C. was known to 

 Ptolemy, while an astrological work prepared about 3700 B.C. con- 

 tains evidence of a long series of pre-existing observations. To 

 the Romans the Chaldeans were known as star-gazers, and the art 

 of augury or divination was much cultivated, making some of the 

 earliest known use of geometrical forms. Herodotus ascribes the 

 origin of the sun-dial to Babylonia. 



BABYLONIAN GEOMETRY. - - In geometry the elementary use of 

 the circle quickly leads to the discovery that a chord equal to the 

 radius subtends one-sixth of the four right angles at the centre, and 

 is thus one side of a regular inscribed hexagon, a figure found on 

 Babylonian monuments. A failure to distinguish between the 

 length of the arc and that of its chord led to the first approximation 

 to the ratio of a circumference to its diameter, TT = 3, which occurs 

 in the Old Testament where King Solomon's molten sea is said to 

 be " ten cubits from the one brim to the other : it was round all 



