1 8 A Short History of Astronomy [CH. I. 



night-hour and in winter shorter, and the length of an hour 

 varied during the year. At Babylon, for example, where 

 this arrangement existed, the length of a day-hour was at 

 midsummer about half as long again as in midwinter, and 

 in London it would be about twice as long. It was there- 

 fore a great improvement when the Greeks, in comparatively 

 late times, divided the whole day into 24 equal hours. 

 Other early nations divided the same period into 1 2 double 

 hours, and others again into 60 hours. 



The next most obvious unit of time is the lunar month, 

 or period during which the moon goes through her phases. 

 A third independent unit is the year. Although the year 

 is for ordinary life much more important than the month, 

 yet as it is much longer and any one time of year is harder 

 to recognise than a particular phase of the moon, the length 

 of the year is more difficult to determine, and the earliest 

 known systems of time-measurement were accordingly 

 based on the month, not on the year. The month was 

 found to be nearly equal to 29! days, and as a period 

 consisting of an exact number of days was obviously con- 

 venient for most ordinary purposes, months of 29 or 30 

 days were used, and subsequently the calendar was brought 

 into closer accord with the moon by the use of months 

 containing alternately 29 and 30 days (cf. chapter n., 19). 



Both Chaldaeans and Egyptians appear to have known 

 that the year consisted of about 365^ days; and the latter, 

 for whom the importance of the year was emphasised by 

 the rising and falling of the Nile, were probably the first 

 na'tion to use the year in preference to the month as a 

 measure of time. They chose a year of 365 days. 



The origin of the week is quite different from that of 

 the month or year, and rests on certain astrological ideas 

 about the planets. To each hour of the day one of the 

 seven planets (sun and moon included) was assigned as a 

 " ruler," and each day named after the planet which ruled 

 its first hour. The planets being taken in the order 

 already given ( 15), Saturn ruled the first hour of the 

 first day, and therefore also the 8th, i5th, and 22nd hours 

 of the first day, the 5th, i2th, and igth of the second day, 

 and so on ; Jupiter ruled the 2nd, 9th, i6th, and 23rd 

 hours of the first day, and. subsequently the ist hour of 



