MEAN NOON. 



pendulum, as will be explained in a future number of the 

 MUSEUM. 



44. When the pendulum has been exactly regulated, it will 

 swing 86400 times between the moments of mean noon on two 

 successive days. The time of 60 swings will be a mean solar 

 minute, and the time of 3600 will be a mean solar hour. The 

 olock thus regulated, being set to 12 at the moment of mean 

 noon, will again point to 12 at mean midnight, and again at 

 the succeeding mean noon, and so on. 



45. From what has been explained, it will be understood that a 

 sidereal day, or the time of the rotation of the earth upon its axis, 

 is somewhat shorter than a common civil day. The exact propor- 

 tion between these chronometric units, which is by no means an 

 easy problem, has, however, been solved by astronomers, and it 

 is found that 100,000000 common or civil days are equal to 

 100,273791 sidereal days, or, if less extreme arithmetical precision 

 be sufficient, it may be stated that in a thousand common days 

 the earth makes 1002| rotations on its axis. 



From these numbers it is easy to express the time of rotation 

 in hours, minutes, and seconds of civil time. To do this we have 

 the proportion 



100,273791 : 100,000000 : : 24 : the time of rotation. 



By the rule of three, therefore, the time of rotation will be 



hours. 



2400,000000 _ 

 T007273W ' 



It appears, therefore, that the time of the earth's rotation falls 

 short of 24 hours, such as those shown by well regulated clocks, 

 by three minutes, fifty-five seconds, and ninety-one hundredths 

 of a second. 



IV. THE WEEK. 



46. Having thus explained fully the meaning of a day, con- 

 sidered as the standard unit of time, and of the subordinate and 

 lesser divisions of hours, minutes, and seconds, it will now be 

 necessary to notice the larger chronometric units. 



The chronometric unit, in the ascending order, which comes 

 next to the day, is the WEEK.* 



47. The opinions of historians and antiquarians are much 

 divided as to the date and prevalence of the custom of counting 

 time by periods of seven days. It is certain, however, that among 

 the oriental nations such a period has been in use from time 

 immemorial. Philo Juda3us, Josephus, and St. Clement of 



* From the Saxon word WEOC, having the same signification. 



139 



