104 HKowx : 



Leap Year, the record is purposely so kept as to accord at peri- 

 odical intervals only with the natural fact. 



If the sun moved eastward on the equattjr at the rate of 

 one degree of arc, corresponding to four minutes of time, per 

 day he would complete his circuit in 360 days ; but he requires 

 a little more than 360 days; — in fact, about three hundred 

 and sixty-fi^'e days and a quarter to come round : and there- 

 fore need not move eastward quite so rapidly. In fact he 

 would, advancing as he does at slightly vaiying velocities but 

 always completing the circuit in an unvarying period move at 

 an average daily rate of 3m. 56sec. (nearly); reaching the 

 meridian that much behind the star he accompanied the day 

 before. 



To the already mentioned causes of difference in speed 

 between this sun in the equator and the real sun is added that 

 of the different arcs traversed by each in a given absolute 

 space of time. Any two meridians, namely, intercept an arc 

 on the equator and an arc on the ecliptic, and these are not 

 equal. Near the equinoxes the arc on the ecliptic is the 

 longer. Near the solstice the arc on the equator is the longer. 



Neglecting, then, certain not necessary details, an imagi- 

 nary sun is supposed to move in the equattjr with un\-arying 

 velocity and to complete the yearly circuit in the same time 

 with the real sun. The interval at which he passes the meri- 

 dian is called an even twenty-four hours ; and the eighty-six 

 thousand, four-hundredth part thereof is the mean solar sec- 

 ond, the second of common life, told off with approximate 

 accuracy by common clocks. 



Let us now digress a moment and at the risk of going over 

 ground more than once lei us recall to memory ivhat is a 

 second f 



Everything else left out of the question, the earth revolves 

 once on its axis in an in\'ariable space of time, marked by the 

 return of a star to the meridian. This is called a mean side- 

 real revolution of the earth. The word is "invariable" and 

 there is vet nothins^' to shew that we mav not understand the 



