COMMON THINGS TIME. 



sun is at different seasons at different distances from the celestial 

 equator, and the more remote it is from the equator the more 

 does a given displacement aifect its return to the meridian, for the 

 same reason exactly as that for which two places on the earth, at 

 a given distance east and west of each other, will have a greater 

 difference of longitude the farther they are from th line. 



33. Seeing, then, that the interval between the successive 

 meridional transits of the sun is subject to variation, and therefore 

 unsuitable for a chronometric unit, but that it would be suitable 

 if the sun's daily easterly displacement were always the same, 

 astronomers have imagined an expedient, which, without sacrificing 

 the advantage of an accordance with the periodical vicissitudes of 

 light and darkness, secures the advantage of complete uniform: ty 

 as to the length of the chronometric unit. 



This is accomplished by the substitution for the real of a 

 fictitious sun, whose daily easterly motion is always the same, and 

 exactly equal to the average daily easterly motion of the real sun, 

 that is, to 59' 8'2". The time, as indicated by this fictitious 

 sun, is called MEAN TIME, the moment when its centre passes the 

 meridian is called MEAN NOON, and the fictitious sun itself is 

 sometimes called the MEAN SUN. 



The variable and unavailable time indicated by the motion of 

 the real sun is called APPARENT TIME, and the moment of the 

 meridional transit of the real sun is called APPARENT NOON. 



34. From what has been stated, it will therefore be understood 

 that the mean and the real suns make a complete circuit of the 

 heavens in exactly the same time, that is, in a year ; so that, 

 starting together from a given point, they will arrive together at 

 the same point at the instant which terminates the year ; but 

 while the easterly daily displacement of the one is always 

 absolutely the same, being 59' 8-2", that of the other is 

 variable, being sometimes greater than 59' 8 '2", sometimes less, 

 and at certain times the same. 



To illustrate the changes of the relative position of the two suns, 

 let us imagine two railway trains to start from London at the same 

 moment, side by side, on two lines of rails, making a trip to 

 Liverpool and back, and to arrive at London, on their return, 

 precisely at the same moment ; but let the speed of one be 

 absolutely uniform, at 30 miles an hour, during the entire 

 journey, while that of the other is subject to variation, being 

 slower in ascending inclines, and faster in descending them. The 

 latter will at some places outstrip, and at others fall behind, the 

 former, and at certain points they will be for a 'moment side by 

 side. The variable train will represent the real, and the uniform 

 train the fictitious or mean SUE. 

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