THE EQUINOXES. 



difficulty in explaining the different senses in which the word 

 YEAR is used. 



If the equinoctial points maintained a fixed position on the 

 heavens, the interval between the moments at which the centre 

 of the sun's disc would pass twice successively through either of 

 them, would be in fact the interval during which the sun makes 

 or appears to make a complete revolution of the heavens. 



This interval is called the SIDEREAL YEAR. 



Astronomers have ascertained the exact length of this year 

 to be 365 d 6 h 9 ra 10-38 8 . 



It appears, therefore, that this long interval has been ascertained 

 to within the hundredth part of a second of its true value. 



96. But the equinoctial points have not a fixed position on the 

 heavens. They are on the contrary, subject to a slow displace- 

 ment from year to year in a direction contrary to the motion of 

 the sun. The amount of this annual displacement is small, being 

 a little less than one minute of a degree, that is, about the 

 thirtieth part of the breadth of the sun's disc. 



Small as this displacement is, it has been very precisely mea- 

 sured ; and its effects, which are of the highest importance, as 

 well in chronology as in astronomy, have been exactly appreciated. 



On account of this removal of the equinoctial point backward, 

 the sun arrives at it after making a revolution of the heavens, 

 sooner than it would have done if it had not been displaced. 

 This must be evident when it is considered that the equinoctial 

 point, displaced in a direction contrary to that of the sun's 

 motion, advances to meet the sun on its return. The sun there- 

 fore arrives at it before it makes a complete revolution of the 

 heavens, and the time of each successive equinox precedes the 

 time at which it would have taken place if the equinoctial point 

 had been stationary. 



This phenomenon has for that reason been called the PRE- 

 CESSION OF THE EQUINOXES. 



97. The effect therefore obviously is, that the interval between 

 two successive equinoxes is less than the sidereal year. 



This interval between two successive equinoxes is called the 



EQUINOCTIAL Or TROPICAL YEAR. 



The sidereal year is of invariable length, and would on that 

 account be well suited to be a standard measure of time. But it 

 has one capital defect, which renders it totally unfit for civil 

 purposes. It is not in accordance with the periodic returns of the 

 seasons by which all mankind measure the year. 



If the equinoctial points were stationary, the sidereal year 

 would also be the equinoctial year, and in that case it would be 

 coincident with the return of the seasons. But in consequence of 



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