LENGTH OF THE YEAR. 55 



and Id. lib. 6m. 58s. more, or in all, 27759d. lib. 6m. 58s., 

 which being divided by 76, gives 365d. 6h. 8m. 47s. for the length 

 of the Siderial year. More recent and exact observations give 

 365d. 6h. 9 m. 11s. 



There are various kinds of years. First, the Siderial year, or 

 the time which it takes the earth to perform exactly one revolution 

 around the sun. This year it is not expedient to use, for the 

 seasons being dependant on the position of the earth with regard 

 to the sun, it is more convenient to have for the length of a 

 year, the time from the commencement of spring to the com- 

 mencement of spring again, and this is a period which, for a 

 reason we will soon explain, is shorter than a siderial year. This 

 year is called a Tropical or Equinoctial year. Again, inasmuch as 

 this year does not consist of an exact number of days, and as it 

 would be excessively inconvenient' to have a year begin at any 

 other time except the commencement of a day, we have the Civil 

 year, which consists of exactly 365 days, and every fourth year, 

 of 366. We have already given the length of the Siderial year, 

 which is the time of a true revolution of the earth in its orbit, 

 but the length of the equinoctial year, or year from beginning 

 of spring, to spring again, is shorter than this. It is obvious 

 that the equinoctial year is the one which most intimately con- 

 cerns us, all agricultural, and other operations, being entirely 

 dependant upon the seasons. 



When we explain, in the next chapter, the cause of the seasons, 

 we shall show why this year, must be shorter than the Siderial 

 year. Meantime we may suppose one of the early philosophers 

 detecting it in this manner. The path of the sun in the heavens 

 being ascertained, it was soon observed that it was inclined at a 

 certain angle, with the apparent diurnal paths of the stars. Thus, 

 if we observe a certain star to-night, (mid-summer,) which rises 

 due east, and watch its diurnal path, or the line which it traces 

 in its apparent motion over the heavens, we will find it a part of 

 a circle, whose centre is the pole of the heavens, near which the 

 pole star is situated, and the star will set due west : at a certain 

 point midway between east and west, it will reach its highest 

 altitude, after which it will begin to set, this highest altitude is 



