ASTRONOMY. 8D 



more than 58 towards the east. HIPPARCHUS 

 discovered the precession of the equinoxes, by a 

 comparison of his own with more ancient obser- 

 vations. 



90. On account of the precession of the equi- 

 noxes, the tropical year, or the time in which 

 the sun moves from the vernal equinox to the 

 vernal equinox again, is less than the time in 

 which he moves from one star to the same star 

 again ; the vernal equinox having gone west- 

 ward so as to meet the sun. 



The time in which the sun goes from one fixed star 

 to the same fixed star again, is called a Siderial 

 Year, and is longer than the tropical year, by the 

 time that the sun takes to move over 50"^ of his 

 orbit. This amounts to .014119 of a day, or 

 20' 19".9 ; so that the siderial year is 365<1 6** 9* 

 1 s . 5. 



91. The Obliquity of the Ecliptic is also sub- 

 ject to change, and appears to have been con- 

 stantly diminishing from the remotest date of 

 astronomical observation : its present rate of 

 diminution is nearly 50" in a century. 



A Chinese observation of the sun's altitude at the 

 solstices, as ancient as the year 1 1 00 before Christ, 



has 



