8 ASTRONOMY. 



same day their cycle of 19 years. Hence the num- 

 ber that denotes the current year of that number, is 

 denominated the Golden Number, having been marked 

 by the Greeks with letters of gold. 



Easter Day. 



Easter Day is always the first Sunday after the 

 full moon, which happens upon, or next after the 

 vernal equinox (the 21st of March). 



The Laws of Kepler. 



1. The first is, that the planets all move in ellip- 

 tical orbits, having the sun for their common centre. 



2. That round this focus the radius vector of each 

 planet describes equal areas in equal times. 



3. That the squares of the periodical times of the 

 planets are as the cubes of their mean distances from 

 the sun. 



The two greatest Astronomical Discoveries. 

 After Newton's discovery of the elliptic orbits of 

 the planets, La Grange's discovery of their periodical 

 inequalities is, without doubt, the noblest truth in 

 physical astronomy; and with respect to the doc- 

 trine of final causes, it may be regarded as the 

 greatest of all.* 



Universal Influence of Gravity. 



In any system, (our solar, for instance,) the com- 

 ponent parts of which are all free to move or be at 

 rest, and which are governed by the usual laws of 

 gravitation, the instant that any one of them is in 

 motion, the others cannot remain quiescent. 



A projectile force is assumed, as impressed on all 

 bodies found revolving round others. 



* Copernicus's true theory of the solar system, was a discovery 

 of the first magnitude, but it was only theory not demonstration. 



