428 ASTRONOMY. 



"when they are nearest to the sun, and slower in the 

 remotest part of their orbits ; and they all observe 

 this remarkable law, that if a straight line be drawn 

 from the planet to the sun, arid this line be sup- 

 posed to be carried along by the periodical motion 

 of the planet, then the areas which are described 

 by this right line and the path of the planet are 

 proportional to the times of the planet's motion. 

 That is, the area described in two days, is double 

 that which is described in one day, and a third part 

 of that which is described in six days ; though the 

 arcs or portions of the orbit described are not in 

 that ratio. 



The planets being at different distances from the 

 sun, perform their periodical revolutions in differ- 

 ent times ; but it has been found that the cubes of 

 their mean distances are constantly as the squares 

 of their periodical times ; viz. of the times of their 

 performing their periodical revolutions. 



These two last propositions were discovered by 

 Kepler, by observations on the planets ; but Sir 

 Isaac Newton demonstrated, that it must have been 

 so on the principle of gravitation which formed 

 the basis of his theory. This law of universal at- 

 traction, or gravitation, discovered by Newton, 

 completely confirms the system of Copernicus, and 

 accounts for all the phenomena which were inex- 

 plicable on any other hypothesis. 



The sun, as the largest body in our system, 

 forms the centre of attraction, round which all the 

 planets move ; but it must not be considered as 

 the only body endued with attractive power, for 

 all the planets also have the property of attraction, 

 and act upon each other as well as upon the .sun. 

 The actual point, therefore, about which they 

 move, will be the common centre of gravity of all 



