Elements of Astronomy. 5 



T/ie Planets. 



The number of planets belonginn; to our system is eleven. 

 Six of these have been known and recognised from time 

 immemorial : namely. Mercury, Venus, the Kavlh, Mars, 

 Jupiter, and Situm. But the remaining; five, which are 

 ■liot visible to the naked eye, hnve lately been discovered by 

 ihe help of the telescope; and are therefore called telescopic 

 planets: namelv, 



Uranus, discovered by Dr. Ilersehel, March 13, 1781. 



Ceres, M. Piazzi, . . January 1, 1601. 



Pallas, M. Olbers, . . March 28, 1S09. 



•Tuno, M. Harding, Septem. 1, 1S03. 



Vesta, M. Olbers". . March 29, 1807. 



All these planets revolve round the Sun, as the centie of 

 motion ; and in performing their revolutions they follow 

 the fundamental laws of the planetary motion so happily 

 discovered bv Kepler 5 and which have been fully confirmed 

 by subsequent observations. These laws are, 



J. The orbit of each planet is an ellipse-) of uhich the 

 Sun occupies 07ie of the foci. 



Thetxlremiiv of the major axis of this ellipse, nearest 

 the Sun, is called the perihelion: the opposite extremity of 

 the same axis is called the aptudion. The line, which joins 

 these two points, is called the line of the apsides. The ra- 

 dius vector is an imaginary line drawn from the centre of 

 the Sun to the centre of the planet, in any part of its orbit. 



The velocity of a planet in its orbit is always greatest at 

 its perihelion. This velocity diminishes as the radius vector 

 increases; till the planet arrives at its aphelion, when its 

 motion is the slowest. It then increases, in an inverse 

 manner, till the planet arrives again at its perihelion. 



II. The areas, described about the Sjpi by the radius vector 

 of the planet, are proportional to the times employed in 

 describing tJiem. 



These laws are sufficient for delerniinincr the motion of 

 the planets round the Sun : but it is necessary to know, for 

 each of these planets, seven cjuantities ; which are called 

 the elements of their elliptical motion. The first five of 

 these elements relate to the motion in an ellipse : the last 

 two relate to the position of the orbit ; smce the planets do 

 not all move in the satDe plane. 



1. The duration of a sidereal revolution of the planet. 



2. Half the itiajor axis of the orbit j or the mean distance 

 of the planet from the Sun. 



3. The eccentricity of the orbit; whence we deduce the 

 greatest equation of the centre. 



A3 4. The 



