SECT. III. PERTURBATIONS. 13 



SECTION III. 



Perturbations, Periodic and Secular Disturbing Action equivalent to 

 three Partial Forces Tangential Force the cause of the Periodic In- 

 equalities in Longitude, and Secular Inequalities in the Form and Position 

 of the Orbit in its own Plane Radial Force the cause of Variations in 

 the Planet's Distance from the Sun It combines with the Tangential 

 Force to produce the Secular Variations in the Form and Position of the 

 Orbit in its own Plane Perpendicular Force the cause of Periodic 

 Perturbations in Latitude, and Secular Variations in the Position of the 

 Orbit with regard to the Plane of the Ecliptic Mean Motion and 

 Major Axis Invariable Stability of System Effects of a Resisting 

 Medium Invariable Plane of the So-lar System and of the Universe 

 Great Inequality of Jupiter and Saturn. 



THE planets are subject to disturbances of two kinds, botli 

 resulting from the constant operation of their reciprocal attrac- 

 tion : one kind, depending upon their positions with regard to 

 each other, begins from zero, increases to a maximum, decreases, 

 and becomes zero again, when the planets return to the same 

 relative positions. In consequence of these, the disturbed planet 

 is sometimes drawn away from the sun, sometimes brought 

 nearer to him : sometimes it is accelerated in its motion, and 

 sometimes retarded. At one time it is drawn above the plane 

 of its orbit, at another time below it, according to the position 

 of the disturbing body. All such changes, being accomplished 

 in short periods, some in a few months, others in years, or in 

 hundreds of years, are denominated periodic inequalities. The 

 inequalities of the other kind, though occasioned likewise by the 

 disturbing energy of the planets, are entirely independent of 

 their relative positions. They depend upon the relative posi- 

 tions of the orbits alone, whose forms and places in space are 

 altered by very minute quantities, in immense periods of time, 

 and are therefore called secular inequalities. 



The periodical perturbations are compensated when the bodies 

 return to the same relative positions with regard to one another 

 and to the sun : the secular inequalities are compensated when 



