94 OUTLINES OF NATURAL PHILOSOPHY. 



an equation to an ellipsis expressing the relatioa 

 between y or F&, (fig. 12.) the radius vector 

 drawn to the focus, and the angle z or AFG 

 which that line makes witli the transverse axis 

 on the side of the centre. 



$f cmilv/ rvunh)') oi< in tujofj sit nK, 



97* The orbit of the sun is therefore an el- 

 lipsis of which the semi-transverse axis is a, 



/yj 



the eccentricity a, and the semi-conjugate 



axs a\~ 



Thus the species of the sun's orbit is determined. 

 For the property of the ellipsis quoted above, see 

 Geometry of Curve Lines, (Prof. LESLIE,) Prop. 

 22. FRISIUS de Anal. Sect. Con. Prob. 29. It 

 may also be easily derived from SIMSON'S Con. 

 Sect. Prop. 4. Book 2. 



98. The position of the transverse axis, or of 

 what is called in astronomy, the Line of the Ap- 

 sides, in respect of the fixed stars, is found, by 

 taking the middle point between two positions 

 of the sun, in which the diurnal motion is the 

 same. 



The 



