ASTRONOMY. 



9 



Obliquity of the Ecliptic. 



It was demonstrated by Euler, that the change in 

 the obliquity of the ecliptic is periodical ; that it is 

 not a constant diminution, but a small and slow 

 oscillation on each side of a mean quantity, 'by which 

 it alternately increases and diminishes in the course 

 of periods which are not of the same length ; but by 

 which, in the course of many ages, a compensation 

 ultimately takes place. 



Inequalities of the Planetary Orbits, 



La Grange found, that the inequalities produced 

 by the mutual action of the planets must in effect be 

 periodical; and that amidst all the changes which 

 arise from their mutual action, two things remain 

 perpetually the same; viz. 



The length of the greater axis of the ellipse de- 

 scribed by the planet, and its periodical time round 

 the sun; or, which is the sarnie thing, the mean dis- 

 tance of each planet from the sun, and its mean 

 motion, remain constant. 



The elliptical figure of a planet's orbit never alters 

 as to length, but only in breadth, bulging out for a 

 series of years, returning gradually to its original 

 figure, and then bulging out again. 



Eccentricity of the Planetary Orbits. 



The orbits of the planets are all ellipses, having 

 the sun for their common focus, and the distance of 

 the focus from the true centre of the ellipse, is what 

 astronomers call the " eccentricity of the orbit." In 

 all the planetary orbits this eccentricity is small, and 

 the ellipse approaches nearly to a circle. These 

 eccentricities, however, continually change in the 

 progress of time, although very slowly ; but in such 

 a manner, that none of them can ever become great. 

 They may vanish or become nothing, when the orbit 



