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distances, eccentricities, and perihelion longitude of the 

 disturbing bodies, with the known eccentricity and longi 

 tude of the planet in question at a given epoch. Hence 

 we perceive that on these circumstances depends the varia 

 tion of the eccentricity and the revolution of the axis of 

 the planet. Thus the secular variation of the eccentricity 

 of the earth s orbit is 0.000045572 of e, the eccentricity 

 which at the epoch (1750) was 0.016814 of the semi-axis 

 major of its orbit ; and it has the negative sine in the 

 expression; consequently the eccentricity is on the decrease, 

 as we before observed. This diminution of the eccentricity 

 amounts to about 18&quot; 79 &quot; yearly (or about 3900 miles). 

 We have already observed that the annual revolving mo 

 tion of the axis of the earth s orbit is IV 53 &quot;, and its 

 period 109,060 years. The examination ef the expres 

 sions for these irregularities shows, as might be expected, 

 that Mars, Venus, and Jupiter bear the most considerable 

 share in producing the variations.* But it is a truly re 

 markable circumstance that the direct action of those 

 planets upon the moon s motion is hardly sensible com 

 pared with their indirect, or, as it is sometimes called, 

 reflected action upon the same body, through the medium 

 of the sun and the earth. For these planets, Mars, Venus, 

 and Jupiter, by altering the eccentricity of the earth s 

 orbit, very sensibly affect the motions of the moon, as we 

 have seen, while directly their action is incomparably less 

 perceptible. 



The perihelion longitudes of all the other planets are 

 increasing, or their orbits advancing, except Venus, 

 whose apsides are retrograde ; and the eccentricities of 

 Venus, Saturn, and Uranus, are decreasing, like that 

 of the earth, whilst those of the other planets are on the 



* Mec. Ccl. liv. ii. eh. 6, 7, 8.; liv. vi. eh. 7. 

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