THE PLANETS. 457 



9. Excentricity of the Planetary Orbits. The form of the 

 elliptical orbits is determined by the greater or less distance 

 of the two foci from the centre of the ellipse. This distance, 

 or the excentricity of the planetary orbits expressed in frac- 

 tional parts of their half major axes, varies from 0-006 in the 

 orbit of Venus (consequently very near the circular form), 

 and 0-076 in that of Ceres, to 0*205 and 0*255 in those of 

 Mercury and Juno. Next in succession to the least excentric 

 orbits of Venus and Neptune follows that of the Earth, whose 

 cxcentricity is now decreasing at the rate of about 0'00004299 

 in 100 years, while the minor axis increases; then come the 

 orbits of Uranus, Jupiter, Saturn, Ceres, Egeria, Vesta, and 

 Mars. The most excentric orbits are those of Juno (0-255), 

 Pallas (0-239), Iris (0-232), Victoria (0-217), Mercury (0*205), 

 and Hebe (0-202). The excentricity is on the increase in 

 the orbits of some planets, as Mercury, Mars, and Jupiter ; 

 on the decrease in those of others, as Venus, the Earth, 

 Saturn, and Uranus. The following table gives the excen- 

 tricities of the large planets for the year 1800, according to 

 Hansen. The excentricities of the fourteen small planets 

 will be given subsequently, together with other elements of 

 their orbits for the middle of the nineteenth century. 



Mercury 0-2056163 



Venus 0-0068618 



Earth 0-0167922 



Mars 0-0932166 



Jupiter 0-0481621 



Saturn . 0-0561505 



which he saw. (Monarquia Indiana, lib. ii. cap. 64.) In 

 the JRaccolta di Mendoza, we find a priest represented 

 (Vues des Cordilleres, pi. Iviii. No. 8, p. 289), who is watch- 

 ing the stars, which is expressed by a dotted line which 

 passes from the observed star to his eye. 



