Elements of Astronomy . 19 



appearance of the ring will occur about once in 15 years; 

 but under circumstances oftentimes very cjifferent. 



[The initrsectioii of the ring and the ecliptic is in 3' 20^ 

 and IP 20-*: consequently, when Saturn is in either of 

 those signs, his ring will be invisible to us. On the con- 

 trary, when he is in 2' 20° or 8' 20% we may see it to most 

 advantage. This was the case towards the end of the year 

 1811. Regard, however, must be had to the position of 

 the Earth.] 



As viewed from the Earth, the motion of Saturn some- 

 times appears retrograde. The mean arc which he describes 

 in this case is about 6° 18': and its duration is nearly 139 

 days. This retrogradation commences, or finishes, when 

 the planet is distant about 108^ 54' from the Sun. 



His mean apparent diameter is 1 T'.G. 



Telescopic Planets. 



Uranus was discovered by Dc Herschel, March 13, 

 1781. who gave it the name of the Georgium.Sidiis. It 

 performs its sidereal revolution in 30688'^ 17'' & l6'',2 ; or in 

 about 84 Julian years: and it is probably situated at the 

 confines of the planetary system. 



Its distance from ihe Sun is upwards of 1800 millions of 

 miles; and its apparent diameter is scarcely 3",9. 



Its mass compared with that of the Sun considered as 

 unity, is -y^-^i 



Six satellites accompany this planet ; which move in 

 orbits nearly perpendicular to the plane of the ecliptic. 



The elements of the four remaining telescopic planets are 

 not yet ascertained with sufficient precision. 



Satellites . 



The number of satellites in our system, at present known, 

 is eighteen : namely, the Moon which revolves round the 

 Earth ; four that belong to Jupiter, seven to Saturn, and 

 six to Uranus. The Moon is the only one visible to the 

 naked eye. 



They all move round their primary planets, as their cen- 

 tre, by the same law sas those primary ones move round the 

 Sun : namely, 



I. The orbit of each satellite is nn ellipse, oftvhich the 

 primary planet occupies one of the foci. 



II. The areas, described alout the primary planet, ly the 

 radius vector of the satellite, are proportional to the limes 

 employed in describing them. 



B 2 in. The 



