5lU tECTURE XLII. 



Older of the signs, and in planes not very remuu from the ecliptic, except, 

 ing those of the Georgian planet, which revolve in planes nearly perpendi- 

 cular to the ecliptic. Each of these planets thus becomes the central lumi- 

 nary of a little system of its oAV'n,in which the motions'and the periods observe 

 the same general laws as prevail in the solar system at large. Of the 28 

 primary and secondary planets, we are indebted to Dr. llerschel for the 

 knowledge of 9; the Georgian planet, with its six satellites, and the two 

 ' innermost moons of Saturn. 



The motions of some of these satellites, in particular of those of Jupiter> 

 and of the niQon, are of considerable importance for the assistance they aftbrd 

 us in determinations of time, and of the relative situations of places. They 

 are subjected to considerable irregularities, but the united labours of various 

 astronomers have enabled us to calculate all their motions with the greatest 

 accuracy. 



The moon performs a complete sidereal revolution in 27 days 7^ hours, 

 and a synodical revolution, during which she returns to the same position 

 with respect to the earth and sun, in 29 days IQ^ hours; a period which 

 constitutes a lunation, or a lunar month. Her orbit is inclined to the 

 ecliptic in an angle of a little more than five degrees, but this inclination is 

 liable to great variations: the place of its nodes is also continually changing, 

 their motion being sometimes retrograde, and sometimes direct, but on the 

 whole the retrograde motion prevails. The form of the moon's orbit is 

 irregularly elliptic, and the velocity of its motion deviates considerably from 

 the Keplerian law of the description of equal areas in equal times; the ap- 

 sides, or the extremities of the greater axis of the ellipsis, which are called 

 the apogee and perigee, have on the whole aidirect motion. From a com- - 

 parison of modern observations with the most ancient, the mean motion of 

 the moon is found to be somewhat accelerated. 



The moon revolves on her own axis with a very equable motion, and the 

 period of her rotation is precisely equal to the mean periodof her revolution 

 round tbe earth; so that she always presents to us the same portion of her 

 surface, excepting the apparent librations produced by her unequal velocities 

 in her orbit, and by the position of her axis, which is inclined I'' A'3' to the ^ 



