OUTLINES OF NATURAL PHILOSOPHY. 



tion of the transverse axis of the orbit, or of the 

 line of the apsides. 



257. The motion of the line of the nodes is 

 produced by that part of the Sun's disturbing 

 force which is in the direction of the straight 

 line joining the centres of the Sun and Earth, 

 and proportional to the distance of the Moon 

 from a plane passing through the centre of the 

 Earth at right angles to the line joining the cen- 

 tres of the Sun and Earth. 



Suppose the Moon to be on the same side of the last- 

 mentioned plane that the Sun is. Then, if in the 

 direction of her motion for the instant just past, 

 there be taken a line equal to the space passed over 

 in that instant, and if in the line drawn through 

 the Moon, perpendicular to the above plane, on 

 the side opposite to the plane, there be taken a 

 part equal to the space which the force urging the 

 Moon from the plane would have made her de- 

 scribe in the same time, then the true path of 

 the Moon will be the diagonal of the parallelogram 

 under these two lines j and the momentary 

 change in the place of the node will be the distance 

 between the point where an arch having the direc- 

 tion of this diagonal, and another having the direc- 

 tion of its side, meet the plane of the ecliptic. The 

 same will happen in other situations of the Moon, 

 and the line of the nodes will thus have a motion 



in 



