THE MOON. 



tance. Now we find by observation that such apparent changes are actually 

 observed in its monthly course around the earth. The moon is subject to a 

 continual and small, though perceptible change of apparent size. We find that 

 it diminishes until it reaches a minimum, and then gradually increases until it 

 reaches a maximum. 



When the apparent magnitude of the moon is least, it is at its greatest dis- 

 tance from us, and when its apparent magnitude is greatest, it is at its least 

 distance from us. The positions in which these distances lie, are directly op- 

 posite. Between these two positions the apparent size of the moon undergoes 

 a regular and gradual change, increasing continually from its minimum to its 

 maximum, and consequently between these positions, its distances must on 

 the other hand gradually diminish from its maximum to its minimum. If we 

 lay down on a chart or plan a delineation of the course or path thus determined, 

 we shall find that it will represent an oval which differs however very little 

 from a circle ; the place of the earth being nearer to one end of the oval than 

 the other. 



The point of the moon's path in the heavens at which its magnitude appears 

 the greatest, and when, therefore it is nearest the earth, is called its perigee ; 

 and the point where its apparent size is least, and where, therefore, its distance 

 i'rom the earth is greatest, is called its apogee. These two points are called the 

 moon's apsides. 



If the positions of these points in the heavens be observed accurately for a 

 length of time, it will be found that they are subject to a regular change ; that 

 is to say, the place where the moon appears smallest, will every month shift 

 its position ; and a corresponding change will take place in the point where 

 it appears largest. The movement of these points in the heavens is found to 

 be in the same direction as the general movement of the planets ; that is, 

 from west to east, or progressive. This effect is called the progression of the 

 moon apsides. 



THE MOON'S NODES. 



If the position of the moon's centre in the heavens be observed from day to 

 day, it will be found that its path is a great circle, making an angle of about 

 5 with the ecliptic. This path consequently crosses the ecliptic at two 

 points in opposite quarters of the heavens. These points are called the 

 moon's nodes. Their positions are ascertained by observing from time to time 

 the distance of the moon's centre from the ecliptic, which is called the moon's 

 latitude ; by watching its gradual diminution, and finding the point at which it 

 becomes nothing ; the moon's centre is then in the ecliptic and its position is 

 the node. TJhe node at which the moon passes from the south to the north of 

 the ecliptic is called the ascending node, and that at which it passes from the 

 north to the south is called the descending node. 



If the positions of these nodes be observed from time to time, it will be 

 found that they are not fixed ; but that they change their positions in the eclip- 

 tic, moving upon that line in a direction contrary to that of the planets, or from 

 east to west. This effect is called the retrogression of the moon's nodes. 



