INTRODUCTION TO ASTRONOMY. 



Iv 



Fig. 21. 



Since attraction is mutual between the moon and the earth, we pro- 

 duce tides in the moon ; and these are more considerable, in proportion 

 as our planet is larger. Neither the moon nor the earth in reality assume 

 an oval form, for the land which intersects the water destroys the regu- 

 larity of the effect. The orbit of the moon being nearly parallel to that 

 of the earth, she is never vertical but to the inhabitants of the torrid zone : 

 in that climate, therefore, the tides are greatest, and they diminish as you 

 recede from it and approach the poles ; but in no part of the globe is the 

 moon immediately above the spot where it is high tide. All matter, by 

 its inertia, offers some resistance to a change of state ; the waters, 

 therefore, do not readily yield to the attraction of the moon, and the 

 effect of her influence is not complete until some time after she has 

 passed the meridian. 



The earth revolves on its axis in about twenty-four hours : if the moon 

 were stationary, therefore, the same part of our globe would, every twenty- 

 four hours, return beneath the moon ; but as during our daily revolution 

 the moon advances in her orbit, the earth must make more than a com- 

 plete rotation in order to bring the same meridian opposite the moon : we 

 are three-quarters of an hour in overtaking her. The tides, therefore, are 

 retarded, for the same reason that the moon rises later, by three-quarters 

 of an hour every day. This, however, is only the average amount of the 

 retardation. The time of the highest tide is modified by the sun's 

 attraction, and is between those of the tides which would be produced by 

 the separate action of the two luminaries. The action of the sun, there- 

 fore, makes the interval different on different days, but leaves the 

 average amount unaffected. 



