Interior of the Globe. 31 



IS oblique to that of the equator, or in the line h M ; and as 

 the distance between the satellite and the earth is only thirty 

 diameters of the latter, the action of the moon M is a little 

 greater on a b, the part of the protuberance next to it, than 

 on the part opposite, c d. The effect of this disturbing action 

 is to draw down the plane of the equator from the direction 

 G E to the direction G ^, and to produce a corresponding 

 angular change in the position of the earth's axis, shifting it 

 from N S to ;i *. This change of position is called the Nuta- 

 tion of the earth's axis (from nutatio, nodding). The name is 

 appropriate, for the motion is constantly varying in amount, and 

 constitutes a sort of tremor or vibration, which runs through its 

 principal phases in eighteen and a-half years, the period in 

 which the moon''s Nodes complete their revolution. The action 

 of the sun is conjoined with that of the moon, but is compara- 

 tively feeble. The secondary effect of this Nutation is the 

 precession of the equinoxes^ or the shifting of the equinoctial 

 points 50' westward annually, which makes the pole of the 

 earth describe a circle of 47° in diameter round the pole of the 

 ecliptic in 25,800 years. 



The thickness of the equatorial protuberance ah cd^ and 

 the magnitude of the angular change in the earth's axis N w, 

 must not be judged of from the figure, in which they are 

 necessarily exaggerated. The equatorial protuberance amount- 

 ing only to 13 miles upon a semi- diameter of 4000, may be 

 compared to a band of writing-paper wrapped round the 

 middle of an orange. The nutation makes the pole N describe 

 a very small circle round its mean place, namely, of about 

 900 feet radius. To give an idea of the extreme minuteness 

 of the change, let us suppose an iron rod 100 feet long, fixed 

 at one end and moveable at the other, to represent one-half of 

 the earth*'s axis. If the moveable end were pulled the twentieth 

 part of an inch to one side, the deviation Avould be proportion- 

 ally as great as that which the lunar nutation produces in the 

 terrestrial axis. 



The earth'*s equatorial diameter exceeds, I have said, the 

 polar only by a 300th part. The moon's attraction, there- 

 fore, may be considered as acting upon the part a, with the 

 aid of a lever, a fraction longer than if the earth had been a 



