1SG9.] Z^6 [LoMaic. 



the eartli lias the same tilt in the moon's plane also, and this would be a 

 further anomaly. 



Possibly these differences of statement may be accounted for from the 

 fact, that, besides our ignorance of the sun's motion, the recession of 

 nodes can have value for us only as the nodes are stations on the earth's 

 orbital plane, and can be noted as crossings of this great highway; and, 

 of course, the ecliptic can be no measure of its own inclination, or revo- 

 lutions, or recession. Certainly the ecliptic does appear to have a tilting 

 motion, completing a revolution in 25,868 years, so that the sun, in that 

 time, will appear to pass over all the stars that are between the tropical 

 circles. And why should we treat this as only an apparent motion of the 

 earth's orbital plane, while admitting that it is real in all other cases'? 



It may help us here if W"e take notice of a class of cases wherein 

 there is a real tilting of the axis of rotation of a body in its orbital plane . 

 They are all cases where a body moves in two planes at once ; as a planet 

 with a satellite, having an inclined orbit, where there is a conflict of two 

 forces, represented by the two planes, and an accommodation between 

 them. Here Vv^e assume that the earth, without the moon, would have no 

 tilt or change of direction of its axis in its own plane. But it is also in the 

 moon's plane, and this has a tilting revolution round the earth ill 19 years. 

 Then this relation of the earth and moon is analogous to their connection 

 by a lever, representing their mutual attraction in the line of the moon's 

 nodes, the fulcrum being their common centre of gravity. If the earth's 

 axis had a fixed position on this lever, it would go with it, and thus iiave 

 a real tilt in its own plane equal to double the inclination of the moon's 

 plane. But it is held by the greater force represented by its own plane 

 and its centre, so that this tilt is A^ery small, called its nutation, having a 

 period of nineteen years, and being only another aspect of the revolution 

 of the moon's nodes. It would perform an ellipse round the ideal pole 

 of the heavens ; but, by its combination with the greater motion of the 

 earth's pole by the recession of its nodes, it becomes a series of 19-year 

 scollops in that ellipse. Here is a case and a cause of tilting in a plane, 

 which no doubt exists in all planets which have satellites, and even in the 

 sun itself, and I think that no other such a case is known to astronomy. 



We know of no cosmical cause for this fixedness of axes of rotation ; 

 but, without it, we could have no science of astronomy, no measure of 

 time, no measure of direction or position beyond the earth itself; for 

 uj)on this depends, directly or indirectly, all our astronomical measures. 

 If ttie earth's orbital plane tilts and revolves, and thus changes the direc- 

 tion of the earth's axis, it is with so slow a movement as not to embarrass 

 tlie observations and calculations of a human lifetime, and scarcely those 

 of human history, but only to mark those immense periods by which 

 eternity is terraced off before and behind us. If this plane does thus 

 revolve, and if its axis is inclined to the axis of the earth, no matter what 

 may be the dip of its tilt, the poles of the two axes will revolve around 

 each other, and always maintain to each other the same angle of incli- 



A. P. S. — VOL. XI — 2C 



