
Secr.IV.] STABILITY OF EARTH'S AXIS, 15 
protuberance along the equatorial regions, the attraction chiefly of 
_ the moon and sun tends to pull the axis aside, or to make it describe 
a conical movement like that of the axis of a top round the vertical. 
Hence each pole points successively to different stars. This move- 
ment, called the precession of the equinoxes, in combination with 
another smaller movement, due to the attraction of the moon (called 
nutation), completes its cycle in 21,000 years. At present the winter 
in the northern hemisphere coincides with the earth’s nearest ap- 
proach to the sun, or perihelion. In 10,500 years hence it will take 
place when the earth is at the farthest part of its orbit from the 
sun, or in aphelion. This movement acquires great importance when 
considered in connexion with the secular variations in the eccentricity 
of the orbit (§ 8). 
§ 4. Change in the Obliquity of the Eeliptie.—The angle at 
which the axis of the earth is inclined to the plane of its orbit does 
not remain strictly constant. It oscillates through long periods of 
_ time to the extent of about a degree and a half, or perhaps a little 
- more, on either side of the mean. According to Dr. Croll,’ this 
oscillation must have considerably affected former conditions of 
climate on the earth, since, when the obliquity is atits maximum, the 
polar regions receive about eight and a half days more of heat than 
they do at present—that is, about as much heat as lat. 76° enjoys at 
this day. This movement must have augmented the geological 
effects of precession, to which reference has just been made, and 
which are described in '§ 8. 
§ 5. Stability of the Harth’s Axis.—That the axis of the 
earth’s rotation has successively shifted, and consequently that the 
poles have wandered to different points on the surface of the globe, 
has been maintained by geologists as the only possible explanation 
of certain remarkable conditions of climate, which can be proved to 
have formerly obtained within the Arctic Circle. Even as far north 
as lat. 81° 45’ abundant remains of a vegetation indicative of a warm 
climate, and including a bed of coal 25 to 30 feet thick, have been 
found zm situ.2 It is contended that where these plants lived the 
ground could not have been permanently frozen or covered for most 
of the year with thick snow. In explanation of the difficulty, it has 
been suggested that the north pole did not occupy its present 
position, and that the locality where the plants occur lay in more 
southerly latitudes. Without at present entering on the discussion 
of the question whether the geological evidence necessarily requires 
“so important a geographical change, let us consider how far a 
shifting of the axis of rotation has been a possible cause of change 
during that section of geological time for which there are records 
among the stratified rocks. 
From the time of Laplace* astronomers have strenuously denied 
1 Croll, Trans. Geol. Soc. Glasgow, ii. 177. 
2 Fielden and Heer, Quart. Journ. Geol. Soc. Nov. 1877. 
3 Mécanique ceéleste, tome v. p. 14. 
