1875.] Variation in the Obliquity of the Ecliptic. 283 
the earth’s axis of rotation, that the rate of this change is 
about r°in 179 years; that it is now going on; and that, 
from the records of the past, this same movement and rate 
appear to have been nearly, if not quite, uniform. 
It is also known from recorded observation that, whilst 
there has been this change in direction of the earth’s axis, 
there is no evidence of any change of the position of the axis 
when referred to the earth itself. The poles of the earth 
appear from recorded latitudes to be situated now where 
they were situated 2000 years ago. 
It is also known that the change in the, direction of the 
earth’s axis takes place in such a manner that it has 
produced, during the past 2000 years, little more than half a 
degree change in the obliquity of the ecliptic. 
These recorded facts Sane before us, we may now advance 
to the next point in our enquiry, which is to investigate how 
this problem has hitherto been examined, and in what 
manner it is still assumed to be disposed of. ’ 
As soon as the revival by Copernicus and Galileo of the 
ancient theory of the earth’s rotation on its axis became 
generally received, it was found necessary to attribute to the 
earth’s axis a movement which should account for the 
decrease of 20” per annum in the polar distance of stars, 
whose right ascension was near 24 hours. At that early 
date, and up to about 100 years ago, the fact of amy decrease 
in the obliquity of the ecliptic was positively denied by the 
then ruling astronomers. Readers of the ‘‘ Philosophical 
Society’s Journal,” from 1700 to 1750, will find that those 
persons who believed in any decrease in the obliquity were 
in the minority, and were treated with but slight respect by 
the paid official astronomers of the day. It being supposed 
that no decrease occurred in the obliquity whilst a move- 
ment of the pole of the heavens occurred of 20’ annually, it 
followed that the only possible course of the pole of the 
heavens to fulfil these assumed conditions was a circular 
course round the pole of the ecliptic as a centre. 
The obliquity ofthe ecliptic being about» 23° 28’, it 
followed that the pole of the heavens traced on the 
sphere a circle having a radius of 23° 28’ at the rate of 
20’ annually. At the period when this supposed move- 
ment was agreed to, theory was in the ascendant; the 
explanation by gravitation of certain celestial movements 
caused every man who hoped to occupy a prominent position 
in the scientific world to be a theorist, and to follow and 
adhere to theories often to the exclusion of facts. The 
theory proposed by Newton to account for the gyration of 
