PTOIiEMY versus COPERNICUS.
117
system, which, dating (in its earliest form) at least from thesecond century before Christ, was so carefully elaborated, sopersistently maintained. By way of contrast to it, let usdescribe the true theory of the Solar System , first publishedby Copernicus , upon his dying bed, in his great work “ReRevolutionists Orbium Ccelestium ,” a.d. 1543 ; and afterwardsmore fully elucidated by the investigations of Kepler, Newton,and others.
Its first assertion is, that the apparent daily motion of thestars from east to west, to which we have already referred, aswell as a corresponding part of the motion of all the otherheavenly bodies, is to be explained by the rotation of the Earth upon its axis in the opposite direction, viz., from west to east,once in every 23 hours 56 minutes ; the deficit of about 4minutes, by which this period falls short of 24 hours, being-caused (as we shall presently explain in Lecture IX.) by themotion of the Earth in its own orbit.
The Sun is taken as the controlling centre of the movementsof all the planets.
In order from it,—see Fig. XXXIII., in which the size ofthe various orbits is indicated as nearly as possible upon acorrect scale,— there revolve
Millions
of
Miles.
Days.
Years.
Miles
per
Hour.
Mercury ...
1 at a 'j) mean dis- (_
') tance of f [)
V about )
( in a )
- period of - 88
( about )
< °r l i) about) t
( with a \
J mean ("i velocity ((of about )
107,000
„ ' 67
„ 224*
s
57 T?
55
78,000
„ 93
„ 367,}
„ 1
55
67,000
„ 142
„ 087
1 . li
55
7,4,000
00
4,333
„ Ilf
55
29,000
„ 887
„ 10,77)9
„ 29*
55
22,000
„ 1,787,
„ 30,088
,, 84
55
15,000
Neptune ...
„ 2,79(>
„ 60,181
„ 164f
12,000