t astronomy than giving Eudoxus the Copernican system? If Mercury
were a black spot in the middle of the sun it would of course move round
the earth in a year, or appear to do so: let it swing a little on one side
and the other of the sun, and the average period is still a year, with
slight departures both ways. The same for Venus, with larger departures.
Say that a person not much accustomed to the distinction might for once
write down the mistake; how are we to explain its remaining in the mind in
a permanent form, and being made a ground for such speculation as that of
the difficulty of observing Mercury leading to a period four times what it
ought to be, corrected in proof and published by an industrious and
thoughtful person? Only in one way: the writer was quite out of his depth.
This one case is conclusive; be it said with all respect for the real
staple of the work and of the author. He knew well the difference of the
systems, but not the effect of the difference: he is another instance of
what I have had to illustrate by help of a very different person, that it
is difficult to reason well upon matter which is not familiar.

(P. 254). Copernicus, in fact, supposed the axis of the earth to be always
turned towards the Sun.^{(169)} [(169). See Delambre, _Hist. Astr. Mod._,
Vol. I, p. 96]. It was reserved to Kepler to propound the hypothesis of the
constant parallelism of the earth's axis to itself.

If there be one thing more prominent than another in the work of Copernicus
himself, in the popular explanations of it, and in the page of
Delambre[288] cited, it is that the _parallelism of the earth's axis_ is a
glaring part of the {166} theory of Copernicus. What Kepler[289] did was to
throw away, as unnecessary, the method by which Copernicus, _per fas et
nefas_,[290] secured it. Copernicus, thinking of the earth's orbital
revolution as those would think who were accustomed to the _solid
orbs_--and much as the stoppers of the moon's rotation do now: why do they
not strengthen themselves with Copernicus?--thought that the earth's axis
would always incline the same end towards the sun, unless measures were
taken to prevent it. He _did_ take measures: he invented a _compensating_
conical motion of the axis to preserve the parallelism; and, which is one
of the most remarkable points of his system, he obtained the precession of
the equinoxes by giving the necessary trifle more than compensation. What
stares us in