201. The inclination of an Axis or Orbit is merely relative, because we
compare it with some other Axis or Orbit which we consider as not
inclined at all. Thus, our Horizon being level to us whatever place of
the Earth we are upon, we consider it as having no inclination; and yet,
if we travel 90 degrees from that place, we shall then have an Horizon
perpendicular to the former; but it will still be level to us. And, if
this Book be held so that the [52]Circle _ABCD_ be parallel to the
Horizon, both the Circle _abcd_, and the Thread or Axis _K_ will be
inclined to it. But if Book or Plate be held, so that the Thread be
perpendicular to the Horizon, then the Orbit _ABCD_ will be inclined to
the Thread, and the Orbit _abcd_ perpendicular to it, and parallel to
the Horizon. We generally consider the Earth’s annual Orbit as having no
inclination, and the Orbits of all the other Planets as inclined to it §
20.
202. Let us now take a view of the Earth in it’s annual course round the
Sun, considering it’s Orbit as having no inclination; and it’s Axis as
inclining 23-1/2 degrees from a line perpendicular to it’s Orbit, and
keeping the same oblique direction in all parts of it’s annual course;
or, as commonly termed, keeping always parallel to itself § 196.
[Sidenote: Fig. I.