Continue both motions, and as the Globe moves through the quarter _B_, the north Pole advances toward the light, and the south Pole recedes as fast from it; the days lengthen in the northern Hemisphere, and shorten in the southern; and when the Globe comes to _G_ the candle will be again over the Equator (as when the Globe was at _E_) and the days and nights will again be equal as formerly: and the north Pole will be just coming into the light, the south Pole going out of it. Thus we see the reason why the days lengthen and shorten from the Equator to the polar Circles every year; why there is no day or night for several turnings of the Earth, within the polar Circles; why there is but one day and one night in the whole year at the Poles; and why the days and nights are equally long all the year round at the Equator, which is always equally cut by the circle bounding light and darkness. [Sidenote: Remark. Fig. III. PLATE V.] 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. A concise view of the seasons.]