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 231⁄2 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].
Fig. I.
A concise view of the seasons.
Let a, b, c, d, e, f, g, h be the Earth in eight different parts of it’s Orbit, equidistant from one another; Ns it’s Axis, N the north Pole, s the south Pole, and S the Sun nearly in the center of the Earth’s Orbit § [18]. As the Earth goes round the Sun according to the order of the letters abcd, &c. it’s Axis Ns keeps the same obliquity, and is still parallel to the line MNs. When the Earth is at a, it’s north Pole inclines toward the Sun, and brings all the northern places more into the light than at any other time of the year. But when the Earth is at e in the opposite time of the year, the north Pole declines from the Sun, which occasions the northern places to be more in the dark than in the light; and the reverse at the southern places, as is evident by the Figure, which I have taken from Dr. Long’s Astronomy. When the Earth is either at c or g, it’s Axis inclines not either to or from the Sun, but lies sidewise to him; and then the Poles are in the boundary of light and darkness; and the Sun, being directly over the Equator, makes equal day and night at all places. When the Earth is at b it is half way between the Summer Solstice and Harvest Equinox; when it is at d it is half way from the Harvest Equinox to the Winter Solstice; at f half way from the Winter Solstice to the Spring Equinox: and at h half way from the Spring Equinox to the Summer Solstice.
Fig. II.
[PLATE V].
The Ecliptic.
The seasons shewn in another view of the Earth, and it’s Orbit.
203. From this oblique view of the Earth’s Orbit, let us suppose ourselves to be raised far above it, and placed just over it’s center S, looking down upon it from it’s north pole; and as the Earth’s Orbit differs but very little from a Circle, we shall have it’s figure in such a view represented by the Circle ABCDEFGH. Let us suppose this Circle to be divided into 12 equal parts called Signs, having their names affixed to them; and each Sign into 30 equal parts called Degrees, numbered 10, 20, 30, as in the outermost Circle of the Figure, which represents the great Ecliptic in the Heavens. The Earth is shewn in eight different positions in this Circle, and in each position Æ is the Equator, T the Tropic of Cancer, the dotted Circle the parallel of London, U the arctic or north polar Circle, and P the north Pole where all the Meridians or hour Circles meet § [198]. As the Earth goes round the Sun the north Pole keeps constantly towards one part of the Heavens, as it keeps in the Figure towards the right hand side of the Plate.
Vernal Equinox.
When the Earth is at the beginning of Libra, namely on the 20th of March, in this Figure (as at g in Fig. I.) the Sun S as seen from the Earth appears at the beginning of Aries in the opposite part of the Heavens[[53]], the north Pole is just coming into the light, the Sun is vertical to the Equator; which, together with the Tropic of Cancer, parallel of London, and arctic Circle, are all equally cut by the Circle bounding light and darkness, coinciding with the six o’clock hour Circle, and therefore the days and nights are equally long at all places: for every part of the Meridian ÆTLa comes into the light at six in the morning, and revolving with the Earth according to the order of the hour-letters, goes into the dark at six in the evening. There are 24 Meridians or hour-Circles drawn on the Earth in this Figure, to shew the time of Sun rising and setting at different Seasons of the Year.
Fig. II.
As the Earth moves in the Ecliptic according to the order of the letters ABCD, &c. through the Signs Libra, Scorpio, and Sagittarius, the north Pole comes more and more into the light; the days increase as the nights decrease in length, at all places north of the Equator Æ; which is plain by viewing the Earth at b on the 5th of May, when it is in the 15th degree of Scorpio[[54]], and the Sun as seen from the Earth appears in the 15th degree of Taurus. For then, the Tropic of Cancer T is in the light from a little after five in the morning till almost seven in the evening; the parallel of London from half an hour past four till half an hour past seven; the polar Circle U from three till nine; and a large track round the north Pole P has day all the 24 hours, for many rotations of the Earth on it’s Axis.