123. When we see a globe hung up in a room we cannot help imagining it to have an upper and an under side, and immediately form a like idea of the Earth; from whence we conclude, that it is as impossible for persons to stand on the under side of the Earth as for pebbles to lie on the under side of a common Globe, which instantly fall down from it to the ground; and well they may, because the attraction of the Earth, being too strong for the attraction of the Globe, pulls them away. Just so would be the case with our Earth, if it were placed near a Globe much bigger than itself, such as Jupiter: for then it would really have an upper and an under side with respect to that large Globe; which, by it’s Attraction, would pull away every thing from the side of the Earth next to it; and only those on the top of the opposite or upper side could keep upon it. But there is no larger Globe near enough our Earth to overcome it’s central attraction; and therefore it has no such thing as an upper and an under side: for all bodies on or near it’s surface, even to the Moon, gravitate towards it’s center.
124. Let any man imagine that the Earth and every thing but himself is taken away, and he left alone in the midst of indefinite Space; he could then have no idea of up or down; and were his pockets full of gold, he might take the pieces one by one, and throw them away on all sides of him, without any danger of losing them; for the attraction of his body would bring them all back by the ways they went, and he would be down to every one of them. But then, if a Sun or any other large body were created, and placed in any part of Space several millions of miles from him, he would be attracted towards it, and could not save himself from falling down to it.
Fig. I.
One half of the Heavens visible to an inhabitant on any part
of the Earth.
Phenomena at the Poles.
[PLATE II].
125. The Earth’s bulk is but a point, as that at C, compared to the Heavens; and therefore every inhabitant upon it, let him be where he will, as at n, e, m, s, &c. sees one half of the Heavens. The inhabitant n, on the North Pole of the Earth, constantly sees the Hemisphere ENQ; and having the North Pole N of the Heavens just over his head, his [[25]]Horizon coincides with the Celestial Equator ECQ. Therefore all the Stars in the Northern Hemisphere ENC, between the Equator and North Pole, appear to turn round the line NC, moving parallel to the Horizon. The Equatoreal Stars keep in the Horizon, and all those in the Southern Hemisphere ESQ are invisible. The like Phenomena are seen by the observer s on the South Pole, with respect to the Hemisphere ESQ; and to him the opposite Hemisphere is always invisible. Hence, under either Pole, only one half of the Heavens is seen; for those parts which are once visible never set, and those which are once invisible never rise. But the Ecliptic YCX or Orbit which the Sun appears to describe once a year by the Earth’s annual motion, has the half YC constantly above the Horizon ECQ of the North Pole n; and the other half CX always below it. Therefore whilst the Sun describes the northern half YC of the Ecliptic he neither sets to the North Pole nor rises to the South; and whilst he describes the southern half CX he neither sets to the South Pole nor rises to the North. The same things are true with respect to the Moon; only with this difference, that as the Sun describes the Ecliptic but once a year, he is for half that time visible to each Pole in it’s turn, and as long invisible; but as the Moon goes round the Ecliptic in 27 days 8 hours, she is only visible for 13 days 16 hours, and as long invisible to each Pole by turns. All the Planets likewise rise and set to the Poles, because their Orbits are cut obliquely in halves by the Horizon of the Poles. When the Sun (in his apparent way from X) arrives at C, which is on the 20th of March, he is just rising to an observer at n on the North Pole, and setting to another at s on the South Pole. From C he rises higher and higher in every apparent Diurnal revolution ’till he comes to the highest point of the Ecliptic y, on the 21st of June, and then he is at his greatest Altitude, which is 231⁄2 degrees, or the Arc Ey, equal to his greatest North declination; and from thence he seems to descend gradually in every apparent Circumvolution, ’till he sets at C on the 23d of September; and then he goes to exhibit the like Appearances at the South Pole for the other half of the year. Hence the Sun’s apparent motion round the Earth is not in parallel Circles, but in Spirals; such as might be represented by a thread wound round a Globe from Tropic to Tropic; the Spirals being at some distance from one another about the Equator, but gradually nearer to each other as they approach nearer to the Tropics.
Phenomena at the Equator.
Fig. I.
126. If the observer be any where on the Terrestrial Equator eCq, as suppose at e, he is in the Plane of the Celestial Equator; or under the Equinoctial ECQ; and the Axis of the Earth nCs is coincident with the Plane of his Horizon, extended out to N and S, the North and South Poles of the Heavens. As the Earth turns round the line NCS, the whole Heavens MOLl seem to turn round the same line, but the contrary way. It is plain that this observer has the Poles constantly in his Horizon, and that his Horizon cuts the Diurnal paths of all the Celestial bodies perpendicularly and in halves. Therefore the Sun, Planets, and Stars rise every day, and ascend perpendicularly above the Horizon for six hours, and passing over the Meridian, descend in the same manner for the six following hours; then set in the Horizon, and continue twelve hours below it. Consequently at the Equator the days and nights are equally long throughout the year. When the observer is in the situation e, he sees the Hemisphere SEN; but in twelve hours after, he is carried half round the Earth’s Axis to q, and then the Hemisphere SQN becomes visible to him; and SEN disappears, being hid by the Convexity of the Earth. Thus we find that to an observer at either of the Poles one half of the Sky is always visible, and the other half never seen; but to an observer on the Equator the whole Sky is seen every 24 hours.
The Figure here referred to, represents a Celestial globe of glass, having a Terrestrial globe within it; after the manner of the Glass Sphere invented by my generous friend Dr. Long, Lowndes’s Professor of Astronomy in Cambridge.
Remark.
127. If a Globe be held sidewise to the eye, at some distance, and so that neither of it’s Poles can be seen, the Equator ECQ and all Circles parallel to it, as DL, yzx, abX, MO, &c. will appear to be straight lines, as projected in this Figure; which is requisite to be mentioned here, because we shall have occasion to call them Circles in the following Article[[26]].