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one half of the heavens is seen; for those parts which
Plate II. other about the equator, and gradually nearer to each
other as they approach toward the tropics.
126. If the observer be any where on the terresmena at trial equator e C q, as suppose at e, he is in the plane the equa- of the celestial equator; or under the equinoctial
EC Q; and the axis of the Earth n C s is coincident with the plane of his horizon, extended out to Nand 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 celestial poies 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, 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. Con. sequently at the equator the days and nights are equally long throughout the year. When the obser. ver 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. 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 Profes.
sor of Astronomy in Cambridge. Remark. 127. If a globe be held sidewise to the eye, at
some distance, and so that neither of its poles can be seen, the equator ECQ, and all circles parallel to it, as D L, Y Zx,ab X, 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 articles of this chapter*.
128. Let us now suppose that the observer has Phenogone from the equator e toward the north pole n, tween the and that he stops at i, from which place he then equator
. sees the hemisphere MEINL; his horizon MCL and poles, having shifted as many degrees from the celestial poles N and S, as he has travelled from under the equinoctial E. And as the heavens seem constantly to turn round the line NC'S as an axis, all those stars which are not as many degrees from the north pole N'as the observer is from the equinoctial, namely, the stars north of the dotted parallel DL, never set below the horizon; and those which are south of the dotted parallel MO never rise above it. Hence the former of these two parallel circles is called the cir. The circle of perpetual apparition, and the latter the circle cles of
perpetual of perpetual occultation : but all the stars between apparition these two circles rise and set every day. Let us im- and occul. agine many circles to be drawn between these two, and parallel to them; those which are on the north side of the equinoctial will be unequally cut by the horizon MCL, having larger portions above the horizon than below it: and the more so, as they are nearer to the circle of perpetual apparition; but the reverse happens to those on the south side of the equinoctial while the equinoctial is divided in two equal parts by the horizon. Hence, by the apparent turning of the heavens, the northern stars describe greater arcs or portions of circles above the horizon than below it; and the greater, as they are farther from the equinoctial toward the circle of perpetual apparition; while the contrary happens to all stars
* The plane of a circle, or a thin circular plate, being turned edge wise to the eye, appears to be a straight line.
south of the equinoctial; but those upon it describe equal arcs both above and below the horizon, and therefore they are just as long above it as below it.
129. An observer on the equator has no circle of perpetual apparition or occultation, because all the stars, together with the Sun and Moon, rise and set to him every day. But, as a bare view of the figure is sufficient to shew that these two circles DL and MO are just as far from the poles N and Sas the observer at i (or one opposite him at o,) is from the equator ECQ; it is plain, that if an observer begins to travel from the equator toward either pole, his circle of perpetual apparition rises from that pole as from a point, and his circle of perpetual occultation from the other. As the observer advances toward the nearer pole, these two circles enlarge their diameters, and come nearer to one another, until he comes to the pole; and then they meet and coincide in the equinoctial. On different sides of the equator, to observers at equal distances from it, the circle of perpetual apparition to one is the circle of perpetual occultation to the other.
Why the 130. Because the stars never vary their distances stars al. from the equinoctial, so as to be sensible in an age, scribe the the lengths of their diurnal and nocturnal arcs are alsame par- ways the same to the same places on the Earth. But allel of *motion,
as the Earth goes round the Sun every year in the and the ecliptic, one half of which is on the north side of Sun a dif- the equinoctial, and the other half on its south side,
the Sun appears to change his place every day; so as to go once round the circle YCX every year, Ø 114. Therefore while the Sun appears to advance northward, from having described the parallel ab 1 touching the ecliptic in X, the days continually lengthen and the nights shorten, until he comes to j, and describes the parallel y z x; when the days are at the longest and the nights at the shortest: for then
as the Sun goes no farther northward, the greatest Plate II.
he describes smaller diurnal and greater noc. turnal arcs or portions of circles every day; which causes the days to shorten and the nights to lengthen, until he arrives again at the parallel a b X; which having only the small part a b above the horizon MCL, and the great part 6 X below it, the days are at the shortest and the nights at the longest : because the Sun recedes no farther south, but returns northward as before. It is easy to see that the Sun must be in the equinoctial E C Q twice every year, and then the days and nights are equally long; that is, 12 hours each. These hints serve at present to give an idea of some of the appearances resulting from the motions of the Earth : which will be more particularly described in the tenth chapter.
131. To an observer at either pole, the horizon Fig. I. and equinoctial are coincident; and the Sun and stars Parallel,
oblique, seem to move parallel to the horizon : therefore such
and right an observer is said to have a parallel position of the spheres, sphere. To an observer any where between either what pole and equator, the parallels described by the Sun and stars are cut obliquely by the horizon, and therefore he is said to have an oblique position of the sphere. To an observer any where on the equator the parallels of motion, described by the Sun and stars, are cut perpendicularly, or at right angles, by the horizon; and therefore he is said to have a right position of the sphere. And these three are all the different ways that the sphere can be posited to the inhabitants of the Earth.