Imágenes de páginas

the poles, and all places on the globe go equally through the light and dark hemispheres, shewing that the days and nights are then equal at all places of the Earth, the poles only excepted; for the Sun is then setting to the north pole, and rising to the south pole.

Continue moving the globe forward, and as it goes through the quarter A, the north-pole recedes still farther into the dark hemisphere, and the south pole advances more into the light, as the globe

comes nearer to s: and when it comes there at F, Winter

the candle is directly over the tropic of Capricorn, solstice. the days are at the shortest, and nights at the longest,

in the northern hemisphere, all the way from the equator to the Arctic circle; and the reverse in the southern hemisphere from the equator to the Antarctic circle; within which circles it is dark to the north frigid zone, and light to the south.

Continue both motions, and as the globe moves through the quarter B, the north pole advances toward the light, and the south pole toward the dark; 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 Vernal when the globe was at E,) and the days and nights equinos. 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 cquator to the polar circles every year; why there is sometimes no day or night for many turnings of the Eurth, 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.

201. The inclination of an axis or orbit is merely Remark, 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 Plate III. no inclination ; and yet, if we travel 90 degrees from Fig. III. that place, we shall then have a horizon perpendicular to the former, but it will still be level 'to us. And if this book be held so that the * circle ABCD be parallel to the horizon, both the circle abcd, and the thread or axis K, will be inclined to it. But if the 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 its annual course round the Sun, considering its orbit as having no inclination, and its axis as inclining 231 degrees from a line perpendicular to the plane of its orbit, and keeping the same oblique direction in all parts of its annual course; or, as commonly termed, keeping always parallel to itself, \ 196.

Let a, b, c, d, e, f, g, h, be the Earth in eight dif- Plate V. ferent parts of its orbit, equidistant from one another: Fig. 1. N's its axis, N its north pole, s its south pole, and S the Sun nearly in the centre of the Earth's orbit, Ø 18.

As the Earth goes round the Sun according

All circles appear elliptical in an oblique view, as is evident by looking obliquely at the rim of a bason. For the true figure of a circle can only be seen when the eye is directly over its centre. The more obliquely it is viewed, the more elliptical it appears, until the eye be in the same plane with it, and then it appears like a straight kine.

Plate V.


to the order of the letters abcd, &c. its axis N skeeps

the same obliquity, and is still parallel to the line A concise M Ns. When the Earth is at a, its north pole inview of the clines toward the Sun S, 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

e 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 с or g, its 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 s, 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.

203. From this oblique view of the Earth's orbit, let us suppose ourselves to be raised far above it, and placed just over its centre S, looking down upon it from its north pole; and as the Earth's orbit differs but

very little from a circle, we shall have its 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 shewn in

heavens. The Earth is shewn in eight different another view of positions in this circle: and in each position Æ is the the Earth equator, T the tropic of Cancer, the dotted circle orbit.

The seasons

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 toward one part of the heavens, as it does in the figure toward the right-hand side of the plate.

When the Earth is at the beginning of Libra, namely, on the 20th of March in this figure (as at & in Fig. I.) the Sun S, as seen from the Earth, appears at the beginning of Aries, in the opposite part of the heavens*, the north pole is just coming into the light, and the Sun is vertical to the equator; Vernal which, together with the tropic of Cancer, parallel equinox. 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 meridians 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.

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 P comes more and more into the light; the days increase as the nights decrease in length at all places north of the equator ; which is plain by viewing the Earth at l on the 5th of May, when it is in the 15th degree of Scorpio t, and the Sun, as

* Here we must suppose the Sun to be no bigger than an ordinary point (as.) because he only covers a circle half a degree in diameter in the heavens; whereas in the figure he hides a whole sign at once from the Earth.

+ Here we must suppose the Earth to be a much smaller point than that in the preceding note marked for the Sun.

Plate V.

seen from the Earth, appears in the 15th degree of

Taurus. For then, the tropic of Cancer T' is in the Fig. 11. light from a little after five in the morning till almost

a 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 its axis.

When the Earth comes to c, at the beginning of Capricorn, and the Sun, as seen from the Earth appears at the beginning of Cancer, on the 21st of June, as in this figure, it is in the position a in Fig. I; and its north pole inclines toward the Sun, so as to bring all the north frigid zone into the light, and the northern parallels of latitude more into the light than the dark from the equator to the polar circle; and the more so as they are farther from the equator. The tropic of Cancer is in the light from five in the morning till seven at night.; the parallel of London from a quarter before four till a quarter after eight;

and the polar circle just touches the dark, so that the Summer Sun has only the lower half of his disc hid from the solstice. inhabitants on that circle for a few minutes about

midnight, supposing no inequalities in the horizon, and no refraction.

A bare view of the figure is enough to shew, that as the Earth advances from Capricorn toward Aries, and the Sun appears to move from Cancer toward Libra, the north pole advances toward the dark,

which causes the days to decrease, and the nights to Autumnal increase in length, till the Earth comes to the begin. Equinox. ning of Aries, and then they are equal as before; for

the boundary of light and darkness cuts the equator and all its parallels equally, or in halves. The north pole then goes into the dark, and continues in it until the Earth goes half way round its orbit; or, from the 23d of September till the 20th of March. In the

« AnteriorContinuar »