the ecliptic at an angle of 66 degrees 31 min. 50 seconds. Let the four globes in the figure represent the Earth in four different equidistant points of her orbit, and let the line joining the globes No. 1 and 3 represent the line of the intersection of the plane of that orbit with the plane of the equator. On the globe of the Earth N is the north pole, and S is the south pole; NS is the axis, AC is the arctic circle, TR the antarctic circle, is the tropic of Cancer, V the tropic of Capricorn; EQ is the equator, forming with the ellipse ECLP at the Earth's center an angle of 23° 28′ 10′′: the Sun is placed in the center of the Earth's orbit and on the line of interscetion of the ecliptic and the plane of the cquator.

When the Earth is in the position No. 1. at the point of intersection of these two planes, her axis, always perpendicular to that of her equator, must of course be likewise perpendicular to the place of her orbit, which passes through the Sun, whose light must then illuminate the whole extent of her body from the north to the south pole; the rays from his centre falling perpendicularly on the Earth's equator : if the Earth revolve on its axis in this position, all parts of her surface will be equally exposed to the Sun's rays, and consequently the day and the night will be of equal length all over the globe; hence, the Earth is at such times said to be in the equinoctial points of her orbit, and the periods when this happens are called the equinoxes; the one occurring about the 20th of March, when the Earth, if observed from the Sun, would be entering the sign Libra, and the other about the 23d of September, when the Earth would appear at the beginning of the sign Aries. But when the Earth is in any point of her orbit the Sun must appear in another point diametrically opposite to it; when therefore the Earth is in the beginning of Libra, the Sun will apparently, as seen from her, be in the begining of Aries; and when the Earth is entering Aries, the Sun will be apparently entering Libra :

hence,

hence, it is in customary language said, that about the 20th of March the Sun enters the sign Aries, and about the 23d of September he enters Libra. The first of these periods is the vernal and the last is the autumnal equinox.

Let now the Earth proceed in her annual course until she arrive in the position No. 2, which is distant from the former one fourth part of her orbit, corresponding to the beginning of the sign Capricorn, when the Sun will appear to be entering the sign Cancer. Her axis invariably preserving the same position in all parts of her orbit, or in other words, being every where parallel to itself, it will now (as may be seen in the figure) have acquired a different position with regard to the Sun, leaning as it were towards him, so that instead of being in the plane, passing through her equator, he will be in one, passing through a point on her surface, 23 degrees to the northward of it, as at ; on which a ray from his centre will fall perpendicularly, and his light still, as in the position No. 1. illuminating one half of the Earth's surface, the illumination will extend beyond the north pole N, just as far as the perpendicular ray from the Sun's centre falls to the northward of the equator, that is 23 degrees; and consequently, instead of reaching to the south pole S, the illumination will fall short of it by 231 degrees, and the boundary between light and darkness will be indicated by the line or circle AR (No. 2). As the plane of this bounding circle passes through the centre of the Earth all places situated on the equator will still have the day and the night of equal length: but all places situated within the arctic circle, described at the distance of 23 degrees round the north pole, will enjoy continual day-light ; and all intermediate places between the equator and the arctic circle will have the day and the night more or less of equal duration, according to their position towards the first or the last of those circles thus if LOD be the parallel of latitude on

which London lies (see No. 2), LO will represent the length or duration of the night, and OD will show the length of the day, when the Earth is entering Capricorn, or when the sun is entering Cancer, which happens about the 21st day of June. The reverse of there appearances occurs in places situated in southern latitude, where the figure shows the nights to be at this juncture all longer than the days, increasing by regular gradation until at the antarctic circle TR, the illuminated part of the Earth's surface ceases, and all places situated within that circle must be involved in constant darkness.

As the Earth in her progress from the vernal equinox, at No. 1, to the position at No. 2, gradually brings the north pole N more and more within the scope of the Sun's rays, so in her course from this last point to the position No. 3, that pole falls regularly back towards the boundary of light and darkness AR, and at last on arriving in the point indicated by No. 3, that boundary coincides with a plane passing through the Earth's axis, and the illumination extends from pole to pole as was the case at the vernal equinox No. 1. When the Earth is in the position No. 2, the progress of the illumination ceases, and hence that period is called the summer solstice (Geography, page 15.) The position of the Earth represented at No. 3, occurs about the 23d of September, when the plane of the circle of light and darkness passing through her axis, the day and the night are of the same length: hence this period is called the autumnal equinox, the Earth then entering the sign Aries, or the Sun apparently entering Libra.

After describing another quarter of her annual orbit the Earth comes to be in the position indicated by No. 4, when entering the sign Cancer the Sun appears to enter the opposite sign Capricorn: this happens about the 22d of December. In this position the axis of the Earth still preserving its direction with respect to itself, the north

pole

pole N is turned away from the Sun and the south pole verges towards him, so that it comes now to be advanced within the illuminated part precisely as much (23 degrees) as six months before in the position No. 2, it was retired. In this state of the globe it is evident that no part contained within the arctic circle AC can by the Earth's diurnal rotation be brought under his rays, while places situated within the antarctic circle TR will never be deprived of his light. All places therefore lying in southern latitude will in this position of the Earth have the day longer than the night proportionally to their distance from the equator, that is to their latitude: the southern hemisphere will consequently be in the enjoyment of summer while the northern is plunged in the depth of winter. The Earth being now arrived at that point of her orbit which is the farthest removed from the plane of her equator she begins to return towards the point where these two planes intersect each other, and the Sun apparently standing still, this period is called the winter solstice. In this manner the Earth continues her course until about the 20th of March, arriving at the first point of Libra, and the Sun appearing to be in the beginning of Aries, the Earth's axis becoming again perpendicular to the line of intersection passing through the Sun, his rays illuminate the whole hemisphere from pole to pole, and the days and nights being of equal. length, the Earth assumes the appearance represented by No 1 in the figure, where we began to consider her annual

course.

The regular succession of night and day, of winter and summer, of spring and harvest, in the other planets of the solar system are produced in the same way as in our Earth.

COMETS. Besides the celestial bodies already described which perform their stated revolutions round the Sun, or round the primary planets to which they are attached,

another

1

another set of bodies are observed to make their occasional appearance in our quarter of the universe, moving with various degrees of rapidity and in paths of a very different nature from those of the solar planets. These bodies are called Comets from a Latin term signifying hair, because they are generally attended by a bright beard or tail, which in whatever part of their course they are observed, is always turned away from the Sun.

The paths in which the Comets move approaches so much to a straight line pointing towards the Sun, that they seem to be intended to fall upon his body; they are however observed to appear a second time on the opposite side of that luminary, receding from him with a velocity at first equal to that with which they approached, and the tail much increased in magnitude and brilliancy. It was long supposed that Comets really did move in straight lines to a point near the Sun's body, from which returning in the same kind of tract they passed beyond the bounds of our system, never more to re-appear; but from observations on a very remarkable Comet which visited us in 1680, Sir Isaac Newton ascertained that its path, in no part a right line, was in fact an ellipse or curve of the same spccies with the paths of the planets, only extremely excentric, the greater diameter bearing a very great proportion to the less, and that the Sun was in one focus of this ellipse while the other was removed at an inconceiveable distance, but still much nearer to the Sun than to any of the fixed Stars, so that the Comet must always continue its revolutions round his body. In proportion to the great elongation of their orbits, the velocity with which comets proceed is subject to great variation, their rapidity when near the Sun being inconceivably greater that when they are in the opposite part of their orbit. Comets have in ignoraut times been supposed to be only temporary meteors formed and kindled in the lower regions of the terrestial atmosphere; many

VOL. II.

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