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through the east and west points of the horizon is called the prime vertical, while that which passes through the north and south points of it is called the meridian. All these secondaries of the horizon pass through the zenith and nadir, which are two points, directly over the head and under the feet of the spectator.

The foregoing explications being given, we now proceed to consider the orbits, nodes, distances and periods of the planets, which are represented in the following table.

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1 20

4332 12 20

494,990,976

9 56

Saturn,

21 50

2 33 30

10759

6 36

907,956,130

10 32

Herschel,

13 30

0 48 10

83 years.

1898,967,700 the earth's dist. 100 mill. 1908,180,000 S by Herschel.

THE SUN.

THE Sun, being by far the largest body belonging to our system, must necessarily possess the center, and by its superior attraction, regulate all the motions of the planets and comets. It would require 1,383,470 globes, each as large as our earth, to be moulded together, to form one as large as the sun. We are not however to suppose that he invariably occupies thé same point of space, so as never to move out of it. While the planets and comets move round him, he moves round the common center of the solar system, which however is not far from his center; because he is so much greater than all the bodies together, which move round the same center of gravity. Hence in common language we generally say that the planets move round the sun, at the center of the system; when, in strictness of speech, both they and the sun revolve round one common center of gravity; which is, however, not so far from his center as to his surface, and which must vary its distance from his center continually, according to the positions of the planets and comets from him.

In estimating the magnitude of the sun, I have considered it as a globe, although to the naked eye, and even through a good telescope, it appears but a circular plane. We arrive at the certainty of its globular figure from the appearance and motion of the spots which are often seen by a telescope upon his surface. They all have a regular motion across his disk from east to west, moving slowly when they first appear on the eastern edge, and gradually quickening their velocity as they approach the center, and then growing slower as they come near to the place of their disap

pearance on the western limb. Besides this, every spot appears broadest about the middle of its passage over the disk, and narrowest towards the edges, although it still preserves the same length. Now these are the appearances, which should necessarily take place, upon the supposition of the globular figure of the sun, and of there being thin broad substances adhering to his surface, or floating in his atmosphere near to his body; and therefore they warrant our conclusion that the sun is not a luminous plane, but a solid sphere. After these spots have passed over the disk of the sun, they dis: appear for the same time, which they spent in passing, and many of them reappear in the same place, and pass over him in the same manner as they did before. This has enabled us also, to know, not only that he has a rotation round his own axis, but also the time of this rotation, which is accomplished in about 25a, 15', 16', from a fixed star to the same again.

It was found, from the mean of a number of observations, that the solar spots return to the same place again, after 27, 12h, 20', but in that time the earth had moved the same way in her orbit 26°, 22′, therefore say as 360°+26°, 22′=386°, 22′: 360°::27a, 12h, 20, 25, 15, 16', the true time of the sun's rotation on his axis, as it would be seen from a fixed star. The observations above referred to, were made in the month of May, when the paths of the spots over the sun's disk were nearly in a right line.

By observations on the apparent paths of the solar spots, astronomers have also found, that the sun's equator is not coincident with the ecliptic, but is inclined to it in an angle of 7°, 30', and that it cuts the ecliptic in the eighth degree of Gemini and Sagittary, which are therefore called the sun's nodes. When the

earth is in either of these nodes, the sun's equator, if visible, would appear to be a straight line. The earth, in the present century is in the 8th degree of Sagittary on the 18th of May, and in the other node on the 19th of November. From the 18th of May, the sun's equator begins to appear as a semi-ellipsis, convex towards the south pole, till the 20th of August, when the ellipsis is widest; from that time it grows narrower, till the ellipsis degenerates into a straight line on the 19th of November. After this time the solar equator becomes convex towards the north pole, with an increasing curvature till the 15th of February, when the curvature is greatest, and then it grows less again till the 18th of May.

As all the solar spots are carried round him in circles parallel to his equator, their apparent paths over his disk will be right lines in May and November, when the earth is in one of the nodes, and in all other months, they will be elliptic curves.

The spots that appear on the face of the sun are by no means permanent; few of them continuing during the time of a complete revolution; some appearing on the eastern edge of the sun and vanishing in a few days, others arising near the middle of the disk, while others split and divide into a number; and on the contrary many contiguous spots have been observed to run together, and form one of larger dimensions. Many have been measured, whose diameters have been found to be three or four times the diameter of the earth. All these spots contain a nucleus of various figures and dimensions, intensely black, and surrounded with an infinite number of small specks, each as black as the neucleus, but very small, separate, and distinct from each other, resembling black sand thinly

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