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Fig. 1, plate 1, represents the solar system, wlierein O denotes the sun ; AB, the circle which the nearest planet, Mercury, describes in moving round it; CD, that in which Venus moves; FG, the orbit of the earth; HK, that of Mars; IN, that of Jupiter; OP, that of Saturn; and QR, that of the Georgium Sidus. Beyond this are the starry heavens.
The sun and the planets are sometimes expresse: by marks or characters, instead of writing their names et length. The characters are as follow: 0 the Sun, $ Mercury, | Venus, the Earth, ô Mars, # Jupiter, h Saturn, y Georgium Sidus.
OF THE SUN.
The sun is the centre of the system, round which the rest of the planets revolve. It is the first and greatest object of astronomical knowledge, and is alone enough to stamp a value on the science to which it belongs. The Sun is the parent of the seasons ; day and night, summer and winter, are among its surprizing effects. All the vegetable creation are the offspring of bis beams; our own lives are supported by its influence. Nature revives, and puts on a . new face, when it approaches nearer to us in spring; and sinks into a temporary death at his departure from us in winter.
Hence the Sun was with propriety called by the ancients cor cæli, the heart of heaven; for, as the heart is the heart of the animal system, so is the Sun the centre of our universe. As the heart is the
fountain of the blood, and the centre of heat and motion; so is the Sun the life and heat of the world, and the first mover of the mundane system. When the heart ceases to beat, the circuit of life is at an end; and if the Sun should cease to act, a total stagnation would take place throughout the whole frame of nature.
The Sun is placed near the centre of the orbits of all the planets, and turns round his axis in 25 days. His apparent diameter, at a mean distance from the earth, is about 32 minutes 12 seconds.
Those who are not accustomed to astronomical calculation, will be surprized at the real magnitude of this luminary; which on account of its distance: from us, appears to the eye not much larger than the moon, which is only an attendant on our earth. When looking at the Sun, they are viewing a globe, whose diameter is 890,000 English miles; whereas the earth is not more in diameter than 7970 miles: so that the Sun is about 1,392,500 times bigger than the earth. Thus, as it is the fountain of light and heat to all the planets, so it also far surpasses them in its bulk.
If the Sun were every where equally bright, his rotation on his axis would not be perceptible; but by means of the spots which are visible on his pure and lucid surface, we are enabled to discover this motion.
When a spherical body is near enough to appear of its true figure, this appearance is owing to the shading upon the different parts of its surface; for
as a flat circular piece of board, when it is properly shaded by painting, will look like a spherical body; so a spherical body appears of its true shape, for the same reason that the plane bɔard, in the present instance, appears spherical. But
But if the sphere be at a great distance, this difference of shading cannot be discerned by the eye, and consequently the sphere will no longer appear of its true shape; the shading is then lost; and it seems like a flat circle.
It is thus with the Sun; it appears to us like a bright flat circle, which flat circle is termed the sun's disc. By the assistance of telescopes, dark spots have been observed on this disc, and found to have a motion from east to west; their velocity is greater when they are at the centre, than when they are near the limb. They are seen first on the eastern extremity, by degrees they come forwards towards the middle, and so pass on till they reach the western edge; they then disappear; and after they have lain hid about the same time that they continued visible, they appear again as at first. By this motion we discover not only the time the sun employs in turning round his axis, but also the inclination of his axis to the plane of the ecliptic.
* The young
observer may view the spots of the sun with a refracting telescope of two or thrce feet, or a reflecting one of 12 inches, 18 inches, or two feet, taking care to guard the eye with a dark glass, to take off the glaring light; or the image or pic. ture of the sun, with his spots, may be thrown into a dark room, through a telescope, and received upon a piece of paper placed acarer or further from the glass at pleasure.
The page of history informs us, that there have been periods, when the sun has wanted of its accustomed brightness, shone with a dim and obscure light for the space of a whole year. This obscurity has been supposed to arise from his surface being at those times covered with spots. Spots have been seen that were much larger than the earth.
The sun is supposed to have an atmosphere, which occasions that appearance which is termed the zodical light. This light is seen at some seasons of the year, either a little after sun-set, or a little before sun-rise. It is faintly bright, and of a whitish colour, resembling the milky way. In the morning it becomes brighter and larger, as it rises above the horizon, till the approach of day, which diminishes its splendor, and renders it at last invisible. Its figure is that of a flat or lenticular spheroid, seen in profile. The direction of its longer axis coincides with the plane of the sun's equator. But its length is subject to great variation, so that the distance of its summit from the sun, varies from 45 to 120 degrees. It is seen to the best advantage about the solstices. It was first described and named by Cassini, in 1683; it was noticed by Mr. Childrey, about the
THE INFERIOR PLANETS, MERCURY AND
OF MERCURY. $
Of all the planets, Mercury is the least; at the same time, it is that which is nearest the sun. It is from his proximity to this globe of light, that he is so seldom within the sphere of our observation, being lost in the splendor of the solar brightness ; yet it emits a very bright white light. It is oftener seen in those parts of the world, which are more southward than that which we inhabit; and oftener to us than to those who live nearer the north pole; for the more oblique the sphere is, the less is the planet's elevation above the horizon.
Mercury never removes but a few degrees from the sun. The measure of a planet's separation, or distance, from the sun, and is called its elongation. His greatest elongation is never more than 28 degrees, or about as far as the moon appears to be from the sun, the second day after new moon.
In some of its revolutions, the elongation is not more than 18 degrees.
Mercury is computed to be 37 millions of miles from the sun, and to revolve round him in 87 days, 23 hours, and nearly 16 minutes, which is the measure of its
about one fourth of our's.
fourth of our's. As from the nearness of this planet to the sun, we neither know the time it revolves round its axis, nor the inclination of that axis to the plane of its orbit, we