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COMETS, MILKY-WAY, ECLIPSES, TIDES.

ASTRONOMY is the science which instructs in the knowledge of the heavenly bodies; as the sun, moon, planets, and fixed stars.

The sun is an immense. globe, the source of light and heat, to which we owe the change of the seasons, the growth of vegetables, and the production of every thing desirable for man.

By the planets are meant stars that revolve round the sun, and receive light from him, being themselves dark opaque bodies the planets, their moons, and the comets form what is called the solar system.

There are seven planets, whose names are Mercury, Venus, the Earth, Mars, Jupiter, Saturn, and the Georgium Sidus; besides four lately discovered, called asteroids, or minor planets; namely, Vesta, Juno, Ceres, and Pallas.

Comets are a kind of planets belonging to our system, and revolving round the sun at very distant periods: they generally appear with tails resembling hair thrown in an opposite direction to the sun; but their real nature and use remain as yet unknown.

The vast sun, to which we are under such sensible obligations for light, heat, life, and vegetation, and without whose genial influence all the earth would become a dark, solid mass of ice, is 900,000 miles in diameter, and the earth is ninety-five millions of miles distant from it.

The sun is, in fact, the centre of a vast system of earths, or globes like the earth, all of which move round his body at immense distances, in periods which include the various seasons to each, and are therefore a year to each. The sun attracts them all, while the laws of motion give them a tendency to go forward in a straight line; and so nicely are the two powers balanced, that they counteract each other, and in consequence the planets move round the sun in an orbit which is nearly circular. The sun has been commonly considered a globe of pure fire, A number of maculæ, or dark spots, by means of a telescope, may however be seen on different parts of his surface.

As the sun is one million times larger than the earth, it is evident that the balance of mutual attraction would not be destroyed if one million of earths moved round the sun, but at present we know only of seven such bodies, some nearer and some farther off than the earth, and some smaller and some larger, called Mercury, Venus, Earth, Mars, Jupiter, Saturn, and Herschel.

The Sun revolving on his axis turns,
And with creative fire intensely burns:
First Mercury completes his transient year.
Glowing, refulgent, with reflected glare;
Bright Venus occupies a wider way,
The early harbinger of night and day;
More distant still our globe terraqueous turns,
Nor chills intense, nor fiercely heated burns ;
Around her rolls the lunar orb of light,
Trailing her silver glories through the night.
Beyond our globe the sanguine Mars displays
A strong reflection of primeval rays;
Next belted Jupiter far distant gleams,
Scarcely enlightened with the solar beams;

With four unfix'd receptacles of light,

He towers majestic through the spacious height:
But farther yet the tardy Saturn lags,
And six attendant luminaries drags;
Investing with a double ring his pace,

He circles through immensity of space.-CHATTERTON.

These several globes so revolving to receive light and • heat from the sun, serve some of them as centres to other

minor globes called moons. These satellites accompany the planet in his tour round the sun, and serve to reflect the sun's light on the planet by night for the use of the inhabitants.

The earth has one moon, 2000 miles in diameter, and quarter of a million of miles distant from the earth. Jupiter, another planet, has four moons: Saturn, seven moons, and

he is also surrounded by a large double ring 30,000 miles distant from his body; and Herschel has six moons.

But if the matter of all these planets and moons were put together, they are not equal to a ten thousandth part of the sun; or rather, it would require ten thousand such masses to make up the bulk of the sun.

The twinkling stars, of which we see so many every clear evening, do not belong to our solar system, but are themselves so many suns to other systems like ours. Each star is supposed to be the centre of its own system, and to have planets, moons, and comets moving round it at immense distances, like those of our solar system!

Bright legions swarm unseen, and sing, unheard

By mortal ear, the glorious Architect,
In this His univeral temple, hung

With lustres, with innumerable lights,

That shed religion on the soul; at once,
The temple and the preacher! O how loud
It calls Devotion! genuine growth of night!
-Devotion! daughter of Astronomy!

An undevout astronomer is mad.-Young.

They are called fixed stars, because they never appear to move; and are so distant, that although the orbit of the earth is twice 95 millions, or 190 millions of miles across, and we are consequently 190 millions of miles nearer to some stars at one time than we are at another time, yet the stars always appear in the same places.

O Nature! all-sufficient! over all!

Enrich me with the knowledge of thy works!
Snatch me to heaven; and show thy wonders there,
World beyond world, in infinite extent,

Profusely scatter'd o'er the blue immense.-THOMSON.

The distance of the nearest of the fixed stars cannot be less than 32 billions of miles, and they are all of them doubtless as far distant from each other. They appear to fill infinite space in shoals of vast systems of stars, and our sun is supposed to be one of that amazing shoal of stars, whose myriads form that bright cloud of light called the Milky-Way.

The student in Nature, who takes an evening's walk to admire the magnificence and the glory of the starry heavens, and who desires to profit by his observations, should learn to class the heavens into particular divisions, and fix on certain points, as a sort of landmarks to direct his attention, By knowing the part of the heavens in which the sun rises, he is able to determine the eastern side; by attending to its situation at noon, he ascertains the south; and by noticing the place of its setting, he determines the western side of the

horizon. He need not be told that the north is opposite to the south. The moment, then, in which he casts his eyes on the sparkling expanse of heaven, he is supposed to be sensible of the bearings of the cardinal points north, south, east, and west.

The next principle to be recognized is, that he sees above his horizon one-half of the whole heavens; that is to say, one-half of the heavens are always visible, or above the horizon, and the other half is below the horizon. He must not expect, therefore, to see all the constellations and planets at once, but only that half which, at the time of observation, is above the horizon.

For the sake of precision and accurate reference, astronomers have supposed the 360 degrees into which geographers divide the surface of the earth, to be extended to the heavens: so that the whole round of the horizon of the heavens is supposed to be 360 degrees, or proportional parts; half is 180 degrees, and a quarter is 90 degrees. And as we see one half of the heavens above the horizon, it is of course 180 degrees from one side of the horizon, in a line passing over our heads, to the directly opposite side; and of course from the point over our heads, called the zenith, it is 90 degrees to the horizon on every side. It may be proper for him at this time to consider, that this general motion of the whole heavens is merely apparent, and is occasioned by the rotation of the earth on its axis in a contrary direction. Of course, if the spectator is moving from west to east, the distant stars will appear to move from east to west. The rising and setting of all the distant heavenly bodies will hence be easily understood. The earth turns completely round every twentyfour hours; every inhabitant of it will therefore be carried round towards all the bodies out of it, and distant from it, every twenty-four hours. Hence the rising and setting of the sun, the succession of day and night, and all the dependent phenomena. This progression of the whole heavens from east to west, the rising of some stars in the east, and the setting of others in the west, are objects which, viewed in this manner, will leave impressions much stronger than the mimic representation of the same phenomena on the celestial globe. The immensity of the great vault of heaven; the still, solemn, uniform motion; the accompanying association of the immeasurable distances; the apparent perpetuity, and the countless numbers of the stars, will fill the mind with reverence and devotion towards the omnipotent, infinite, and eternal Author of the whole.

It happens that there is a star so near the north pole that,

for all ordinary purposes, it may be taken for the north pole itself; and this star may always be found very easily, by means of two other stars which point to it in a right line. During the winter months, these stars, which are in the constellation of the Great Bear, are to be found with the other stars of that remarkable constellation on the eastern side of the pole. They are about six degrees asunder, and the nearest is five times that space, or thirty degrees, from the polar star, at which they seem to point, and are thence called the Pointers.

The north pole star being thus found, it will be a pleasing employment to observe that all the stars appear to move round it, according to their several distances, while it constantly stands still. An hour's contemplation of this star, and of the motions of the rest of the heavens, while it remains an immoveable centre, will teach more to the uninformed in astronomy than a thousand lessons or lectures in the closet.

On a winter's evening, the other remarkable objects in view will be the Pleiades, or seven stars, in the south-east; and below them, a little to the east, the grand constellation of Orion; and still lower, the dog-star, Sirius, the brightest of all the fixed stars. The three bright stars together in a line, called the Belt of Orion, are at about equal distances from the Pleiades and Sirius, that is, about twenty-five degrees from each. Besides remembering this distance, and that of the Pointers, before mentioned, for the sake of occasional comparisons, it will be useful to recollect that the most northern of the three stars in the Belt of Orion is exactly over the equator, so that from that star to the north pole star is exactly ninety degrees.

The Pleiades are in the Zodiac, on the south side, and so is the red star, Aldebaran, near them; and the two bright stars about forty degrees to the left, called Castor and Pollux, or the Twins, are also in the Zodiac, and about five degrees north of the sun's place on the 12th of July. On such an evening the Milky-Way will be seen in the west, as a light cloud, but in truth a mass or shoal of stars, almost infinite in number, but indistinct from their distance.

Next to the sun, the moon is that of the heavenly bodies which the most interests our curiosity. She is 240,000 miles distant from the earth, 2000 miles in diameter, and 6300 miles in circumference. She accompanies the earth in its annual orbit, and during the period goes herself nearly thirteen times round it in an orbit of her own; and turns round her axis exactly in the time she goes round the earth. If a

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