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ASTRONOMY.

ASTRONOMY is a science which treats of the motions and distances of the heavenly bodies, and of the appearances thence arising.

A great variety of opinions prevailed, at different times, among philosophers of former ages, concerning the order and arrangement of the several parts of the universe, or of the positions of those bodies which appear in the heavens. The most eminent Astronomers of the present day suppose the universe composed of an infinite number of systems or worlds; that in every system there are certain bodies moving in free space, and revolving at different distances around a sun, placed in or near the centre of the system, and that these suns and other bodies are the stars which are seen in the heavens.

The STARS are distinguished into two kinds, viz. fixed and wandering. The fixed stars are supposed to be suns in the centre of their systems, shining with their own light, and preserving always the same situations with respect to each other: they are usually divided, according to their apparent splendour, into different classes, called magnitudes; the brightest being denominated stars of the first magnitude; the next to them in brightness, of the second magnitude; and so on to those stars that are scarcely visible to the naked eye, which are termed stars of the sixth or seventh magnitude. In order to assist the memory, Astronomers have divided the heavens into parcels, called constellations: these are a number of fixed stars, lying contiguous, which are supposed to be circumscribed by the outline of some animal, or other imaginary figure. Stars which are not included within the constellations, are called unformed stars. The stars in each constellation are generally distinguished by letters of the Greek alphabet, and some of the principal have proper names: thus the star marked 2, in the constellation of Taurus, is called Aldebaran.

The wandering stars are those bodies within our system which revolve round the sun; they appear luminous, by reflecting the light they receive from the sun, and are of three kinds, namely, primary planets, secondary planets, and comets.

PRIMARY PLANETS are those bodies which, in revolving round the sun, respect him only as the centre of their revolution; their motions are regularly performed in tracks or paths, called ORBITS, which are nearly circular and concentric with each other.

A SECONDARY PLANET, called also a SATELLITE or MOON, is a body which, while it is carried round the sun, also revolves round a primary planet, which it respects as its centre.

COMETS are a kind of planets which move round the sun in very eccentric orbits, and in various directions, having vast atmospheres about them, and tails of a hairy or nebulous appearance, especially when they are near the sun.

OF THE SOLAR SYSTEM.

The SOLAR SYSTEM is that in which our earth is placed, and in which the sun is supposed to be fixed near the centre, with several bodies similar to our earth revolving round him at different distances. This hypothesis, which is the only one that can explain all the phenomena of the heavenly bodies, is said to have been first taught by Pythagoras; after whose time it remained many ages in oblivion, until it was revived, in the beginning of the sixteenth century, by Copernicus, and has since been fully established on the firm basis of demonstration by the immortal Newton.

There are eleven primary planets in our system, accompanied by at least sixteen satellites. The names of the primary planets, with their characters, reckoned in order from the Sun O, are as follow:-Mercury, Venus f, Earth, Mars 8, Vesta, Juno, Ceres ?, Pallas &, Jupiter 4, Saturn h, and Uranus, Herschel, or Georgium Sidus (See Plate VI. Fig. I.)

Mercury and Venus are called inferior planets, because their orbits are within the Earth's; the others are called superior planets, as their orbits include that of the Earth.

The primary planets, accompanied by their satellites, revolve round the sun from west to east, in various portions of time, which are called their periodic revolution, or annual motion; the planes of their orbits all pass through the centre of the sun, but are inclined more or less to that of the earth, crossing it in two points, called Nodes.

Although to an observer placed in the sun, the planets would appear to move in due order about him, from west to east, yet, since the earth is not in the centre of the system, their apparent motions in the heavens are very irregular: sometimes they appear to move from west to east, and then to stand still; then they seem to move from east to west, and after standing some time, they again move from west to cast, and so on continually. The motion of a planet from west to east is called the direct motion, or according to the order of the signs. The contrary motion from east to west is called retrograde; and when the planet appears to stand still, it is said to be stationary.

The situation of a planet in the heavens, as it would appear if seen from the sun, is called its heliocentric place; and as seen from the earth, its geocentric place.

The angular distance of a planet from the sun is termed its elongation. When a planet is in the same part of the heavens with the sun, those bodies are said to be in conjunction; and when their angular distance is 180°, in opposition. The same terms are used with respect to any other two celestial bodies.

We now proceed to give a more particular description of the sun and the planets.

The SUN, that great fountain of heat, light, and vegetation, is an immense spherical body, placed near the centre of the orbits of all the planets; its diameter is about 882,000 English miles, and it turns round its axis in

MERCURY is, of all the primary planets, the nearest to the sun: his diameter is about 3140 miles, and mean distance from that luminary above 36 millions of miles. His periodic revolution is performed in 87 days

23 hours.

To a spectator on the earth this planet keeps so near the sun, that we can seldom discern him without the aid of telescopes: he appears sometimes a little before sunset, and at other times a little after sunrise.

VENUS is the next planet in the order of the system, and is distinguished by her superior brilliancy; her diameter is 7687 miles, her mean distance from the sun nearly 68 millions of miles, and her periodic revolution is performed in 224 days 17 hours.

This planet appears to us always near the sun, although she recedes from him almost double the distance of Mercury; when she is in that part of her orbit which is west of the sun, she rises before him in the morning, and is called Lucifer, or the morning star; and when she is in the eastern part of her orbit, she shines in the evening after he sets, and is called Vesper, or the evening star.

The EARTH, or planet which we inhabit, is about 7916 English miles in diameter, and is about 95 millions of miles from the sun; it performs a revolution through its orbit in 365 days 6 hours, which period is called a year, and revolves round its axis, from west to east, in 24 hours, which occasions the apparent diurnal motion of the sun, and all the heavenly bodies round it, from east to west, in the same time; it is of course the cause of their rising and setting, of day and night. The axis of the earth is inclined 23° 28′ from a perpendicular to the plane of its orbit, and keeps in a direction parallel to itself throughout its annual course, which causes the return of spring and summer, autumn and winter. Thus the diurnal motion gives us the grateful vicissitude of day and night; and the annual motion, the regular succession of the seasons.

VI. Figure 2.)

(See Plate

The earth is attended by a satellite called the Moon, whose diameter is about 2160 miles, her mean distance from the centre of the earth above 237,000 miles; she goes round her orbit in 27 days 8 hours, revolving round her axis in the same time; but the interval between each new moon is 29 days; the former of these periods is termed a periodic month, and the latter a synodic month, or lunation.

As the moon, like the other planets, is an opaque body, and borrows her light from the sun, only one hemisphere is enlightened by the solar rays: hence she puts on various appearances, called phases, during her monthly course round the earth, as her illuminated side is more or less turned towards us: when she is in the same part of the heavens as the sun, her dark side being turned towards us, she is invisible, and this part of her period is called the change, or time of new moon; in a few days after, as she advances to the eastward of the sun, we see a small part of her enlightened face, and she assumes a horned appearance, the cusps or points being turned from the sun towards the east. When she has advanced 90° to the eastward of the sun, we then see half her illuminated face, and she is then said to be in her first quarter; as she proceeds on her journey, more of her enlightened

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