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the stars change their places. The sun itself is observed to turn round. If we investigate still further, we shall discover that these brilliant orbs, which occupy the heavens, are all of them worlds, some of them larger and some smaller, all moving in their appointed courses, and all fulfilling the will of the Almighty Architect who made them.
The study of the heavenly bodies is called astronomy : geography, strictly speaking, is a description of the earth. However interesting and wonderful the subject of astronomy may be, we can only attend to it here, so far as may be necessary to complete our view of the globe we inhabit.
2. Fixed Stars. Constellations. The stars nearest to our earth are grouped into signs or constellations, for the convenience of description ; but the more distant appear to the eye as nebulous patches or streaks of diluted light, which optical instruments have enabled us to resolve partially into clusters of stars, and these analogy teaches us to regard as myriads of suns ; while imagination, ranging through illimitable space, pictures still more remote orbs, whose light has for ages travelled the vast profound, without yet reaching the abodes of man. The great mass of these stars appear to us to remain in the same relative situation, and have therefore been called Fixed Stars, although it is probable that they are all in a state of motion. Their number seems to be beyond calculation, their distance from the earth is too great to be measured by human skill ; but mathematical considerations show us, that it cannot be so 'small as nineteen trillions of miles ; how much larger it may be we know not.
3. Solar System. But the celestial bodies with which we are most familiarly acquainted, are those which are called planets or wanderers, and which revolve around the sun as their common centre of gravity. These bodies, of which the terraqueous globe is one, together with several secondary planets which revolve around the larger, an unknown number of comets, and the Sun itsell, the great central mass, form what is called the Solar System.
(1.) Primary Planets. Those planets which revolve around the sun are called primary planets, and the number at present known is ten, beside our Earth, but it is not at all improbable that there may be others yet undiscovered. Four of them are remarkably large and brilliant. Venus, Mars, Jupiter, and Saturn ; another, Mercury, is also visible to the naked eye as a large star, but, on account of its nearness to the sun, is seldom conspicuous; a sixth, Uranus or Herschel, is barely discernible without a telescope ; and four others, Ceres, Pallas, Vesta, and Juno, are not visible to the naked eye. Five of these planets, Mercury, Venus, Mars, Jupiter, and Saturn, have been known and named from the earliest ages of history; Uranus or Herschel was discovered by Herschel in 1781, and the other four have been discovered still more recently. Saturn is distinguished from the other planets by its being surrounded by two luminous rings concentric with itself, which are visible only by means of a telescope.
(2.) Satellites. The smaller bodies, which revolve round several of the primary planets, and probably serve as moons to their inhabitants, are called secondary planets or satellites. The number of satellites certainly known is fourteen, four more are suspected, and it is not impossible that the number may even be larger ; of these, one, the moon, belongs to the earth, four to Jupiter, seven to Saturn, and certainly two, probably five or six, to Uranus.
(3.) Comets. A third class of bodies belonging to the solar system, are the comets, with the nature of which we have but slight acquaintance. Their actual number is unknown, but is supposed to amount to some thousands. They generally consist of a large and splendid, but ill-defined nebulous mass of light, called the head, which is usually much brighter towards the centre, and offers the appearance of a vivid nucleus, like a star or planet.
planet. From the head, and in a direction opposite to that in which the sun is situated from the comet, appear to diverge two streams of light, which grow broader and more diffused at a distance from the head, and which sometimes unite at a little distance behind it, and some
times continue distinct for a great part of their course ; this is called the tail Some comets, however, have no tail, and some have as many as five or
six. The length of these appendages is sometimes immense ; that of the comet of 1680 was 40 million leagues. These bodies revolve round the sun in very elliptic orbits, some moving with great velocity, others with extreme slowness, some in a direct, and others in a retrograde course ; but it has been shown that their motions are regulated by the same general laws as those of the planets, and the times of the revolutions of several are known.
4. Rotation of the Sun and Planets. From the phenomena of the spots, which, by the aid of a telescope, are visible on the sun's disc, we are led to the conclusion, that that body revolves from east to west on an axis, in about twenty-five days and a half. If a spot is carefully watched, when first discovered on the eastern edge of the disc, it will be found to move gradually toward the centre, and thence toward the western edge, until at length it entirely disappears in that direction ; and in about fourteen days it is often seen again on the eastern side. That the moon, and Mercury, Venus, Mars, Jupiter, and Saturn, have each a rotation from west to east is inferred in like manner from the spots that are seen to traverse their discs. The moon always presents the same side to the earth, and must, therefore, revolve on her axis in the same time in which she is carried round the earth by her revolution in her orbit. In the remaining planets no appearances have been discovered which enable us to ascertain whether or not they revolve on axes; though from analogy it is highly probable that they do.
5. Apparent Annual Motion of the Sun. Zodiac.
Zodiac. Beside the apparent daily motion of the sun round the earth, he also appears to move eastward among the fixed stars. The circle which the plane of the path or orbit thus annually described, marks out on the sphere of the heavens, is called the ecliptic. It passes through twelve constellations, which are called the twelve signs. This has given rise to the division of the ecliptic into twelve equal parts, which are also called signs, and each of which of course contains 30 degrees, or one-twelfth of a whole circle of 360 degrees. The twelve signs are contained in a zone or belt of the starry heavens, 47 degrees in breadth, called the Zodiac. The names of these constellations, with the characters by which they are usually denoted, are as follows:
6. Equinox. Solstice. A circle of the celestial sphere, whose plane passes through the earth's centre, and is perpendicular to its axis, is called the celestial equator; this circle is cut by the ecliptic in two opposite points, called the equinoctial points. The first point of Aries coincides with the point of the vernal equinox, and the first point of Libra with that of the autumnal equinox. As the sun on leaving the point of the vernal equinox advances in the ecliptic, his meridian altitude above our horizon daily increases, and hence arises a gradual increase in the length of the day, until the sun reaches his greatest altitude, and the day acquires its greatest length ; the point of the ecliptic at which the maximum takes place is called the Summer Solstice. Having reached this point the sun now turns back towards the equator, which he crosses at the point of the autumnal equinox ; his meridian altitude and the length of the days are now gradually diminishing, and the point at which they become least is called the Winter Solstice. The vernal equinox falls about the 21st of March, the summer solstice on the 21st of June, the autumnal equinox about the 23d of September, and the winter solstice the 22d of December.
Sun, Mercury, Venus, Earth, Mars, Vesta, Juno,
25,05 882,000 Ceres,
1681.40 262,000,000 87.97 36,000,000 1,01 3,200 Pallas,
1686:54 263,000,000 224.70 68,000,000 .97 7,800 | Jupiter,
4332-58 | 484,000,000 365.26 95,000,000 1 7,920 Saturn,
10,759-22 900,000,000 686.98 143,000,000 1.03 4,100 Herschel or Uranus, 30,686.82 1820,000,000 1325.74 224,000,000
27.32 1592.66 | 233,000,000 ?
35,000 27:32 2,160
The following illustration of the relative magnitudes and distances of the parts of our system, as stated in the above table, is given by the celebrated astronomer Herschel. "Choose," says he, “any well-levelled field or bowling-green. On it place a globe, two feet in diameter ; this will represent the sun ; Mercury will be represented by a grain of mustard-seed, on the circumference of a circle 164 feet in diameter for its orbit ; Venus a pea, on a circle 284 feet in diameter ; the Earth also a pea, on a circle of 430 feet; Mars a rather large pin's head, on
a circle of 654 feet ; Juno, Ceres, Vesta, and Pallas, grains of sand, in orbits of from 1000 to 1200 feet. Jupiter, a moderate-sized orange in a circle nearly half a mile across ; Saturn, a small orange, in a circle of four fifths of a mile ; and Uranus, a full-sized cherry or small plum upon the circumference of a circle, more than a mile and a half in diameter.”
II. THE EARTH, ITS FIGURE, DIMENSIONS, &c. 1. The Earth. The earth is a planet, of a globular shape, and forming very nearly a perfect globe or sphere. Like the other planets it has two motions ; its rotation on its own
axis, completed in about 23 hours and 56 minutes, causes the apparent daily revolution of the sun and heavens round the earth, and thus produces the alternation of day and night; its motion in its orbit, which carries it round the sun in about 365 days and 6 hours, produces the alternation of the seasons. That the earth is round, is proved by several considerations, such as the shape of its shadow on the moon during an eclipse; the fact that it is constantly circumnavigated, so that a ship, by steering in a general direction, either eastward or westward, arrives again at its point of departure, and also by the
appearances, exhibited by vessels at sea. At a short distance the hull of the vessel is sunk below the horizon ; a little further the lower sails disappear, and then the topsails.
2. Axis, Poles. The axis of the earth is an imaginary line passing through its centre, and about which it revolves; the extremities of the axis are called the poles ;
the north pole is called ArcIllustration of the sphericity of the Earth.
tic pole, from its being in the direction of the Great Bear (in Greek Arctos); the south pole, the Antarctic.
3. Equator, Meridian. An imaginary great circle, passing round the earth from east to west, and equally distant from both poles, is called the equator ; imaginary great circles drawn round the earth from north to south, passing through the poles, and intersecting the equator at right angles, are called meridians.
4. Latitude and Longitude. The relative position of a place on the earth's surface is determined by its distance north or south of the equator, and its distance east or west of any given meridian, called the prime meridian ; on English and American maps and globes the meridian of Greenwich in England, is generally assumed as the prime meridian ; but American geographers often adopt that of Washington, and other nations those of their respective capitals. Distance from the equator is called latitude; distance from the prime meridian is called longitude.
5. Degrees, fc. The geographical measure of distance is a degree or 360th part of a great circle of the earth ; the degree is divided into 60 minutes and the minute into 60 seconds. Circles passing round the earth parallel to the equator at given distances from each other, whether of one, five, or ten degrees, are called parallels of latitude, and serve to show at what distance from the equator are the points through which they pass. In the same way meridians are drawn round the earth's surface from north to south at the same distances.
6. Tropics. In spring and autumn, the sun appears to move round the earth over the equatorial regions ; but in summer the sun appears to be 23} degrees ncrth, and in winter the same distance south of the equator. Circles passing round the earth at these points are called tropics. That at the south of the equator being the tropic of Capricorn, and that at the north the tropic of Cancer.
7. Polar Circles. When the sun is in the tropic of Cancer, he is not visible round the Antarctic pole for a distance of 23} degrees ; and on the contrary, when he is in the tropic of Capricorn, he is not visible for the same distance round the Arctic pole. Circles drawn round the poles at that distance, are called respectively the Arctic and Antarctic circle.
8. Zones. These circles and the tropics divide the surface of the earth into five bands or zones; that which lies between the tropics, on both sides of the equator is called the torrid zone ; the band between the tropic of Cancer and the Arctic circle, is called the northern temperate zone, and that included within the Arctic circle the northern frigid zone. Between the tropic of Capricorn and the Antarctic circle, is the southern temperate zone, and within the Antarctic circle, the southern frigid zone.
9. Dimensions and Divisions of the Earth. The mean diameter of the earth is 7,912 miles ; its circumference at the equator nearly 24,900 miles ; and its area 198 million square miles. The surface is divided into land and water ; the former occupies about 50 million square miles, and the latter 148 millions, being nearly three fourths of the whole surface.
10. Representation of the Earth. In order to obtain a correct conception of the appearance of the surface of the earth, representations of that surface, on a small scale, ought to be presented to the eye. There are two ways in which the earth may be thus represented to our view ; viz. by globes and by maps. 11. Globes. The appearance of the surface of the earth may be delineated upon the sur
face of a solid having the same figure as the earth.
The necessity of maps arises from large globes being very expensive and inconvenient for use ; while on small ones sufficient details cannot be introduced.
Maps are constructed by making a projection of
the globe, on the plane of some particular circle, supposing the eye placed at some particular point, according to the rules of perspective.
In maps three things are required ; First to show the latitude and longitude of places, which is done by drawing a certain number of meridians and parallels of latitude ; Secondly, the shape of the countries must be exhibited as accùrately as possible ; for real accuracy cannot be obtained by projection, because the map is on a plane surface, whereas the earth is globular ; Thirdly, the bearings of places, and their distances from each other must be shown.
In all maps, the upper part is the north, the lower the south, the right the east, and the left the west. On the right and left the degrees of latitude are marked.
III. LAND AND WATER.
1. Continents. The land surface of the arth is divided into three vast masses called continents, and numerous smaller tracts, called islands. The eastern continent comprises three great divisions called Asia, Europe, and Africa, lying chiefly upon the north of the equator ; the western continent comprises two divisions called North America and South America ; and the southern continent or New Holland lies to the south of the equator. The eastern continent has an area of 31,500,000 square miles; the western of 14,500,000; and the southern of 4,200,000.
2. Islands. Smaller portions of land scattered over the ocean, or otherwise surrounded by water, are called islands ; some of these form considerable masses, as Borneo and Madagascar, the largest known islands. A number of islands lying near each other is called a group or cluster ; several groups lying near each other are often called an archipelago. Small islands are also called islets ; keys are rocky islets, which are sometimes numerous along the coasts of continents or large islands.