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torial diameter is about 7925 miles; the polar diameter, 7899 miles; the ratio of the polar to the equatorial diameter being 305 to 306. Her mean diameter is about 7913 miles; her circumference about 25,000 miles; and her surface nearly 197 millions of square miles.

6. Place of the Earth in the Solar System.That portion of the universe, of which the sun is the centre, is called the Solar System. It consists of the sun himself, and a number of planets and comets which revolve around him. The sun is an immense globe, nearly one million four hundred thousand times larger than the earth, placed near the centre of the system, and dispensing light and heat to the dependent bodies. The planets are either primary or secondary. The primary planets revolve round the sun in elliptical orbits: they are eleven in number, viz. Mercury, Venus, the Earth, Mars, Vesta, Juno, Ceres, Pallas, Jupiter, Saturn, and Uranus. The secondary planets revolve round other planets, by whom they are also carried round the sun they are called satellites or moons, and are eighteen in number. The Earth has one, Jupiter four, Saturn seven, and Uranus six. The distance of the earth from the sun is 95 millions of miles.

7. Motions of the Earth.-The earth has two principal motions. Once every 24 hours she revolves on her axis-an imaginary line, passing through her centre from north to south, the extreme points of which are called poles. In consequence of the earth's sphericity, one-half of her surface is enlightened by the sun, which causes day; while the other half is involved in the darkness of night. In consequence

of her diurnal rotation, every portion of her surface enjoys the succession of day and night. This rotation of the earth on her axis, which is from west to east, gives the sun and all the heavenly bodies the appearance of moving from east to west; and hence we speak of their rising, culminating, and setting, in that direction.

8. The earth also travels round the sun, in the course of a year, in an orbit or path which is, strictly, an ellipse; but at the same time so nearly circular that we may regard it as such. The length of this year is very nearly 365 days 6 hours.

9. Geographical Circles.-Geographers find it convenient to suppose various circles to be drawn on the earth's surface. Most of these are derived from astronomy. They are drawn on maps and artificial globes.

10. Those which have the same centre as the earth itself are called great circles; these divide the globe into two equal parts or hemispheres. All others are called small circles; they have independent centres, and divide the earth into two unequal parts. Every circle, great or small, is supposed, by English mathematicians, to be divided into 360 equal parts, called degrees; each degree is subdivided into 60 equal parts, called minutes; and each minute into 60 equal parts, called seconds. Degrees are marked (°), minutes ('), and seconds ("). These degrees vary in length according to the magnitude of the circle. On a great circle, a degree is equal to 60 geographical miles, or about 69 English miles.

11. That great circle, which at every point is

equally distant from the poles, is called the Equator. It divides the earth into the northern and southern hemispheres. The earth's orbit, which is the sun's apparent annual path, is another great circle, called the Ecliptic. The earth's axis is not perpendicular to her orbit, but is inclined to it at an angle of about 6610; consequently the ecliptic is inclined to the equator at an angle of about 2310. This angle is not, strictly, invariable; but here we shall treat it as such. These two circles intersect one another at points which are known as the equinoctial points. These points are not, strictly, fixed; but here we shall treat them as such.

12. If now we follow the sun in his apparent path from the vernal equinox, we shall see that he will recede northward from the equator until he has reached the distance of 2310, as measured on a line passing from the north to the south pole through the equator. At this point he will appear, first, to stand still for a while, and then to turn back southwards towards the equator. Hence this point is called the summer solstice; and the circle which passes through it, parallel to the equator, is called the tropic of Cancer.1 He will next appear to cross the equator

1 The early astronomers referred the sun's apparent motion among the stars to certain constellations, included in a zone or band, which they called the Zodiac. In consequence of a certain slow motion, called the precession of the equinoxes, the signs of the ecliptic no longer correspond to the constellations: but this does not affect geographical phenomena. The signs of the Zodiac are

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at the autumnal equinox, and thence to travel southward, as far as 2310, measured as before. Here, therefore, we have the winter solstice and the tropic of Capricorn, from which the sun apparently returns towards the spring or vernal equinox.

13. Since the sun, in consequence of the earth's sphericity, enlightens half the globe at any one given time, when he is at the summer solstice he will enlighten the globe 234° beyond the north pole. The circle which marks this illumination is called the arctic circle. In like manner, when the sun is at the winter solstice, he will mark out a corresponding circle, 23° from the south pole, called the antarctic circle.

14. Every great circle passing through the poles, and consequently cutting the equator at a right angle, is called a meridian. Each such circle is called the meridian of the places through which it passes.

15. The earth is apparently situated in the centre of an immense concave sphere, in which the sun, moon, planets, and stars perform their respective apparent courses. A portion only of this sphere is visible at any one time to a fixed spectator: the circle which bounds his view is called the horizon. If he is situated in the northern or southern hemisphere, this will be a small circle, and is called the sensible horizon. If he is situated at the equator, it will be a great circle, and is called the rational horizon. The point immediately or vertically over his head is called the zenith; the point vertically beneath his feet the nadir.

16. The Seasons.-We can now explain the phenomena of the seasons, so far as they depend

on astronomical causes; availing ourselves of the following diagram :—

.S

Here we have the sun in the centre, and the earth, in her orbit, in four positions, 90° apart; viz. on March 21, the vernal equinox; June 21, the summer solstice; September 21, the autumnal equinox; and December 21, the winter solstice. The earth's axis, it will be observed, is always parallel to itself. In the positions V and A, this is inclined neither towards nor from the sun, which accordingly shines over half of the northern and half of the southern hemispheres; in other words, from pole to pole. As the earth revolves on her axis, every point of her surface will describe half its diurnal course in light and half in darkness, so that the duration of day and night will be equal over the whole world. Hence these points are called equinoxes. In the position S, the northern half of the earth's axis is turned towards the sun, which accordingly shines over the

1 The earth's orbit is here supposed to be circular; but seen in perspective by an eye situated towards the bottom of the page, and somewhat elevated above it.

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