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THE SEASONS.- It is the annual revolution of the Earth, in connection with its slanting axis, that causes the change of the seasons of the year. The differences of the seasons are not dependent, as sometimes is supposed, upon the Earth's nearness to or remoteness from the Sun. In point of fact the Earth is nearest to the Sun during the winter, and farthest from the Sun during the summer, of the Northern Hemisphere.

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But in summer the inclination of the Earth's axis brings the Northern Hemisphere more directly under the Sun's rays, and that for a longer time each day, than in winter. Now it is on the different aspects which the Earth presents to the Sun, making the rays of the latter at one time more nearly perpendicular, at another more oblique, that the phenomena of the seasons really depend.

As the Earth moves round the Sun, it brings each day a new point of its surface directly under the Sun's vertical rays. These points trace out a great circle on the Earth's surface called the Ecliptic, the plane of which cuts the plane of the Equator obliquely (at an angle of 231 degrees).

The two points at which the Ecliptic cuts the Equator mark the two Equinoxes (equal-night points). These occur in the middle of Spring and of Autumn-about the 20th March and the 22nd September. The two intervening middle points, at which the Ecliptic is farthest from the Equator, mark the middle of Summer and of Winter-about the 21st June and 21st December, the longest and the shortest days in the northern year. When the Sun reaches these points, so little change is perceptible in his position for several days, that he is supposed to stand still. These points are therefore called the Solstices (sun-standing points), of mid-summer and mid-winter respectively. As the Sun in his apparent course, after travelling from the Equator, at these points turns towards the Equator again, the circles drawn round the globe parallel to the Equator, and through these points, are called the Tropics (from Greek trepo, I turn), viz., the Tropic of Cancer (northern) and the Tropic of Capricorn (southern). (See Fig. 4.)

At the Summer Solstice, as the Sun's vertical ray is 23 degrees above the Equator, his most oblique ray will reach 23 degrees beyond the North

Pole on the one hand, and will fall 23 degrees short of the South Pole on the other. For some time before and after the summer solstice, the north polar region has no night, and the south polar region has no day. (See Fig. 3.)

Through these points two other circles are drawn round the globe, namely, the Arctic Circle in the north, and the Antarctic Circle in the south.

These four circles divide the globe into five great regions called Zones, or belts, distinguished by differences of climate and of animal and vegetable life. That between the Tropics is called the Torrid (or hot) Zone; those around the Poles are called the Frigid (or cold) Zones. the Temperate Zones.

NORTH FRIGID

ZONE

NORTH

TEMPERATE ZONE

TORRID

ZONE

SOUTH TEMPERATE

ZONE

SOUTH

FRICID ZONE

The intervening regions are called

THE MOON. When the Moon comes to exactly the same part of the sky as the Sun, it hides or eclipses the whole or part of the Sun's disc.* Hence it must be nearer us than the Sun. When the Moon comes to a star, it hides the star. Hence the Moon is nearer us than the stars. It is the nearest of the heavenly bodies. Its average distance from the Earth is 237,000 miles.

Again: when the Moon crosses the Sun, it is a black, dark circle, or part of one. Hence the Moon is not a luminous body like the Sun, but a dark one like the ground we tread on. But, like other dark bodies, it can reflect, or throw back light that falls on it. When the Moon comes to the Sun, you may see it slowly moving across the Sun from the west side to the east. Then a day or so after, in the evening, you may see it a little way east of the Sun, a thin, bright crescent. The bright crescent is always on the side next the Sun; but though the rest is not bright, it is all there. As soon as the Sun is fairly set, you may see the rest of the Moon as a dim gray body.

As the Moon moves away from the Sun, the crescent becomes broader; but the inner edge of the crescent, as well as the outer, always appears circular. This shows that the Moon is not a flat disc, but a globe. A week after crossing the Sun, it appears a half-moon; then it is a quarter of the circle of the heavens from the Sun.

During the second week the Moon appears greater than a half-moon, or gibbous; and at the end of that week it is right opposite the Sun, and is a full-moon. It then begins to come nearer the Sun, and becomes gibbous again. At the end of the third week it is half-moon again, or in its third quarter, as it is called. For the next week it is a crescent, always growing

* This is called an eclipse of the Sun. An eclipse of the Moon is caused by the Sun throwing the shadow of the Earth upon the Moon. This can only occur at full moon, and when the Sun, the Earth, and the Moon are in the same line.

less, as it moves nearer the Sun. At last it is lost sight of in the Sun's brightness, until it reäppears as a new-moon again.

The time taken by the Moon in going round the Earth is called a Month (Old English, Monath; that is, Moon's time). It is exactly 27 days, 7 hours, 43 minutes and 11 seconds. But as the Earth is at the same time moving forward, and carrying the Moon with it, it is 2 days more; or, in all, 29 days, 12 hours, 44 minutes, before the Moon goes through all the above changes and is exactly opposite the Sun again. These changes are called the Phases of the Moon.

As the Moon is a dark body, only one half of it at a time can be bright— the half towards the Sun. At full-moon we are between the Moon and the Sun, as at e in the figure, where S is the Sun and E the Earth. The same side of the Moon is turned to us and to the Sun, and we see the whole of the bright side. When it is gibbous, we see most of the bright side, and some of the

E

PHASES OF THE MOON.

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us is turned towards the Sun, and is bright, as at c and g. When it is a crescent, we see most of the dark side, and only a little of the bright, as at b and h. Lastly, at newmoon only the dark side is towards us, as at a. A body appears smaller the farther off it is; but the full-moon's apparent size does not vary much. Its distance from the Earth must therefore be always nearly the same; and its path, or orbit, must be nearly a circle. A body which appears of the Moon's size at the distance of 237,000 miles, must be 2153 miles in diameter. The Moon's bulk, therefore, is 1-49th of that of the Earth.

THE PLANETARY SYSTEM.-The heavenly body next to the Moon in brightness is the planet Venus. Like the Moon, if watched from night to night, it will be seen to approach or go further from the Sun, and move among the stars, but in a very different time and manner from the Moon. It never appears opposite the Sun, nor farther from him than 47°; but there it stands for a time. It keeps by the Sun, and moves round the Sun, not round the Earth. If looked at with a large telescope when it is farthest from the Sun, it appears like a half-moon, with the bright side to the Sun. Thus, like the Earth and Moon, it is a dark body, and shines by reflecting the light of the Sun.

As it begins to approach the Sun, the half-moon shape wanes into a crescent; therefore it is coming more nearly into a line with us and the Sun. But while the crescent grows narrower, its length, or the apparent diameter of Venus, becomes much greater; and therefore Venus is then coming much nearer us. At last, it goes so near the Sun, that it is lost in his brightness. But on rare occasions, at intervals of about 113 and 8 years,

it is seen as a black spot crossing the face of the Sun. This is called a transit of Venus; it will happen in 1874 and 1882. This shows that the path, or orbit, of Venus round the Sun lies inside the path of the Earth round the Sun, for Venus then comes between us and the Sun. Because Venus thus moves among the stars, it is called a planet (from a Greek word signifying a wanderer); and because it moves in a path between us and the Sun, it is called an inferior or inner planet.

Another and smaller planet, called Mercury, also shows phases like the Moon, and keeps near the Sun, (much closer than Venus,) so that it is not often well seen; and it also makes transits. Hence it moves round the Sun

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in a path inside that of Venus, and is the nearest planet to the Sun, so far as is certainly known,

Another planet is easily known by its very red light. It is called Mars. It never crosses between us and the Sun. Hence its path must lie outside the Earth's; it is therefore called a superior or outer planet. Another planet, Jupiter, is known, in the same way as Mars, to be an outer or superior planet. In the telescope Mars sometimes appears gibbous; but Jupiter shows no phases whatever. It must therefore be farther off than Mars.

There is one other planet, Saturn, that we can see with the naked eye, and its orbit is next outside that of Jupiter. Another planet, Uranus, is barely visible to the keenest eye on the clearest night. It revolves outside Saturn. The farthest out yet known is Neptune.

The distances of the planets from the Sun in millions of miles are :— -Mercury, 35 millions; Venus, 66; Earth, 91; Mars, 139; Jupiter, 476; Saturn, 872; Uranus, 1754; Neptune, 2746.

Between Mars and Jupiter,-at a distance of from 200 to 300 millions of miles from the Sun,-there have been discovered 131 small planets called Planetoids or Asteroids, and new ones are being discovered every year. The largest are Vesta, Juno, Ceres, and Pallas.

Other planets besides the Earth have moons or satellites. Jupiter has four, Saturn eight, Uranus six, and Neptune two. Saturn has three rings, consisting of immense numbers of small satellites or bodies, revolving very swiftly round him, and close together.

QUESTIONS.-To what system does the Earth belong? What is its form? How much is the Earth compressed at each pole? What is such a flattened globe called? Mention proofs of its spherical form. What does the circumference of the Earth measure? What, its diameter? Where is its diameter greatest? At what distance is it from the Sun ?

What motions has the Earth? What depend on these motions? By what circumstance are the effects of these motions modified?

What

What causes day and night? causes the variety in the length of the day? When are day and night equal all over the world?

What causes the change of the seasons? When is the Earth farthest from the Sun? Why is it warmest then? On what do the phenomena of the seasons really depend? What are the Equinoxes? When do they occur? What are the intervening midpoints called? Why are they called Solstices? What are the circles drawn through these points called? Why? When is there

no night at the North Pole? What fixes the points through which the Arctic and Antarctic Circles are drawn? What are the regions into which these four circles divide the globe called? What zone is between the tropics? What zones are around the poles? What are the intervening zones called?

What phase does the moon show during its first, second, third, and fourth quarters respectively? Find by looking at it, or from an almanac, in what phase the moon is now. How do we know that the moon moves round the Earth? or that Venus and Mars do not? Why does the moon show phases? Which of the planets show them? and why? Which do not? and why? What is the moon's size and distance? Name the inferior or inner planets. How do we know that they are so? Name the superior or outer planets. How do we know that they are so? Which is the nearest planet to the Sun, so far as known? Which is farthest from it? Which planets are now visible in the evening? which in the morning?

NEARLY READY, FOR ADVANCED CLASSES.

PHYSICAL GEOGRAPHY. BY CHARLES BIRD, B.A., F.R.A.S., Science Master in the Bradford Grammar School. T. Nelson and Sons, London, Edinburgh, and New York.

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