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The course in which the earth moves, or the orbit of the earth, is nearly a circle, of which the sun is the centre; and an imaginary plane passing through the centre of the sun and this orbit is called the plane of the earth's orbit, as we might call the floor on which our top spun, the plane of the top's orbit round the lamp.
The axis of the earth is inclined to the plane of its orbit, just as we supposed the axis of the top to be; and is always inclined in the same direction.
When the earth is at that point in its orbit at which the axis is most inclined towards the sun, the days are longer than the nights in the northern hemisphere, and at the parts near the north pole it is always day. This is midsummer in the northern hemisphere. In the southern hemisphere, on the contrary, the nights are longer than the days, and in the south polar regions it is always night. It is midwinter.
When the earth is at the opposite point in its orbit, the contrary takes place. It is midwinter in the northern hemisphere, and midsummer in the southern.
These two points in the earth's orbit are called, respectively, the summer and the winter Solstice. The summer solstice is on June 21st, and the winter solstice on December 21st.
When the axis of the earth is not inclined towards or from the sun, the day and night are equal all over the globe.
These points in the earth are called the vernal, or spring, and the autumnal Equinox. The spring equinox is on March 21st, and the autumnal equinox on September 21st.
As the earth moves in its orbit from the summer to
the winter solstices, the days in the northern hemisphere shorten and the nights lengthen, there being one time between the two when night and day are exactly equal; and as the earth moves from the winter to the summer solstice, the nights shorten and the days lengthen, there being one time between the two when day and night are equal.
The converse takes place in the southern hemisphere. It is spring in the southern hemisphere when it is autumn in the northern, and it is summer in the southern when it is winter in the northern. Christmasday is in the height of summer in New Zealand.
It will be easily seen that the nearer a place is to the poles, the longer the days are in summer, and the shorter in winter, and exactly at the poles the sun is always above the horizon during one half of the year, and always below during the other half.
In countries near the equator at some times of the year, the sun at noon is immediately overhead, or, as it is called, vertical. This makes these places very hot, because the more nearly vertical the sun is, the greater heat it gives.
Such countries are called tropical. In parts of the earth more distant from the equator, the sun is never quite vertical, but is more nearly so in summer than in winter. In northern latitudes, the sun is always in the south at noon; in southern latitudes, in the north, being higher or lower in the sky according to the time of year.
It is hotter in summer than winter, for reasons which both depend upon the inclination of the earth's axis:-1st, because the days are longer; and 2nd, because the rays of the sun are more nearly vertical.
Were the axis of the earth vertical, there would be no change of seasons throughout the year. The alternation of spring, summer, autumn, and winter, with the succession of fruit and flowers, and all the varied bounties of the circling year, are some of the wonderful results of what has been well called "the simple but stupendous contrivance" of the inclination of the earth's axis to the plane of its orbit;-a contrivance, like the rest of God's works, full of wisdom and goodness, and well adapted to lead the thoughtful mind to admire, love, and praise the Almighty and All-wise Creator of the universe.
WHEN Spring unlocks the flowers to paint the laughing
When Summer's balmy showers refresh the mower's toil; When Winter binds in frosty chains the fallow and the flood,
In God the earth rejoiceth still, and owns his Maker good. The birds that wake the morning, and those that love the shade;
The winds that sweep the mountain or lull the drowsy glade;
The Sun that from his amber bower rejoiceth on his way, The Moon and Stars, their Master's name in silent pomp display.
Shall Man, the lord of nature, expectant of the sky,
Shall Man, alone unthankful, his little praise deny?
No, let the year forsake his course, the seasons cease to be, Thee, Master, must we always love, and, Saviour, honour Thee.
The flowers of Spring may wither, the hope of Summer fade,
The Autumn droop in Winter, the birds forsake the shade ; The winds be lull'd-the Sun and Moon forget their old decree,
But we in Nature's latest hour, O Lord! will cling to Thee.
THE TERRESTRIAL GLOBE.
A terrestrial globe is a globe upon which are drawn the various countries, lands, and seas of the earth, in their proper positions and proportions.
Suppose you wished to copy a pattern from one sheet of paper to another: the most convenient way would be to draw upon each sheet a number of lines, equally distant from each other, both ways of the paper. If the two sheets were lined exactly alike, you could easily find out where to put every point in the pattern by measuring its distance from some one of the lines.
If you had to make the same pattern but half the size of the original, you must draw the same number of lines on the blank paper as on the copy, but the distance between each two must be half as great; and by making proper measurements from the lines you could find the right places for each point in the pattern.
This is the way you copy maps. There are certain lines drawn across the paper, and the distances
between these represent so many miles; and if you are making a small copy of a large map, all you have to do is to get your first lines right, and the rest will be easy. In every map there should be a line, along which is marked what length represents a mile, five miles, ten miles, and the like. This line is called the scale of the map.
Now, if we want to map out a globe, we must discover some way of dividing it by lines from which to make our measurements.
The equator furnishes us with one line from which to start.
We may draw upon our globes circles parallel to the equator, at equal distances from each other and from the equator to either pole, and if we know how large the earth is, we know how many miles the distances between any two of these circles will represent.
Thus, if the earth were 24,000 miles round, from pole to equator would be 6000 miles. If we draw five such circles as have been described between equator and pole, the distance between each circle measured upon theglobe will be 1000 miles; and if we supposed Cape Verde to be 1000 miles north of the equator (which is nearly the case), we should have to place it somewhere upon the parallel circle next above the equator.
We may also draw upon our globe, any number of great circles passing through the poles. Let one pass through the spot where we mean to mark Greenwich. This is the meridian of Greenwich. Then if we know
*The teacher is particularly advised to exercise his pupils in simple illustrations of the use of scales in map-drawing before he proceeds further with the lesson. This may be easily done upon a black board from any large map hanging in the school-room.