LESSON IV.
ROTATION OF THE SUN, THE EARTH, AND THE REST OF THE PLANETS ON THEIR AXES.—DAY AND NIGHT.—INCLINATION OF THE EARTH’S AXIS.—SEASONS.
§ 20. Besides the progressive motion of the Planets, of which we have spoken in the last Lesson, they have yet a peculiar motion like a wheel turning on its Axle-tree. This motion is called the rotation of the Planets on their Axes; because each of them seems to move round a straight line passing through its centre; like a ball turning round a piece of wire run through the middle of it.—Moreover it is customary to call such a straight line imagined to be drawn through the centre of a Planet—the Axis of that Planet.
The double motion of the Planets,—progressive and rotary,—is perhaps one of the most difficult things for young pupils properly to understand, without some popular illustration. If the teacher, therefore, has no orrery to show this motion to his pupils, he may compare it to a screw which is turned round whilst it suffers at the same time a progressive motion; or perhaps with more propriety to a spinning-top, which is continually turning on its Axis, while at the same time it describes large circles.[4]
§ 21. If you have well understood what has just been said, you will be able to comprehend, that our Earth, while it is performing its great journey round the Sun in Three Hundred and Sixtyfive days, is, at the same time, every Twentyfour hours turning on its Axis. This rotary motion of our Earth on its Axis is the cause of the successive changes of day and night; that portion of the Earth which is turned toward the Sun having always day, when the other, which is turned away from him, has night.
This is again a wise dispensation of God’s providence. For if the Earth would always keep the same relative position to the Sun, then that portion of it, which would then be turned toward the Sun, would have continual day, whilst the other, which would then be turned away from him, would be enveloped in perpetual darkness. But as it is now arranged by the Earth’s rotation on its Axis, most every portion of its surface must at least once every Twentyfour hours be turned toward the Sun and receive from him light and heat. Without this, one great half of our Earth would have a perpetual winter, destructive to plants and animals, while an everlasting summer would scorch the other half and render it equally unfit for the support of man.
The following Diagram, Plate [IX], may serve to give you an idea of the Earth’s rotation round its Axis, and the alternate succession of Day and Night, resulting from it. When the Earth is situated as represented in the Diagram, then that portion of it, which is marked A, will have Day, because it is turned toward the Sun; and the portion marked B, will have Night. But in the course of the next Twelve hours the order will be reversed. The portion which is marked B, will be turned toward the Sun and have day, whilst the portion A will be turned from him, and have night.
§ 22. The rotation of the Earth on its Axis is also the cause of the Rising and Setting of the Sun. For no portion of our Earth is at once turned toward or from the Sun; but moves toward or from it by degrees (as you may see by slowly turning a ball near the flame of a candle). This gradual motion of each portion of the Earth’s surface toward or from the Sun, makes the Sun himself appear to us as rising and going down; while, in fact, we ourselves are turning towards, or receding from him. This is a kind of deception similar to that which you experience when slowly gliding down a river; when the objects on shore have the appearance of receding from you, while in fact, it is you, yourself, who are travelling away from them.
No. IX.