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 therotation 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]
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, thatour 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. Thisrotarymotion 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 leastonceevery 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.
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 leastonceevery 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, PlateIX, 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 theRisingandSetting 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.
No. IX.
No. IX.
The gradual rise and setting of the Sun is another excellent provision of nature. Were we from the darkness of night atonceexposed to the luminous rays of the Sun, it would dazzle our eyes and render them unfit to distinguish a single object.—It is only by a gradual transition from darkness to light that we are able to accustom our eyes to the brilliancy of noon.
The gradual rise and setting of the Sun is another excellent provision of nature. Were we from the darkness of night atonceexposed to the luminous rays of the Sun, it would dazzle our eyes and render them unfit to distinguish a single object.—It is only by a gradual transition from darkness to light that we are able to accustom our eyes to the brilliancy of noon.
§ 23. There is another peculiarity in the situation of our Earth with regard to the Sun, which you have not yet learned. The Earth’s Axis is not even (parallel) with that of the Sun; but is somewhat inclined towards it, as represented in the last Diagram. To this is owing theChange of the Seasons. For on account of the inclination of the Earth’s Axis, the Sun’s rays fall, sometimes nearly perpendicular upon us, while at other times they are striking us more obliquely[5]. This is the principal cause of those changes of temperature which we are in the habit of distinguishing by the names,Spring,Summer,AutumnandWinter.
In winter the Sun’s rays strike us most obliquely; it is therefore the coldest season of the year. In summer they are most perpendicular;—Summer therefore is the hottest season. Spring and autumn are standing in the middle between these two. From spring till mid-summer the Sun’s rays are striking us more and more perpendicularly; from mid-summer till winter more and more obliquely.
In winter the Sun’s rays strike us most obliquely; it is therefore the coldest season of the year. In summer they are most perpendicular;—Summer therefore is the hottest season. Spring and autumn are standing in the middle between these two. From spring till mid-summer the Sun’s rays are striking us more and more perpendicularly; from mid-summer till winter more and more obliquely.
A similar change of temperature is felt every day from Sun-rise (when the Sun-beams are most oblique) till noon, (when they are most perpendicular); and from noon again towards evening or Sun-set, when they are again oblique.
§ 24. It has been mentioned (Lesson IV, § 20), that each Planet in our Solar System is regularly turning on its Axis. But all of them do not perform this rotation with equalvelocity. The Planets which are farther from the Sun are turning quicker than those which are near him. Jupiter, for instance, turns on its Axis twice as fast as our Earth. The nights in Jupiter, therefore, do not last half as long as ours.
This is, in some degree, necessary. For in proportion as a planet is further from the Sun, it receives less light and heat, which deficiency is, in part, made up by a more frequent exposition to his rays.
This is, in some degree, necessary. For in proportion as a planet is further from the Sun, it receives less light and heat, which deficiency is, in part, made up by a more frequent exposition to his rays.
§ 25. TheSunhimself is also known to turn regularly on his Axis, and to complete one whole rotation in about Twentysix of our days. This we have been able to perceive from the spots which have been discovered on its surface, and which gradually move toward and disappear on one side, when in a short time after they appear again on the other.
§ 26. The Moon, and the Satellites of the other Planets have no rotary motion; but have always the same side turned towards their Planets. Thus the moon keeps constantly the same side turned towards the Earth; but her monthly motion round the Earth (Lesson III, § 17) is equal to a rotation on her axis; because by this means every part of her is, at least, once every Twentyseven days turned toward the Sun; as you may see from the Moon’s phases, represented on plateVII. A day in the Moon, therefore, is equal to Twentyseven of our days; because the Moon moves in Twentyseven of our days round the Earth, which is equal to turning once on her axis.
RECAPITULATION OF LESSON IV.
QUESTIONS.
[§ 20.] Is the progressive motion of the Planets round the Sun the only one which they are performing? What other motion have they besides this? What is this motion called? Why is it called so? What do you call a straight line imagined to be drawn through the centre of a Planet?
To what may the double motion of the Planets’ progression and rotary be compared?
To what may the double motion of the Planets’ progression and rotary be compared?
[§ 21.] What time does the Earth need to turn once on its axis? How many times does the Earth turn on its axis during its whole journey round the Sun?[6]What is this rotary motion of our Earth the cause of? Which portion of the Earth has day? Which night?
What would be the case, if the Earth was always to keep the same relative position with regard to the Sun? What is the advantage derived from the rotation of the Earth on its Axis. Would the Earth be habitable without it or not?[The pupil ought now to explain plateIX.]
What would be the case, if the Earth was always to keep the same relative position with regard to the Sun? What is the advantage derived from the rotation of the Earth on its Axis. Would the Earth be habitable without it or not?
[The pupil ought now to explain plateIX.]
[§ 22.] What is the rotation of the Earth on its axis further the cause of? What does the gradual motion of each portion of our Earth towards the Sun produce?
What would be the case, if from the darkness of night we were at once exposed to the luminous rays of the Sun? Could our eyes endure the brightness of noon without agradualtransition from darkness to light?
What would be the case, if from the darkness of night we were at once exposed to the luminous rays of the Sun? Could our eyes endure the brightness of noon without agradualtransition from darkness to light?
[§ 23.] What other peculiarity is there in the situation of the Earth with regard to the Sun? What is this the cause of? Do the rays of the Sun strike us at all times equally perpendicular or obliquely? What changes of temperature are thereby created?
How do the rays of the Sun strike us in winter? What season, therefore, is it? How do the rays of the Sun strike us in summer? What season, therefore, is summer with regard to temperature? How are the rays of the Sun striking us from spring till mid-summer? How, from mid-summer till winter? What similar change of temperature do we experience every day from Sun-rise till noon, and from noon till evening or Sun-set?
How do the rays of the Sun strike us in winter? What season, therefore, is it? How do the rays of the Sun strike us in summer? What season, therefore, is summer with regard to temperature? How are the rays of the Sun striking us from spring till mid-summer? How, from mid-summer till winter? What similar change of temperature do we experience every day from Sun-rise till noon, and from noon till evening or Sun-set?
[§ 24.] Do all Planets turn on their axes with equal velocities? Which Planets turn quicker, those which are nearer or those which are further from the Sun? Give an instance. How long are Jupiter’s nights in comparison to our’s?
Why is this, in some degree, necessary?
Why is this, in some degree, necessary?
[§ 25.] Is the Sun himself also known to turn on its axis like the Planets? How many days does he need for one complete rotation? By what means have we been able to observe this motion?
[§ 26.] Have the Moon and the Satellites of the other Planets also arotarymotion? In what position does the Moon remain with regard to the Earth? But what is her monthly revolution round the Earth equal to? Why? What is a day in the Moon equal to? Why?
FOOTNOTES:[4]The propriety of this comparison becomes still more evident, when we reflect that the Axis of the top is generally somewhat inclined to the plane.[5]If the termsperpendicularandoblique, should not be perfectly understood by the pupils, it will be easy for the teacher to explain their meaning.[6]If the Earth requires 365 days to travel round the Sun; and each day has twentyfour hours; then the Earth will, during the whole of this time, turn Three Hundred and Sixtyfive times on its axis.
[4]The propriety of this comparison becomes still more evident, when we reflect that the Axis of the top is generally somewhat inclined to the plane.
[4]The propriety of this comparison becomes still more evident, when we reflect that the Axis of the top is generally somewhat inclined to the plane.
[5]If the termsperpendicularandoblique, should not be perfectly understood by the pupils, it will be easy for the teacher to explain their meaning.
[5]If the termsperpendicularandoblique, should not be perfectly understood by the pupils, it will be easy for the teacher to explain their meaning.
[6]If the Earth requires 365 days to travel round the Sun; and each day has twentyfour hours; then the Earth will, during the whole of this time, turn Three Hundred and Sixtyfive times on its axis.
[6]If the Earth requires 365 days to travel round the Sun; and each day has twentyfour hours; then the Earth will, during the whole of this time, turn Three Hundred and Sixtyfive times on its axis.