LESSON III.
EXPLAINING THE MOTION OF THE PLANETS AND COMETS ROUND THE SUN.
§ 14. You have learned, in the preceding lesson, that the Earth and the other Planets are regularly moving round the Sun; but you must know that the time which each requires to complete a whole revolution, that is, the time which each needs to move once round, cannot be the same with all; as you may easily judge yourself: for the Planets which are next to the Sun will, of course, have a much smaller journey to perform, than those which are further from it. Thus Mercury, which is the first Planet in order from the Sun, will naturally come round much sooner than Jupiter, which is placed at a much greater distance from it.
That you may the easier understand this, the following Plate, No. VI, will represent to you our Solar System:
The Sun from which proceeds all light and heat, is placed in the centre. Then come the Planets Mercury and Venus; Third in order is our Earth with its Satellite the Moon; and so on. The rest of the Planets in the same order in which they are represented on PlateII. You will also perceive there the orbs of two Comets, distinguished as you were told, by a tail oflight. The Planets Jupiter, Saturn and Herschel are each represented with their Moons, and the Planet Saturn with its luminous ring.
No. VI.
No. VI.
No. VI.
Now you will easily understand that Ceres, Vesta, Juno, Pallas, Mars, Jupiter, Saturn and Herschel, have each a much greater distance to travel, than our Earth; but that, on the contrary, Venus and Mercury can complete their revolution in a much shorter time.[2]
§ 15. The time which our Earth needs to travel once completely round the Sun, (to finish one revolution) is called aYear. Such a year has three hundred and Sixtyfive days;—and each of these days has again Twentyfour hours.
You see from this that the revolution of the Earth round the Sun gives usa means of measuring time, by which we are able to bring order and regularity into our business and transactions of life. For the making of clocks and watches is but a late invention, and we should be left entirely in the dark as regards the history of former ages, and a great many people would, at this present moment, be incapable of forming a correct estimate of time, if Providence had not given to all this appropriate means of measuring it.
§ 16. While the Earth needs a whole year for one revolution round the Sun; Mercury requires but Eightyone days, and Venus only about two thirds of One of our years. Mars, on the contrary, needs for one of his revolutions almost Two years; Vesta almost Four; Juno, Ceres and Pallas over Four years; Jupiter almost Twelve, Saturn over Twentynine, and Herschel nearly Eightyfour of our years![3]—And if these Planets, as we have reason to believe, are inhabited by beings endowed with human understanding and faculties, numbering their years as we do ours—by the revolution of their Planets round the Sun—how different from ours must be the Period of their existence!!
§ 17. During the time that the Earth is performing her journey round the Sun, the Moon, our constant attendant, is continually moving round the Earth,and completes one of these revolutions in little more than Twentyseven days.—Very important and interesting to us are the changes in appearance which she exhibits during each of these revolutions.—You probably will know, that the Moon does not appear to us, at all times, the same. Sometimes she is hardly at all visible, (at least not with the naked eye); at other times only a small rim of her is seen, which by degrees becomes larger and larger, until finally she appears in her full round form. After this she begins again to diminish, changes again into a small luminous rim, and finally disappears entirely from our sight. These successive changes in the Moon’s appearance are calledthe Moon’s Phases, or thewaxing and waning of the Moon. The time during which the Moon is not seen is calledNew Moon; the time during which she exhibits her full shape is called theFull Moon; and the different periods of her waxing and waning (when she appears to us in the form of a crescent) are calledQuarters. Thus we speak of theFirstand of theLastQuarter of the Moon. The First Quarter takes place after New Moon; the last Quarter after Full Moon.
The following diagram, Plate No.VII, may serve to represent to you the Moon’s phases as seen from our Earth.When the Moon is ina, then the light of the Sun falls just on that side of it which is turned from the Earth. It is then, we have New Moon. When inb, a small brim of the Moon is seen, because a small portion of its lighted surface is then turned towards the Earth.—When inchalf of her lighted surface is turned towards us, and we have the First Quarter. Inda still greater portion of the Moon’s lighted surface is visible, and ine, we have Full Moon, because her whole lighted surface is then turned towards the Earth. Infthe moon commences to wane (to grow smaller,) and ingthe last quarter commences; finally, when passed through the pointh, we have ina, again New Moon. For familiar illustration you may also take a white ivory ball, holding it before a lighted candle, which may take the place of the Sun. When the ball is in a straight line between your eye and the candle it will appear to you all dark; because the lighted part is then entirely turned toward the candle (away from you), and you have the same case which is represented to you in the diagram, when the Moon is ina. But if you move the ball a little to the right, you will perceive a streak of light, similar to the First Quarter represented in the Diagram, when the Moon is inc. If moved still farther to theright, so that the whole lighted part of the ball is seen, it will resemble the Full Moon; represented in the Diagram, when the Moon is ine.
The following diagram, Plate No.VII, may serve to represent to you the Moon’s phases as seen from our Earth.
When the Moon is ina, then the light of the Sun falls just on that side of it which is turned from the Earth. It is then, we have New Moon. When inb, a small brim of the Moon is seen, because a small portion of its lighted surface is then turned towards the Earth.—When inchalf of her lighted surface is turned towards us, and we have the First Quarter. Inda still greater portion of the Moon’s lighted surface is visible, and ine, we have Full Moon, because her whole lighted surface is then turned towards the Earth. Infthe moon commences to wane (to grow smaller,) and ingthe last quarter commences; finally, when passed through the pointh, we have ina, again New Moon. For familiar illustration you may also take a white ivory ball, holding it before a lighted candle, which may take the place of the Sun. When the ball is in a straight line between your eye and the candle it will appear to you all dark; because the lighted part is then entirely turned toward the candle (away from you), and you have the same case which is represented to you in the diagram, when the Moon is ina. But if you move the ball a little to the right, you will perceive a streak of light, similar to the First Quarter represented in the Diagram, when the Moon is inc. If moved still farther to theright, so that the whole lighted part of the ball is seen, it will resemble the Full Moon; represented in the Diagram, when the Moon is ine.
No. VII.
No. VII.
No. VII.
No. VIII.
No. VIII.
No. VIII.
§ 18. While the Moon is moving round the Earth, it often occurs that she is placed in a direct line between ourselves and the Sun. In this case a greater or less part of the Sun is concealed from us, which causes a diminution of light or a partial darkness on our Earth. This we call anEclipse of the Sun. (Such an Eclipse took place in 1831, and you will probably have an opportunity of seeing many more). If, on the contrary, the Earth is placed in a direct line between the Sun and the Moon, then the Moon will be obscured by our Earth. This is called anEclipse of the Moon. The following two figures on Plate No. VIII will serve for an illustration.
You will easily perceive from them that if the Moon (as represented in Figure I) is placed in a direct line between the Sun and ourselves, it must necessarily conceal from us part of that luminary; and in this state cast ashadeupon our Earth.But if theEarthis placed in a direct line between the Sun and the Moon, (as represented in Figure II), then the Moon will be much more obscured, because the Earth is much larger than the Moon, and will therefore cast a much greater shade upon her.
You will easily perceive from them that if the Moon (as represented in Figure I) is placed in a direct line between the Sun and ourselves, it must necessarily conceal from us part of that luminary; and in this state cast ashadeupon our Earth.
But if theEarthis placed in a direct line between the Sun and the Moon, (as represented in Figure II), then the Moon will be much more obscured, because the Earth is much larger than the Moon, and will therefore cast a much greater shade upon her.
§ 19. It remains for us to speak of that class of bodies known by the name ofComets, (see Lesson I, § 6). Of these an unknown number belongs to our Solar System.—(Some philosophers have estimated their number to be about Twentyone; others think it must amount to several hundred). They move round the Sun in exceedingly long ovals, having their transparent tails always turned away from that luminary. What is most remarkable about them is the astonishing degree of heat to which they are exposed on account of passing so near the Sun, and the astonishing velocity with which they travel.
The Comet which appeared in the year 1680, is supposed to sustain a heat nearly Two Thousand times greater than that of red hot iron, and to move at the rate of several Hundred Thousand miles an hour!!
The Comet which appeared in the year 1680, is supposed to sustain a heat nearly Two Thousand times greater than that of red hot iron, and to move at the rate of several Hundred Thousand miles an hour!!
RECAPITULATION OF LESSON III.
QUESTIONS.
[§ 14.] Do all Planets need the same time to complete a whole revolution round the Sun? Why not?
If the pupils are old enough to understand the use of Dividers, it will perhaps be well for the teacher to let them draw the Solar System on a piece of paper.—If not, he ought to let them explain PlateIV, or an orrery, if one be at hand.
If the pupils are old enough to understand the use of Dividers, it will perhaps be well for the teacher to let them draw the Solar System on a piece of paper.—If not, he ought to let them explain PlateIV, or an orrery, if one be at hand.
[§ 15.] What is the time called, which our Earth needs for a complete revolution round the Sun? How many days are there in a year? How many hours are there in a day?
What is the revolution of the Earth round the Sun, the means of?
What is the revolution of the Earth round the Sun, the means of?
[§ 16.] What time does Mercury require for a complete revolution round the Sun? What time does Venus require for the same purpose? What time does Ceres, Vesta, Juno and Pallas need? What, Jupiter, Saturn, and Herschel?
[§ 17.] What motion does the Moon make whilst the Earth is travelling round the Sun? How many days does the Moon need for a complete revolution round the Earth? Does the Moon during its revolution round the Earth always exhibit the same shape? What changes then does she gradually undergo? What do you call the successive changes in the Moon’s appearance? What do you call the time during which the Moon is not seen? What, that, during which she exhibits her full shape? What, the different periods of her waxing and waning? When does the First Quarter take place? When the last?
[The teacher might now require the explanation of PlateV; the elder pupils may draw the Diagram.]
[The teacher might now require the explanation of PlateV; the elder pupils may draw the Diagram.]
[§ 18.] What does frequently occur during the Moon’s motion round the Earth? What is the consequence of the Moon’s position in a direct line between the Sun and ourselves? What is such a diminution of light in consequence of the Moon being placed between us and the Sun, called? What takes place when theEarth is placed between the Sun and the Moon? What is such an obscuration of the Moon, in consequence of the Earth’s position between her and the Sun, called?
[The younger pupils ought now to explain PlateVIII; the elder pupils ought to draw the Diagram on a slate or paper.]
[The younger pupils ought now to explain PlateVIII; the elder pupils ought to draw the Diagram on a slate or paper.]
[§ 19.] Is the number of Comets belonging to our Solar System precisely ascertained? How many are theresupposedto belong to our System? In what manner do they move round the Sun? What remarkable property do they possess?
FOOTNOTES:[2]If the teacher has an orrery at hand, if it be even of the most simple construction, he may exhibit it now. But it is the author’s belief that a considerable portion of the pupil’s interest is lost, if he be acquainted with it, at the beginning of the study; or, as it is the custom in some schools, if an orrery is hung up among the charts and maps of the school room. The pupil ought not to see the orrery, until he knows that it is but afaint illustrationof the infinite grandeur of the heavens. Nothing detracts so much from our estimation of things as a too familiar acquaintance with them, before we know their real value.[3]The exact numbers are given in Table II, at the end of the book.
[2]If the teacher has an orrery at hand, if it be even of the most simple construction, he may exhibit it now. But it is the author’s belief that a considerable portion of the pupil’s interest is lost, if he be acquainted with it, at the beginning of the study; or, as it is the custom in some schools, if an orrery is hung up among the charts and maps of the school room. The pupil ought not to see the orrery, until he knows that it is but afaint illustrationof the infinite grandeur of the heavens. Nothing detracts so much from our estimation of things as a too familiar acquaintance with them, before we know their real value.
[2]If the teacher has an orrery at hand, if it be even of the most simple construction, he may exhibit it now. But it is the author’s belief that a considerable portion of the pupil’s interest is lost, if he be acquainted with it, at the beginning of the study; or, as it is the custom in some schools, if an orrery is hung up among the charts and maps of the school room. The pupil ought not to see the orrery, until he knows that it is but afaint illustrationof the infinite grandeur of the heavens. Nothing detracts so much from our estimation of things as a too familiar acquaintance with them, before we know their real value.
[3]The exact numbers are given in Table II, at the end of the book.
[3]The exact numbers are given in Table II, at the end of the book.