Part 4

§ 15. The time which our Earth needs to travel once completely round the Sun, (to finish one revolution) is called a Year. 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 us a 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!—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 called the Moon’s Phases, or the waxing and waning of the Moon. The time during which the Moon is not seen is called New Moon; the time during which she exhibits her full shape is called the Full Moon; and the different periods of her waxing and waning (when she appears to us in the form of a crescent) are called Quarters. Thus we speak of the First and of the Last Quarter 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 in a, 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 in b, a small brim of the Moon is seen, because a small portion of its lighted surface is then turned towards the Earth.—When in c half of her lighted surface is turned towards us, and we have the First Quarter. In d a still greater portion of the Moon’s lighted surface is visible, and in e, we have Full Moon, because her whole lighted surface is then turned towards the Earth. In f the moon commences to wane (to grow smaller,) and in g the last quarter commences; finally, when passed through the point h, we have in a, 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 in a. 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 in c. If moved still farther to the right, 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 in e.

N^o. VII. ]

N^o. 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 an Eclipse 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 an Eclipse 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 a shade upon our Earth.

But if the Earth is 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 of Comets, (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!!

RECAPITULATION OF LESSON III.

QUESTIONS.

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 Plate IV, or an orrery, if one be at hand.

What is the revolution of the Earth round the Sun, the means of?

FOOTNOTES:

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 a faint illustration of 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.

The exact numbers are given in Table II, at the end of the book.

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.

§ 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.