Conversations on Natural Philosophy, in which the Elements of that Science are Familiarly Explained Part 18

_Mrs. B._ They are those celestial bodies, which revolve like our earth, about the sun; they are supposed to resemble the earth also in many other respects; and we are led by a.n.a.logy, to suppose them to be inhabited worlds.

_Caroline._ I have heard so, but do you not think such an opinion too great a stretch of the imagination?

_Mrs. B._ Some of the planets are proved to be larger than the earth; it is only their immense distance from us, which renders their apparent dimensions so small. Now, if we consider them as enormous globes, instead of small twinkling spots, we shall be led to suppose that the Almighty would not have created them merely for the purpose of giving us a little light in the night, as it was formerly imagined; and we should find it more consistent with our ideas of the Divine wisdom and beneficence, to suppose that these celestial bodies should be created for the habitation of beings, who are, like us, blessed by his providence. Both in a moral, as well as a physical point of view, it appears to me more rational to consider the planets as worlds revolving round the sun; and the fixed stars as other suns, each of them attended by their respective system of planets, to which they impart their influence. We have brought our telescopes to such a degree of perfection, that from the appearances which the moon exhibits when seen through them, we have very good reason to conclude that it is a habitable globe: for though it is true that we cannot discern its towns and people, we can plainly perceive its mountains and valleys: and some astronomers have gone so far as to imagine that they discovered volcanos.

_Emily._ If the fixed stars are suns, with planets revolving round them, why should we not see those planets as well as their suns?

_Mrs. B._ In the first place, we conclude that the planets of other systems (like those of our own) are much smaller than the suns which give them light; therefore at a distance so great as to make the suns appear like fixed stars, the planets would be quite invisible. Secondly, the light of the planets being only reflected light, is much more feeble than that of the fixed stars. There is exactly the same difference as between the light of the sun and that of the moon; the first being a fixed star, the second a planet.

_Emily._ But the planets appear to us as bright as the fixed stars, and these you tell us are suns like our own; why then do we not see them by daylight, when they must be just as luminous as they are in the night?

_Mrs. B._ Both are invisible from the same cause: their light is so faint, compared to that of the sun, that it is entirely effaced by it: the light emitted by the fixed stars may probably be as great as that of our sun, at an equal distance; but they being so much more remote, it is diffused over a greater s.p.a.ce, and is in consequence proportionally lessened.

_Caroline._ True; I can see much better by the light of a candle that is near me, than by that of one at a great distance. But I do not understand what makes the planets s.h.i.+ne?

_Mrs. B._ What is that which makes the gilt b.u.t.tons on your brothers coat s.h.i.+ne?

_Caroline._ The sun. But if it was the sun which made the planets s.h.i.+ne, we should see them in the day-time, when the sun shone upon them; or if the faintness of their light prevented our seeing them in the day, we should not see them at all, for the sun cannot s.h.i.+ne upon them in the night.

_Mrs. B._ There you are in error. But in order to explain this to you, I must first make you acquainted with the various motions of the planets.

You know, that according to the laws of attraction, the planets belonging to our system all gravitate towards the sun; and that this force, combined with that of projection, will occasion their revolution round the sun, in orbits more or less elliptical, according to the proportion which these two forces bear to each other.

But the planets have also another motion: they revolve upon their axis.

The axis of a planet is an imaginary line which pa.s.ses through its centre, and on which it turns; and it is this motion which produces day and night. It is day on that side of the planet which faces the sun; and on the opposite side, which remains in darkness, it is night. Our earth, which we consider as a planet, is 24 hours in performing one revolution on its axis; in that period of time, therefore, we have a day and a night; hence this revolution is called the earth's diurnal or daily motion; and it is this revolution of the earth from west to east which produces an apparent motion of the sun, moon and stars, in a contrary direction.

Let us now suppose ourselves to be beings independent of any planet, travelling in the skies, and looking upon the earth from a point as distant from it as from other planets.

_Caroline._ It would not be flattering to us, its inhabitants, to see it make so insignificant an appearance.

_Mrs. B._ To those accustomed to contemplate it in this light, it could never appear more glorious. We are taught by science to distrust appearances; and instead of considering the fixed stars and planets as little points, we look upon them either as brilliant suns, or habitable worlds; and we consider the whole together as forming one vast and magnificent system, worthy of the Divine hand by which it was created.

_Emily._ I can scarcely conceive the idea of this immensity of creation; it seems too sublime for our imagination;--and to think that the goodness of Providence extends over millions of worlds throughout a boundless universe--Ah! Mrs. B., it is we only who become trifling and insignificant beings in so magnificent a creation!

_Mrs. B._ This idea should teach us humility, but without producing despondency. The same Almighty hand which guides these countless worlds in their undeviating course, conducts with equal perfection, the blood as it circulates through the veins of a fly, and opens the eye of the insect to behold His wonders. Notwithstanding this immense scale of creation, therefore, we need not fear that we shall be disregarded or forgotten.

But to return to our station in the skies. We were, if you recollect, viewing the earth at a great distance, in appearance a little star, one side illumined by the sun, the other in obscurity. But would you believe it, Caroline, many of the inhabitants of this little star imagine that when that part which they inhabit is turned from the sun, darkness prevails throughout the universe, merely because it is night with them; whilst, in reality, the sun never ceases to s.h.i.+ne upon every planet.

When, therefore, these little ignorant beings look around them during their night, and behold all the stars s.h.i.+ning, they cannot imagine why the planets, which are dark bodies, should s.h.i.+ne; concluding, that since the sun does not illumine themselves, the whole universe must be in darkness.

_Caroline._ I confess that I was one of these ignorant people; but I am now very sensible of the absurdity of such an idea. To the inhabitants of the other planets, then, we must appear as a little star?

_Mrs. B._ Yes, to those which revolve round our sun; for since those which may belong to other systems, (and whose existence is only hypothetical) are invisible to us, it is probable that we also are invisible to them.

_Emily._ But they may see our sun as we do theirs, in appearance a fixed star?

_Mrs. B._ No doubt; if the beings who inhabit those planets are endowed with senses similar to ours. By the same rule we must appear as a moon to the inhabitants of our moon; but on a larger scale, as the surface of the earth is about thirteen times as large as that of the moon.

_Emily._ The moon, Mrs. B., appears to move in a different direction, and in a different manner from the stars?

_Mrs. B._ I shall defer the explanation of the motion of the moon till our next interview, as it would prolong our present lesson too much.

Questions

1. (Pg. 71) What revolution does the earth perform in a year?

2. (Pg. 71) Had the earth received a projectile force only, at the time of its creation, how would it have moved?

3. (Pg. 72) What do the lines A B, and A C, represent in fig. 1. plate 6?

4. (Pg. 72) What have you been taught respecting a body acted upon by two forces at right angles with each other?

5. (Pg. 72) How does the force of gravity change the diagonal into a curved line?

6. (Pg. 72) Describe the operation of the forces of projection and of gravity as ill.u.s.trated by the parallelograms in the figure?

7. (Pg. 72) What is the law respecting the time required for motion in the diagonal?

8. (Pg. 73) What portion of a year is represented by the three diagonals in the figure?

9. (Pg. 73) How will what you have learned respecting motion in a curve, apply to the earth's motion?

10. (Pg. 73) In what form are you directed to cut a piece of card to aid in ill.u.s.trating the two forces acting upon the earth?

11. (Pg. 73) How must you apply it to this purpose? (fig. 2. plate 6.)

12. (Pg. 73) If these two forces did not exactly balance each other, what would result?

13. (Pg. 73) Does the earth revolve in a circular orbit?

14. (Pg. 73) What results from its motion in an ellipsis?

15. (Pg. 74) What is represented by the lines A C, A B, in fig. 3. plate 6?

16. (Pg. 74) Were the projectile force to carry the earth from B to D, (fig. 3.) what would result?

17. (Pg. 74) When it has arrived at E, what angle will be formed by the lines representing the two forces?

18. (Pg. 74) What effect will the accelerated motion then produce?

19. (Pg. 75) What is the form of the earth's...o...b..t, and what circ.u.mstances produce this form?

20. (Pg. 75) What is the consequence as regards the regularity of the earth's motion?

21. (Pg. 75) What law governs as regards the s.p.a.ces pa.s.sed over, and how is this explained by fig. 4. plate 6?

22. (Pg. 75) What is meant by _perihelion_, and by _aphelion_?

23. (Pg. 75) What is the difference of the distance of the earth from the sun, in these two points?