Part 9

_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 space, 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 shine?

_Mrs. B._ What is that which makes the gilt buttons on your brothers coat shine?

_Caroline._ The sun. But if it was the sun which made the planets shine, 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 shine 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 passes 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 shine upon every planet. When, therefore, these little ignorant beings look around them during their night, and behold all the stars shining, they cannot imagine why the planets, which are dark bodies, should shine; 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 illustrated 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 illustrating 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 orbit, and what circumstances 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 spaces passed 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?

24. (Pg. 76) At what season of the year is it nearest to, and at what furthest from the sun?

25. (Pg. 76) What is the mean distance of the earth from the sun?

26. (Pg. 76) Why is but little effect produced, as regards temperature, by the change of distance?

27. (Pg. 76) Has it any influence on the sun's apparent size?

28. (Pg. 76) Are the summer and winter, half years, of the same length; what is their difference, and what is the cause?

29. (Pg. 76) What are the planets?

30. (Pg. 77) What circumstances render it probable that they are habitable globes?

31. (Pg. 77) What is believed respecting the fixed stars?

32. (Pg. 77) What discoveries have been made in the moon?

33. (Pg. 77) What prevents our seeing the planets, if there are any, which revolve round the fixed stars?

34. (Pg. 77) What prevents our seeing the stars and planets in the day-time?

35. (Pg. 78) What other motions have the earth and planets, besides that in their orbits?

36. (Pg. 78) What is the imaginary line called, round which they revolve?

37. (Pg. 78) How does this occasion night and day?

38. (Pg. 78) In what direction does the earth turn upon its axis, and what apparent motion of the sun, moon, and stars is thereby produced?

39. (Pg. 79) What must be the appearance of the earth to an inhabitant of one of the planets?

40. (Pg. 79) What the appearance of the sun to the inhabitants of planets in other systems?

41. (Pg. 79) What the appearance of the earth to an inhabitant of the moon?

CONVERSATION VII.

OF THE PLANETS.

OF THE SATELLITES OR MOONS. GRAVITY DIMINISHES AS THE SQUARE OF THE DISTANCE. OF THE SOLAR SYSTEM. OF COMETS. CONSTELLATIONS, SIGNS OF THE ZODIAC. OF COPERNICUS, NEWTON, &c.

MRS. B.

The planets are distinguished into primary and secondary. Those which revolve immediately about the sun are called primary. Many of these are attended in their course by smaller planets, which, revolve round them: these are called secondary planets, satellites, or moons. Such is our moon which accompanies the earth, and is carried with it round the sun.

_Emily._ How then can you reconcile the motion of the secondary planets to the laws of gravitation; for the sun is much larger than any of the primary planets; and is not the power of gravity proportional to the quantity of matter?

_Caroline._ Perhaps the sun, though much larger, may be less dense than the planets. Fire you know, is very light, and it may contain but little matter, though of great magnitude.

_Mrs. B._ We do not know of what kind of matter the sun is made; but we may be certain, that since it is the general centre of attraction of our system of planets, it must be the body which contains the greatest quantity of matter in that system.

You must recollect, that the force of attraction is not only proportional to the quantity of matter, but to the degree of proximity of the attractive body: this power is weakened by being diffused, and diminishes as the distance increases.

_Emily._ Then if a planet was to lose one-half of its quantity of matter, it would lose one half of its attractive power; and the same effect would be produced by removing it to twice its former distance from the sun; that I understand.

_Mrs. B._ Not so perfectly as you imagine. You are correct as respects the diminution in size, because the attractive force is in the same proportion as the quantity of matter; but were you to remove a planet to double its former distance, it would retain but one-fourth part of its gravitating force; for attraction decreases not in proportion to the simple increase of the distance, but as the squares of the distances increase.

_Caroline._ I do not exactly comprehend what is meant by the squares, in this case, although I know very well what is in general intended by a square.

_Mrs. B._ By the square of a number we mean the product of a number, multiplied by itself; thus two, multiplied by two, is four, which is therefore the square of two; in like manner the square of three, is nine, because three multiplied by three, gives that product.

_Emily._ Then if one planet is three times more distant from the sun than another, it will be attracted with but one-ninth part of the force; and if at four times the distance, with but one-sixteenth, sixteen being the square of four?

_Mrs. B._ You are correct; the rule is, that _the attractive force is in the inverse proportion of the square of the distance_. And it is easily demonstrated by the mathematics, that the same is the case with every power that emanates from a centre; as for example, the light from the sun, or from any other luminous body, decreases in its intensity at the same rate.

_Caroline._ Then the more distant planets, move much slower in their orbits; for their projectile force must be proportioned to that of attraction? But I do not see how this accounts for the motion of the secondary, round the primary planets, in preference to moving round the sun?

_Emily._ Is it not because the vicinity of the primary planets, renders their attraction stronger than that of the sun?

_Mrs. B._ Exactly so. But since the attraction between bodies is mutual, the primary planets are also attracted by the satellites which revolve round them. The moon attracts the earth, as well as the earth the moon; but as the latter is the smaller body, her attraction is proportionally less; therefore, neither the earth revolves round the moon, nor the moon round the earth; but they both revolve round a point, which is their common centre of gravity, and which is as much nearer to the earth than to the moon, as the gravity of the former exceeds that of the latter.

_Emily._ Yes, I recollect your saying, that if two bodies were fastened together by a wire or bar, their common centre of gravity would be in the middle of the bar, provided the bodies were of equal weight; and if they differed in weight, it would be nearer the larger body. If then, the earth and moon had no projectile force which prevented their mutual attraction from bringing them together, they would meet at their common centre of gravity.

_Caroline._ The earth then has a great variety of motion, it revolves round the sun, round its own axis, and round the point towards which the moon attracts it.

_Mrs. B._ Just so; and this is the case with every planet which is attended by satellites. The complicated effect of this variety of motions, produces certain irregularities, which, however, it is not necessary to notice at present, excepting to observe that they eventually correct each other, so that no permanent derangement exists.

The planets act on the sun, in the same manner as they are themselves acted on by their satellites; for attraction, you must remember, is always mutual; but the gravity of the planets (even when taken collectively) is so trifling compared with that of the sun, that were they all placed on the same side of that luminary, they would not cause him to move so much as one-half of his diameter towards them, and the common centre of gravity, would still remain within the body of the sun. The planets do not, therefore, revolve round the centre of the sun, but round a point at a small distance from its centre, about which the sun also revolves.

_Emily._ I thought the sun had no motion?

_Mrs. B._ You were mistaken; for besides that round the common centre of gravity, which I have just mentioned, which is indeed very inconsiderable, he revolves on his axis in about 25 days; this motion is ascertained by observing certain spots which disappear, and reappear regularly at stated times.

_Caroline._ A planet has frequently been pointed out to me in the heavens; but I could not perceive that its motion differed from that of the fixed stars, which only appear to move.

_Mrs. B._ The great distance of the planets, renders their apparent motion so slow, that the eye is not sensible of their progress in their orbits, unless we watch them for some considerable length of time: but if you notice the nearness of a planet to any particular fixed star, you may in a few nights perceive that it has changed its distance from it, whilst the stars themselves always retain their relative situations. The most accurate idea I can give you of the situation and motion of the planets in their orbits, will be by the examination of this diagram, (plate 7. fig. 1.) representing the solar system, in which you will find every planet, with its orbit delineated.

_Emily._ But the orbits here are all circular, and you said that they were elliptical. The planets appear too, to be moving round the centre of the sun; whilst you told us that they moved round a point at a little distance from thence.

_Mrs. B._ The orbits of the planets are so nearly circular, and the common centre of gravity of the solar system, so near the centre of the sun, that these deviations are too small to be represented. The dimensions of the planets, in their proportion to each other, you will find delineated in fig. 2.

Mercury is the planet nearest the sun; his orbit is consequently contained within ours; his vicinity to the sun, prevents our frequently seeing him, so that very accurate observations cannot be made upon Mercury. He performs his revolution round the sun in about 87 days, which is consequently the length of his year. The time of his rotation on his axis is not known; his distance from the sun is computed to be 37 millions of miles, and his diameter 3180 miles. The heat of this planet is supposed to be so great, that water cannot exist there but in a state of vapour, and that even quicksilver would be made to boil.

_Caroline._ Oh, what a dreadful climate!

_Mrs. B._ Though we could not live there, it may be perfectly adapted to other beings, destined to inhabit it; or he who created it may have so modified the heat, by provisions of which we are ignorant, as to make it habitable even by ourselves.

Venus, the next in the order of planets, is 68 millions of miles from the sun: she revolves about her axis in 23 hours and 21 minutes, and goes round the sun in 244 days, 17 hours. The orbit of Venus is also within ours; during nearly one-half of her course in it, we see her before sun-rise, and she is then called the morning star; in the other part of her orbit she rises later than the sun.

_Caroline._ In that case we cannot see her, for she must rise in the day time?

_Mrs. B._ True; but when she rises later than the sun, she also sets later; so that we perceive her approaching the horizon after sun-set: she is then called Hesperus, or the evening star. Do you recollect those beautiful lines of Milton?

Now came still evening on, and twilight gray Had in her sober livery all things clad; Silence accompanied; for beast and bird, They to their grassy couch, these to their nests Were slunk, all but the wakeful nightingale; She all night long her amorous descant sung; Silence was pleas'd; now glowed the firmament With living sapphires. Hesperus that led The starry host, rode brightest, till the moon Rising in clouded majesty, at length Apparent queen unveil'd her peerless light, And o'er the dark her silver mantle threw.

The planet next to Venus is the Earth, of which we shall soon speak at full length. At present I shall only observe that we are 95 millions of miles distant from the sun, that we perform our annual revolution in 365 days 5 hours and 49 minutes; and are attended in our course by a single moon.

Next follows Mars. He can never come between us and the sun, like Mercury and Venus; his motion is, however, very perceptible, as he may be traced to different situations in the heavens; his distance from the sun is 144 millions of miles; he turns round his axis in 24 hours and 39 minutes; and he performs his annual revolution, in about 687 of our days: his diameter is 4120 miles. Then follow four very small planets, Juno, Ceres, Pallas and Vesta, which have been recently discovered, but whose dimensions, and distances from the sun, have not been very accurately ascertained. They are generally called asteroids.

Jupiter is next in order: this is the largest of all the planets. He is about 490 millions of miles from the sun, and completes his annual period in nearly 12 of our years. He turns round his axis in about ten hours. He is above 1200 times as big as our earth; his diameter is 86,000 miles. The respective proportions of the planets cannot, therefore, you see, be conveniently delineated in a diagram. He is attended by four moons.

The next planet is Saturn, whose distance from the sun, is about 900 millions of miles; his diurnal rotation is performed in 10 hours and a quarter: his annual revolution is nearly 30 of our years. His diameter is 79,000 miles. This planet is surrounded by a luminous ring, the nature of which, astronomers are much at a loss to conjecture: he has seven moons. Lastly, we observe the planet Herschel, discovered by Dr. Herschel, by whom it was named the Georgium Sidus, and which is attended by six moons.

_Caroline._ How charming it must be in the distant planets, to see several moons shining at the same time; I think I should like to be an inhabitant of Jupiter or Saturn.

_Mrs. B._ Not long I believe. Consider what extreme cold must prevail in a planet, situated as Saturn is, at nearly ten times the distance at which we are from the sun. Then his numerous moons are far from making so splendid an appearance as ours; for they can reflect only the light which they receive from the sun; and both light, and heat, decrease in the same ratio or proportion to the distances, as gravity. Can you tell me now how much more light we enjoy than Saturn?

_Caroline._ The square of ten is a hundred; therefore, Saturn has a hundred times less--or to answer your question exactly, we have a hundred times more light and heat, than Saturn--this certainly does not increase my wish to become one of the poor wretches who inhabit that planet.