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 circ.u.mstances 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 quant.i.ty 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 quant.i.ty of matter in that system.

You must recollect, that the force of attraction is not only proportional to the quant.i.ty 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 quant.i.ty 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 quant.i.ty 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.

[Ill.u.s.tration: PLATE VII.]

_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...o...b..t 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.