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.