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.