TUTOR. The planets are opaque, that is dark bodies, which receive their light from the sun; and, as I told you, revolve about him. The first, or that nearest the sun, is called Mercury, the next Venus, then the Earth, Mars, Jupiter, Saturn, and Georgian, or the Georgium Sidus.[3] These are called primary planets. [Footnote 3: Their characters are, Sun, Merc. Venus, Earth, Mars, Jup. Saturn, Georgian, ☉ ☿ ♀ ♁ ♂ ♃ ♄ ♅ .] PUPIL. Are there then any others? TUTOR. Yes. There are fourteen others, which move round their respective primaries as their centers, and with them round the sun, and are called secondaries, satellites or moons. PUPIL. Have all the primaries secondaries? TUTOR. Only four of them have moons. The earth, I need not tell you, has one; Jupiter has four; Saturn seven, besides a stupendous ring which surrounds his body; and Georgian two. PUPIL. In what time, and at what distances, from the sun, do the planets perform their periodical revolutions? TUTOR. _Mercury_ revolves about the sun in 88 days, at the distance of 36 millions of miles. _Venus_, at the distance of 68 millions of miles, completes her revolution in 224 days. _Earth_, on which we live, at the distance of 95 millions of miles, performs its period in one year.[4] [Footnote 4: The motion of the earth in its orbit is at the rate of 68 thousand miles an hour.] _Mars_, at the distance of 145 millions of miles, in little less than two of our years. _Jupiter_, at the distance of 494 millions of miles, in near 12 years. _Saturn_, at the distance of 906 millions of miles, in about 30 years. _Georgian_, discovered a few years since by Dr. Herschell, performs its period at the distance of 1812 millions of miles, in about 83 years.[5] [Footnote 5: As the distances of the planets, when marked in miles, are a burthen to the memory, astronomers often express their mean distances in a shorter way, by supposing the distance of the earth from the sun to be divided into ten parts. Mercury may then be estimated at four of such parts from the sun, Venus at seven, the Earth at ten, Mars at fifteen, Jupiter at fifty-two such parts, Saturn at ninety-five, and Georgian 190 parts. See Plate I. Fig. 1. These are calculated by multiplying the respective distances of the planets by 10, and dividing by 95, the mean distance of the earth from the sun; and may be set off by any scale of equal parts.] PUPIL. What proportion does the earth bear in magnitude to the other planets? TUTOR. The earth is fourteen times as large as Mercury, very little larger than Venus, and three times as large as Mars. But Jupiter is more than fourteen hundred times as large as the earth; Saturn above a thousand times as large, exclusive of his ring; and Georgian eighty-two times as large. PUPIL. Have you any thing else, Sir, to remark concerning the planets?