Well, Sir! I suppose this early visit is in consequence of my promise, and your anxiety to become an astronomer.

Pupil. It is, Sir.—And as astronomy is a science of which I have a very imperfect idea, I must beg of you to explain it to me.

Tutor. That I shall do with pleasure. But you surely cannot wholly forget what I have formerly told you. However, as I mean to treat the subject as if you had no previous knowledge of it, you will have an opportunity from what you can recollect, to make such remarks, and ask such questions, as may appear most material to you.

Pupil. I thank you, Sir, it is just what I wish.

Tutor. By astronomy then is meant a knowledge of the heavenly bodies, the sun, moon, planets, comets, and stars, respecting their nature, magnitudes, distances, motions, &c.

Pupil. I fear I shall find it a difficult study.

Tutor. Have patience.——

Pupil. This gives me encouragement, and, if you will have patience with me, I will endeavour to profit by your instructions.——Pray, Sir, what is the sun?

Tutor. The sun, the source of light and heat, has been considered a globe of fire, round which seven other spherical bodies revolve at different distances from him, and in different periods of time, from west by south to east. These are the planets^[1].

Pupil. Any round ball is a globe, is it not?

Tutor. A sphere or globe is defined a round solid body, every part of whose surface is equally distant from a point within called its center; and a line drawn from one side through the center to the opposite side, is called its diameter.

Pupil. You say the sun has been considered a globe of fire. Is he not now thought to be so?

Tutor. ^[2]Doctor Herschell, from some late observations, is of a different opinion.—But what think you of his magnitude?

Pupil. I really cannot conjecture.—This I know, that when I saw him through the fog the other day, he appeared about the size of a common plate.

Tutor. You must not always judge by appearances. You will find that there is a material difference between his real and apparent magnitude, which I think you will be convinced of when I tell you, that he is no less than 95 millions of miles from our earth.

Pupil. Ninety-five millions of miles! You astonish me.

Tutor. You will, I dare say, be no less surprized at being told, that he is more than a million of times as large as our earth.

Pupil. It is almost incredible! And what are the planets?

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.

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]

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]

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?

Tutor. There are several other things I intend to make you acquainted with, namely, their nature, appearances, motions, &c. At present I shall only say, that Mercury and Venus are called ^[6]inferior planets, their orbits or paths described in going round the sun, being within that of the earth; and the other four, whose orbits are without the earth’s orbit, ^[7]superior planets.

Pupil. There is one thing more I wish to know, if——

Tutor. I suppose you were going to say if not too much trouble; that is quite unnecessary, as you well know that where I see a desire to learn, teaching is to me a pleasure.—What is it?

Pupil. That you will be so kind as to inform me what the comets are, and if they have any motion?

Tutor. The knowledge we have of comets is very imperfect, as they afford few observations on which to ground conjecture. They are generally supposed to be planetary bodies, forming a part of our system: for, like the planets, they revolve about the sun, but in different directions, and in extremely long elliptic curves, being sometimes near the sun, at others staying far beyond the orbit of the outermost planet; whereas the orbits of the planets are nearly circular. The period of one, which appeared in 1680, is computed to be 575 years.

Pupil. Whence do they derive their name?

Tutor. From Cometa, a hairy star, because they appear with long tails, somewhat resembling hair: some, however, have been seen without this appendage, as well defined and round as planets.

Pupil. You say our system: what am I to understand by it?

Tutor. The word system, in an astronomical sense, means a number of bodies moving round one common center or point: and, because the planets and comets revolve about the sun, it is called the Solar System (Plate I. fig. 2.); and we say our system, as the earth is one of the planets. Other systems have been invented for solving the appearances and motions of the heavenly bodies, a description of which I shall leave till I next see you.