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. [Footnote 6: Perhaps with more propriety _interior_ or _inward_.] [Footnote 7: _Exterior_ or _outward_.] 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. ------------------------------------------------------------------------ DIALOGUE II. PUPIL. I am afraid, Sir, I am come before you are prepared for me: but the very great pleasure I received yesterday, induced me to be with you as early as possible. TUTOR. I am glad to see you, and happy to find you are so well pleased with your difficult study. It will, I assure you, give you more exalted ideas of the Deity than any that I know of. The Psalmist was undoubtedly of this opinion when he said, The Heavens declare the glory of God, and the Firmament sheweth his handy work. PUPIL. I will no longer call it a difficult, but a pleasing study, and feel myself ashamed at having used the expression. I shall now beg you to explain to me the different systems.