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.
[1]. From Planeta, roving or wandering.