The ancients, in reducing astronomy to a science, combined the fixed stars into constellations, allowing several stars to make one constellation: and, for the better distinguishing and observing them, they reduced the constellations to the forms of animals, or to the images of some known things, by which means they were enabled to signify to others any particular star they meant to notice. Job mentions two of the constellations, namely, Orion and Pleiades, which shews the study of astronomy to be very ancient.

Pupil. Pray, Sir, how may I know them?

Tutor. By studying the use of the cÆlestial globe, on which they are drawn.

Pupil. Will you be kind enough to instruct me, Sir?

Tutor. At some future time I probably may: at present you are not prepared for it.

Pupil. I am satisfied.—Have you any thing more to remark of the constellations, Sir?

Tutor. Yes. The situation of the planets, as they are continually changing their places, could not be pointed out without first dividing the stars into constellations: hence, necessity was the mother of invention.

Pupil. And I think a very ingenious one.—If I may be allowed a comparison, I will suppose the different kingdoms of the world on my dissected map, to represent so many constellations; then, if I hear of London, I know it is in England; if of Paris, in France; of Lisbon, in Portugal; and so on. These I would compare with stars of the first magnitude, being the chief cities of their respective kingdoms; inferior cities, stars of the second magnitude; principal towns of the third, &c.

Tutor. A very apt comparison indeed. Now if you hear of a traveller setting off from London to Dover, thence to Calais, Paris, Bern, and so on to Rome, you know that he must go through part of England, Flanders, France, Switzerland, and Italy, passing many towns and villages on his way.

Pupil. That is very evident.

Tutor. Very well, then; in like manner would the planets, if seen from the sun, be traced from star to star, from constellation to constellation, through their whole periods.

Pupil. It is not possible to view them from the sun, surely, is it?

Tutor. No, certainly.

Pupil. Why then do you say if seen from the sun?

Tutor. Because it is there only their motions can appear uniform; as seen from the earth they apparently move very irregularly.—Suppose you were in the center of a circular course; and, whilst a horse was going round, you kept your eye on him: cannot you conceive that you should see him run round the course in a regular manner, moving the whole time the same way?

Pupil. It is not at all difficult to conceive.

Tutor. Again. Imagine yourself placed at a considerable distance on the outside of the course, where you could see the horse the whole time he was going round, would he appear to move as uniformly as before?

Pupil. Certainly not: on the opposite side of the course his motion would be the same as when I stood in the center of it; when he was approaching me, I should scarcely see him move; in that part of the course next to me he would move in a direction contrary to what he did at first; and again when going from me, his motion would be scarcely visible.

Tutor. This I think will give you a tolerable idea of the irregular motion of the inferior planets, as seen from the earth. When farthest from us their motion is said to be direct; when nearest to us retrograde, because they appear to be moving back again; and, when approaching, or going from us, we say they are stationary; because, if then observed in a line with any particular star, they will continue so for a considerable time: now these appearances could not happen if they moved round the earth.

Pupil. Nothing can be plainer: for if the earth were in the center we should always see them move the same way.

Tutor. When the planet is nearest to us, that is in a line between us and the sun, we say it is in its inferior conjunction; when farthest from us, and the sun is between us and the planet, in its superior conjunction. But the superior planets have alternately a conjunction and an opposition.

Pupil. A conjunction, I suppose, when the sun is between the earth and the planet, and an opposition when the earth is between the sun and the planet; that is, when the planet is nearest to us, and appears to be opposite to the sun?

Tutor. You are right.—Therefore, when in conjunction it rises and sets, nearly with the sun; but in opposition, it rises nearly when the sun sets, and sets when he rises.

Pupil. Why do you say nearly, Sir?

Tutor. Because it cannot be exactly, but when the sun, earth, and planet are in a right line, which seldom happens.

Pupil. How do you account for this, Sir?

Tutor. At present I fear you will not be able to comprehend what I wish to explain, as I must use a term you are unacquainted with. The reason is, that the planets are very seldom in or near their nodes at their conjunctions or oppositions.

Pupil. I do not indeed understand what you mean by the word nodes.

Tutor. It will be explained to you in due time, and I shall conclude this evening with a few more remarks relative to the appearance of the planets.

Pupil. Any thing you please, Sir.

Tutor. You know that the planets, being opaque bodies, receive their light from the sun; and that only that part which is turned to the sun can be enlightened by him, whilst the opposite side must remain in darkness.

Pupil. This is self-evident: if I hold my ball to the candle it will have the same effect.

Tutor. Tell me then how you think they will appear as seen from the earth.

Pupil. If, when you shewed me Venus, she had not appeared perfectly round, I should say that, both before and after her superior conjunction I should see her nearly with a full face; when stationary, only half enlightened, like the moon at first quarter; because, an equal portion of the dark and bright parts will be turned towards us; the bright part will be decreasing till her inferior conjunction, when the dark side will be turned towards us, and consequently invisible; the light will then increase; and, when she is again stationary, she will appear like the moon at last quarter.

Tutor. When seen through a telescope she has the different appearances you have mentioned; and when I next see you I will shew you that both Venus and Mercury may sometimes be seen when in their inferior conjunctions; the superior planets always appear with nearly a full face.

Pupil. How are the planets distinguished from each other?

Tutor. Mercury, from his vicinity to the sun, is seldom seen, being lost in the splendor of the solar brightness. When seen, he emits a very bright white light.

Venus, known by the names of the morning and evening star, is the brightest, and to appearance, the largest of all the planets; her light is of a white colour, and so considerable, that in a dusky place she projects a sensible shade. She is visible only for three or four hours in the morning or evening, according as she is before or after the sun.

Mars is the least bright of all the planets. He appears of a dusky reddish hue, and much larger at some periods than at others, according as he is nearer to, or farther from us.

Jupiter is distinguished by his peculiar magnitude and light. To the naked eye he appears almost as large as Venus, but not altogether so bright.

Saturn shines but with a pale feeble light, less bright than Jupiter, though less ruddy than Mars.

The Georgium Sidus cannot be readily perceived without the assistance of a telescope.