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.