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. ------------------------------------------------------------------------ DIALOGUE V. TUTOR. Before I proceed to explain what I promised you, it is necessary you should be informed that the earth as seen from the sun, in its periodical revolutions, will describe a circle among the stars which astronomers call the _ecliptic_, and sometimes _the sun’s annual path_, because the sun, as seen from the earth, always appears in that line. PUPIL. Do not all the planets move in the ecliptic? TUTOR. No.—On account of the obliquity of their orbits, they are, in every revolution, one half of their periods above the ecliptic, and the other half below it. PUPIL. I think I comprehend your meaning; but shall be obliged to you, Sir, if you can make it clearer to me.