Tutor. Undoubtedly: and as only one half of a spherical body can at any time be enlightened by a luminous body, that part of the earth only which is turned to the sun, can receive the benefit of his enlivening rays, when it will be day; whilst the opposite part will be involved in darkness, and it will be night.
Pupil. I perceive it must be so. But, if the earth move in the manner you describe, I cannot conceive how it is that we are not sensible of its motion.
Tutor. If the motion of the earth were irregular it would be perceptible; but as it meets with no obstruction the motion must be so uniform as not to be perceived.
Pupil. Had I recollected this, I need not have given you this trouble.—But I am continually meeting with fresh difficulties.
Tutor. You have only to mention what they are, and I shall take a pleasure in removing them.
Pupil. I thank you, Sir; and shall be obliged to you to inform me, how the motion of the earth can cause the sun to appear to move?
Tutor. When in a carriage which went smoothly on the road, or in a boat whose motion was scarcely perceptible on the water, did you never fix your attention on the objects you passed?
Pupil. Yes, often, Sir.
Tutor. And had you not known that you really moved, and that the trees, &c. were immoveable in the ground, what then would have been your opinion?
Pupil. That the trees, &c. moved in a direction contrary to that in which I was moving.