TUTOR. If there were no atmosphere, the sun would yield no light but when our eyes were directed towards him; and the heavens would appear dark and as full of stars as on a dark winter’s night; but the atmosphere being strongly illuminated by the sun, reflects the light back upon us, and makes the whole heavens to shine so strongly, that the faint light of the stars is obscured, and they are rendered invisible. PUPIL. I find then the atmosphere is of more use than I imagined. But how is it the cause of the twilight? TUTOR. The atmosphere is about 45 miles above the surface of the earth, therefore the sun’s rays falling upon the higher parts of it before rising, by reflection causes a faint light, which increases till he appears above the horizon; and in the evening it decreases after he sets, till he is 18 degrees below the horizon, where the morning twilight begins, and the evening twilight ends. PUPIL. By the horizon, I think you mean that distant boundary of our sight where the heavens and the earth seem to join all around us, as it appears from an eminence. TUTOR. The very same. ’Tis that imaginary circle which intercepts from our view the sun, moon, and stars each night; and when, by the rotation of the earth, they appear to descend below it, we say they are set; as on the contrary, each morning, when they appear above it, we say they rise. “To find the spacious line, cast round thine eyes, “And where the earth’s high surface joins the skies, “Where stars first set, and first begin to shine, “There draw the fancy’d image of this line.” PUPIL. A very pleasing description, indeed. TUTOR. You will remember that this is called the _rational horizon_; but that which respects land and water is called the _sensible horizon_. The former divides the heavens into two equal parts, and is 90 degrees distant from a point directly over our heads, called the _zenith_, and the opposite point of the heavens directly under our feet, called the _nadir_.—But I must resume the subject of the atmosphere. PUPIL. Had I not thought you had finished your description of the atmosphere, I should not have presumed to interrupt you. TUTOR. What I have told you respecting the horizon is necessary for you to be acquainted with; therefore, the suspension is immaterial.—You must, I make no doubt, have observed the sun and moon at rising and setting to appear larger than when higher above the horizon. PUPIL. I have, frequently, Sir. TUTOR. And cannot you tell the reason of it? PUPIL. No, Sir. TUTOR. The reason is this: In viewing them, when near the horizon, you see them through a thicker medium than when they are higher, that is, you see them through a greater quantity of the atmosphere; and you not only see them larger, but really above the horizon whilst they are actually below it. PUPIL. How do you account for this, Sir?