Chapter VIII: Section 7: Cause of “sunrise” and “sunset.”

Although the Sun is at all times above and parallel to the Earth’s surface, he appears to ascend the firmament from morning until noon, and to descend and sink below the horizon at evening. This arises from a simple and everywhere visible law of perspective. A flock of birds, when passing over a flat or marshy country, always appears to descend as it recedes; and if the flock is extensive, the first bird appears lower, or nearer to the horizon than the last. When a balloon sails from an observer without increasing or decreasing its altitude, it appears gradually to approach the horizon. The farthest light in a row of lamps appears the lowest, although each one has the same altitude. Bearing these phenomena in mind, it will easily be seen how the Sun, although always parallel to the surface of the Earth, must appear to ascend when approaching, and descend after leaving the meridian or noon-day position. Let the line A B, Fig. 29, represent a portion of the Earth’s surface; C D of the Sun’s path, and H H, the line of sight. The surface of the Earth, A B, will appear to ascend from B to H, forming the horizon. When the Sun is traversing the line C D, in the direction of the arrows, he will appear to emerge from the horizon H, and to gradually ascend the line H D. When in the position 1, he will _appear_ to be at the point 2; and when at 3, the apparent position will be at 4; but when he arrives upon the meridian D, his apparent and actual, or noon-day position, will be the same. But now, from the point D, the Sun will appear to descend, as in Fig. 30, and when he has passed from D to 1, he will appear at 2, and when really at 3 will appear at 4; and thus continuing his course in the direction D C, he will reach the horizon at H, and disappear or “set” to the observer at H A. Thus “Sunrise” and “Sunset” are phenomena dependent entirely upon the fact that horizontal lines parallel to each other appear to approach or converge in the distance, the surface of the Earth being horizontal, and the line-of-sight of the observer and the Sun’s path being parallel with it, necessarily produce the observed phenomena.