178. The Atmosphere refracts the Sun’s rays so, as to bring him in sight every clear day, before he rises in the Horizon; and to keep him in view for some minutes after he is really set below it. For, at some times of the year, we see the Sun ten minutes longer above the Horizon than he would be if there were no refractions: and about six minutes every day at a mean rate.
Fig. IX.
[PLATE II].
179. To illustrate this, let IEK be a part of the Earth’s surface, covered with the Atmosphere HGFC; and let HEO be the[[43]] sensible Horizon of an observer at E. When the Sun is at A, really below the Horizon, a ray of light AC proceeding from him comes straight to C, where it falls on the surface of the Atmosphere, and there entering a denser medium, it is turned out of its rectilineal course ACdG, and bent down to the observer’s eye at E; who then sees the Sun in the direction of the refracted ray edE, which lies above the Horizon, and being extended out to the Heavens, shews the Sun at B § [171].
Fig. IX.
180. The higher the Sun rises, the less his rays are refracted, because they fall less obliquely on the surface of the Atmosphere § [172]. Thus, when the Sun is in the direction of the line EfL continued, he is so nearly perpendicular to the surface of the Earth at E, that his rays are but very little bent from a rectilineal course.
The quantity of refraction.
181. The Sun is about 321⁄4 min. of a deg. in breadth, when at his mean distance from the Earth; and the horizontal refraction of his rays is 333⁄4 min. which being more than his whole diameter, brings all his Disc in view, when his uppermost edge rises in the Horizon. At ten deg. height the refraction is not quite 5 min. at 20 deg. only 2 min. 26 sec.; at 30 deg. but 1 min. 32 sec.; between which and the Zenith, it is scarce sensible: the quantity throughout, is shewn by the annexed table, calculated by Sir Isaac Newton.
182. A Table shewing the Refractions of the Sun, Moon, and Stars; adapted to their apparent Altitudes.
| Appar. Alt. | Refraction. | Ap. Alt. | Refraction. | Ap. Alt. | Refraction. | ||||||
|---|---|---|---|---|---|---|---|---|---|---|---|
| D. | M. | M. | S. | D. | M. | S. | D. | M. | S. | ||
| 0 | 0 | 33 | 45 | 21 | 2 | 18 | 56 | 0 | 36 | ||
| 0 | 15 | 30 | 24 | 22 | 2 | 11 | 57 | 0 | 35 | ||
| 0 | 30 | 27 | 35 | 23 | 2 | 5 | 58 | 0 | 34 | ||
| 0 | 45 | 25 | 11 | 24 | 1 | 59 | 59 | 0 | 32 | ||
| 1 | 0 | 23 | 7 | 25 | 1 | 54 | 60 | 0 | 31 | ||
| 1 | 15 | 21 | 20 | 26 | 1 | 49 | 61 | 0 | 30 | ||
| 1 | 30 | 19 | 46 | 27 | 1 | 44 | 62 | 0 | 28 | ||
| 1 | 45 | 18 | 22 | 28 | 1 | 40 | 63 | 0 | 27 | ||
| 2 | 0 | 17 | 8 | 29 | 1 | 36 | 64 | 0 | 26 | ||
| 2 | 30 | 15 | 2 | 30 | 1 | 32 | 65 | 0 | 25 | ||
| 3 | 0 | 13 | 20 | 31 | 1 | 28 | 66 | 0 | 24 | ||
| 3 | 30 | 11 | 57 | 32 | 1 | 25 | 67 | 0 | 23 | ||
| 4 | 0 | 10 | 48 | 33 | 1 | 22 | 68 | 0 | 22 | ||
| 4 | 30 | 9 | 50 | 34 | 1 | 19 | 69 | 0 | 21 | ||
| 5 | 0 | 9 | 2 | 35 | 1 | 16 | 70 | 0 | 20 | ||
| 5 | 30 | 8 | 21 | 36 | 1 | 13 | 71 | 0 | 19 | ||
| 6 | 0 | 7 | 45 | 37 | 1 | 11 | 72 | 0 | 18 | ||
| 6 | 30 | 7 | 14 | 38 | 1 | 8 | 73 | 0 | 17 | ||
| 7 | 0 | 6 | 47 | 39 | 1 | 6 | 74 | 0 | 16 | ||
| 7 | 30 | 6 | 22 | 40 | 1 | 4 | 75 | 0 | 15 | ||
| 8 | 0 | 6 | 0 | 41 | 1 | 2 | 76 | 0 | 14 | ||
| 8 | 30 | 5 | 40 | 42 | 1 | 0 | 77 | 0 | 13 | ||
| 9 | 0 | 5 | 22 | 43 | 0 | 58 | 78 | 0 | 12 | ||
| 9 | 30 | 5 | 6 | 44 | 0 | 56 | 79 | 0 | 11 | ||
| 10 | 0 | 4 | 52 | 45 | 0 | 54 | 80 | 0 | 10 | ||
| 11 | 0 | 4 | 27 | 46 | 0 | 52 | 81 | 0 | 9 | ||
| 12 | 0 | 4 | 5 | 47 | 0 | 50 | 82 | 0 | 8 | ||
| 13 | 0 | 3 | 47 | 48 | 0 | 48 | 83 | 0 | 7 | ||
| 14 | 0 | 3 | 31 | 49 | 0 | 47 | 84 | 0 | 6 | ||
| 15 | 0 | 3 | 17 | 50 | 0 | 45 | 85 | 0 | 5 | ||
| 16 | 0 | 3 | 4 | 51 | 0 | 44 | 86 | 0 | 4 | ||
| 17 | 0 | 2 | 53 | 52 | 0 | 42 | 87 | 0 | 3 | ||
| 18 | 0 | 2 | 43 | 53 | 0 | 40 | 88 | 0 | 2 | ||
| 19 | 0 | 2 | 34 | 54 | 0 | 39 | 89 | 1 | 1 | ||
| 20 | 0 | 2 | 26 | 55 | 0 | 38 | 90 | 0 | 0 | ||
[PLATE II].
The inconstancy of Refractions.
A very remarkable case concerning refraction.