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.
[Sidenote: 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.
[Sidenote: The quantity of refraction.]
181. The Sun is about 32-1/4 min. of a deg. in breadth, when at his mean
distance from the Earth; and the horizontal refraction of his rays is
33-3/4 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.| Refrac- ||Ap. | Refrac- ||Ap. | Refrac- |
| Alt. | tion. ||Alt.| tion. ||Alt.| tion. |
+-------+---------++----+---------++----+---------+
| 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 |
+-------+---------++----+---------++----+---------+
[Sidenote: PLATE II.
The inconstancy of Refractions.