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" It commences in the morning and ends in the evening, when the sun is 18° below the horizon. "
Spherical trigonometry - Page 241
by Benjamin Martin - 1736
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A treatise on astronomy

Olinthus Gilbert Gregory - 1802 - 590 pages
...May 4, 1801 ? % It was observed in Art. 107 that, at a medium, the twilight begins in the morning, and ends in the evening, when the sun is 18° below the horizon : therefore if tt (in fig. 4, PI. II.) represent a parallel to the horizon drawn 18° below it, or,...
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Elements of Astronomy: For the Use of Schools and Academies : with Questions

John Hubbard Wilkins - Astronomy - 1822 - 158 pages
...This faint light before sun rise and after sun set, is called twilight. It commences in the morning and ends in the evening, when the sun is 18° below the horizon. But the sun's apparent daily course is more oblique to the horizon at some places than at others; and...
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Elements of Astronomy: Illustrated with Plates, for the Use of Schools and ...

John Hubbard Wilkins - Astronomy - 1825 - 151 pages
...This faint light before sun rise and after sun set, is called twilight. It commences in the morning and ends in the evening, when the sun is 18° below the horizon. But the sun's apparent daily course is more oblique to the horizon at some places than at others ;...
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A Manual of Spherical and Practical Astronomy: Spherical astronomy

William Chauvenet - Astronomical instruments - 1863 - 764 pages
...for the moon. TIME OF THE BEGINNING AND ENDING OF TWILIGHT. 155. Twilight begins in the morning or ends in the evening when the sun is 18° below the horizon, and consequently the zenith distance is then £ = 90° + 18°, or h = — 18°, with which we can find...
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A Manual of Spherical and Astronomy: Embracing the ..., Part 32, Volume 1

William Chauvenet - Astronomical instruments - 1864 - 726 pages
...for the moon. TIME OF THE BEGINNING AND ENDING OF TWILIGHT. 155. Twilight begins in the morning or ends in the evening when the sun is 18° below the horizon, and consequently the zenith distance is then £ — 90° + 18°, or k = — 18°, with which we can...
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