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The Use of
in the Zenith, and laying it over any par-ticular Place.
In all those Places that are i8 Degreesbelow the Western Side of the Horizon, theTwilight is just beginning in the Morning,or the Day breaks. And in all those Placesthat are 18 Degrees below the Eastern Sideof the Horizon, the Twilight is ending,and the total Darkness beginning.
The Twilight is in all those Places whoseDepression below the Horizon does not ex-ceed 18 Degrees. And> All those Places that are lower than i8Degrees have dark Night.
The Depression of any Place below theHorizon is equal to the Altitude of its An-tipodes, which may be easily found by theQuadrant of Altitude.
Prob. XV. The Day of the Month beinigiven ; to shew , at one View, the Lengthoj Day and Night in all Places upon theEarth at that Time ; and to explain hotthe Vicijjitudes _ of Day and Night arereally made by the Motion of the Earthround her Axis in 24 Hours , the Sunsanding still.
The Sun always illuminates one half ofthe Gobe, or that Hemisphere which isnext towards him, while the other remainsin Darkness: And if (as by the last Problem)
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