All those places that are above the horizon, have the Sun above them, just so much as the places themselves are distant from the horizon; which height may be known by fixing the quadrant of altitude in the zenith, and laying it over any particular place. In all those places that are 18 degrees below the Western side of the horizon, the twilight is just beginning in the morning, or the day breaks. And in all those places that are 18 degrees below the Eastern side of the horizon, the twilight is ending, and the total darkness beginning. The twilight is in all those places whose depression below the horizon does not exceed 18 degrees. And, All those places that are lower than 18 degrees, have dark night. The depression of any place below the horizon is equal to the altitude of its _Antipodes_, which may be easily found by the quadrant of altitude. PROB. XV. _The Day of the Month being given; to show, at one view, the length of Days and Nights in all places upon the Earth at that time; and to explain how the vicissitudes of Day and Night are really made by the motion of the Earth round her axis in 24 hours, the Sun standing still._ The Sun always illuminates one half of the globe, or that hemisphere which is next towards him, while the other remains in darkness: And if (as by the last problem) we elevate the globe according to the Sun’s place in the ecliptic, it is evident, that the Sun (he being at an immense distance from the Earth) illuminates all that hemisphere, which is above the horizon; the wooden horizon itself, will be the circle terminating light and darkness; and all those places that are below it, are wholly deprived of the solar light. The globe standing in this position, those arches of the parallels of latitude which stand above the horizon, are the _Diurnal Arches_, or the length of the day in all those latitudes at that time of the year; and the remaining parts of those parallels, which are below the horizon, are the _Nocturnal Arches_, or the length of the night in those places. The length of the diurnal arches may be found by counting how many hours are contained between the two meridians, cutting any parallel of latitude, in the Eastern and Western parts of the horizon.