After the same manner the globe being rectified, you may distinguish those Stars that are to the Southward of you, and be soon acquainted with all the Stars that are visible in our hemisphere.
SCHOLIUM.
The globe being rectified to the latitude of any place, if you turn it round its axis, all those Stars that do not go below the horizon during a whole revolution of the globe, never set in that place; and those that do not come above the horizon never rise.
Prob. XXXVII. The latitude of the place being given; to find the Amplitude, Oblique Ascension and Descension, Ascensional Difference, Semi-diurnal Arch, and the time of continuance above the horizon, of any given point in the heavens.
Having rectified the globe for the latitude, and brought the given point to the meridian, set the index to the hour of 12; then turn the globe until the given point be brought to the Eastern side of the horizon, and that degree of the equinoctial which is cut by the horizon at that time, will be the Oblique Ascension; and where the given point cuts the horizon, is the Amplitude Ortive: If the globe be turned about until the given point be brought to the Western side of the horizon, it will there show the Amplitude Occasive; and where the horizon cuts the equinoctial at that time, is the Oblique Descension.
The time between the index at either of these two positions, and the hour of 6; or half the difference between the oblique ascension and descension is the Ascensional Difference.
If the place be in North latitude and the declination of the given point be (North/South) the ascensional difference reduced into time, and (added to/subtracted from) 6 o’Clock, gives the Semi-diurnal Arch; the complement whereof to a semicircle, is the Semi-nocturnal Arch. If the place be in South latitude, then the contrary is to be observed with respect to the declination.
The semi-(diurnal/nocturnal) arch being doubled, gives the time of continuance (above/below) the horizon. Or the time of continuance above the horizon, may be found by counting the number of hours contained in the upper part of the horary circle, betwixt the place where the index pointed when the given point was in the Eastern or Western parts of the horizon. If the given point was the Sun’s place, the index pointed the time of his rising and setting, when the said place was in the Eastern and Western parts of the horizon, as in [Prob. 18]. Or the time of Sun-rising may be found by adding or subtracting his ascensional difference, to or from the hour of six, according as the latitude and declination are either contrary or the same way.