its altitude at Havana, in the island of Cuba, and also the hour of the night. PROBLEM XVIII. The day of the month and hour when any star rises or sets being given, to find the latitude of the place of observation. RULE Bring the sun's place in the ecliptic to the brazen meridian, and set the index of the hour circle to 12; then, if the given time be in the forenoon, turn the globe eastward till the index has passed over as many hours as the time wants of noon; but, if the given time be in the afternoon, turn the globe westward till the index has passed over as many hours as the time is past noon; the globe being kept in this position, elevate or depress the pole till the centre of the given star coincides with the horizon; then, the elevation of the pole will show the latitude. EXAMPLES. 1. In what latitude does Rigel rise at half past six o'clock in the evening, on the 10th of December? Answer. In latitude 410 north, nearly. 2. In what latitude does Mirach in Bootes, rise at half past 12 o'clock at night, on the 10th of December? 3. In what latitude does Betelgueze in Orion, rise at 10 o'clock at night, on the 21st of January ? PROBLEM XIX. Two stars being given, the one on the meridian, and the other at the east or west part of the horizon, to find the latitude of the place. RULE. Bring the star which was observed to be on the meridian, to the brazen meridian; keep the globe from turning on its axis, and elevate or depress the pole till the other star comes to the eastern or western part of the horizon; then the degrees from the elevated pole, will be the latitude required. EXAMPLES. 1. When Vega in Lyra was on the meridian, a Arieties was rising; required the latitude. Answer. 40° 42' north, which is the latitude of New York. 2. When the two pointers of the Great Bear, marked a and ẞ, were on the meridian, Vega in Lyra was observed to be rising; required the latitude of the place of observation. 3. In what latitude are Sirius, and ẞ in Canis Major rising, when Algenib in Perseus, is on the meridian ? PROBLEM XX. The latitude of a place, the day of the month, and two stars, that have the same azimuth, being given, to find the hour of the night. RULE. Rectify the celestial globe, (by Prob. VII.) screw the quadrant of altitude upon the brazen meridian over the given degree of latitude; turn the globe on its axis from east to west, till the two given stars coincide with the graduated edge of the quadrant; then, the hours passed over by the index, will show the time from noon; and the common azimuth of the two stars will be found on the horizon. EXAMPLES. 1. On the 21st of November, what is the hour at New-York when Capella and Castor have the same azimuth, and what is the azimuth? Answer. At 9 o'clock in the evening, and the azimuth will be 61 from the north towards the east. 2. At what hour, at London, on the 1st of May, will Altair in the Eagle, and Vega in the Harp, have the same azimuth, and what will that azimuth be? 3. On the 21st of December, what is the hour at Dublin when Algenib in Perseus, and ẞin the Bull's horn, have the same azimuth, and what is that azimuth? PROBLEM XXI. The latitude of a place, the day of the month, and two stars, that have the same altitude, being given, to find the hour of the night. RULE. Rectify the celestial globe, (by Prob. VII.) and screw the quadrant of altitude upon the brazen meridian over the given latitude; then, turn the globe on its axis till the two given stars coincide with the graduated edge of the quadrant, and the hours passed over by the index will be the time from noon when the two stars have the same altitude. EXAMPLES. 1. At what hour at New-York, on the 20th of April, will yin the Dragon, and a in the Serpent, have each 35° of altitude? Answer. At half past 10 o'clock in the evening. 2. At what hour at London, on the 2nd of September, will Markab in Pegasus, Alpherast in the head of Andromeda, have each 30 degrees of altitude? |