Prob. III. To find the Difference of Longitude betwixt any two given Places.
Bring each of the given places successively to the meridian, and see where the meridian cuts the equator each time; the number of degrees contained betwixt those two points, if it be less than 180 degrees, otherwise the remainder to 360 degrees, will be the difference of longitude required. Or,
Having brought one of the given places to the meridian, bring the index of the hour circle to 12 o’clock; then having brought the other place to the meridian, the number of hours contained between the place the index was first set at, and the place where it now points, is the difference of longitude in time betwixt the two places.
Thus the difference of longitude betwixt Rome and Constantinople will be found to be 19 degrees, or 1 hour and a quarter; betwixt Mexico and Pekin in China, 240 degrees, or 9⅓ hours.
Prob. IV. Any Place being given to find all those Places that are in the same Latitude with the same Place.
The latitude of any given place being marked upon the meridian, turn the globe round its axis, and all those places that pass under the same mark are in the same latitude with the given place, and have their days and nights of equal lengths. And when any place is brought to the meridian, all the inhabitants that lie under the upper semicircle of it, have their Noon or mid-day at the same point of absolute time exactly.
Prob. V. The day of the Month being given; to find the Sun’s Place in the Ecliptic, and his Declination.
1. To find the Sun’s Place: Look for the day of the month given in the kalendar of months upon the horizon, and right against it you’ll find that sign and degree of the ecliptic which the Sun is in. The Sun’s place being thus found, look for the same in the ecliptic line which is drawn upon the globe, and bring that point to the meridian, then that degree of the meridian, which is directly over the Sun’s place, is the Declination required; which is accordingly either North or South, as the Sun is in the Northern or Southern signs. Thus,