The zenith is a point in the heavens exactly overhead and is the superior pole of our hori zon. The nadir is a point in the heavens exactly under our feet, being the inferior pole of our horizon, and the zenith or superior pole of the horizon of our antipodes.
The pole of any circle is a point on the surface of the globe 90 degrees distant from every part of the circle. Thus the poles of the world are 90 degrees from every part of the equator; the poles of the ecliptic (on the celes tial globe) are 90 degrees from every part of the ecliptic, and 23° 28' from the poles of the equinoctial. The equinoctial points are in the signs of Aries and Libra, where the ecliptic cuts the equinoctial. The vernal equinox is called the first point of Aries, and the autumnal the first point of Libra. When the sun is in either of these points the days and nights on every part of the globe are equal to each other. The solstitial points are in Cancer and Capricorn. When the sun enters Cancer it is the longest day to all the inhabitants north of the equator, and the shortest day to those on the south side. When the sun enters Capricorn it is the shortest day to those who live in north latitude, and the longest day to those who live in south lati tude. The latitude of a place on the terrestrial
globe, or its distance from the equator in de grees and minutes, or geographical miles, is reckoned on the brass meridian from the equator toward the pole. The quadrant of alti tude is a thin piece of brass divided upward from 0 to 90 degrees, downward from 0 to 18 degrees; when used it is generally screwed to the brass meridian. The upper divisions de termine the distances of places on the earth, the distances of the celestial bodies, their lati tudes, etc.; and the lower divisions are applied to finding the beginning, the end and duration of twilight. The longitude of a place on the terrestrial globe is the distance of the meridian of that place from theprime meridian, reckoned in degrees and parts of a degree, on the equator. Longitude is either east or west, according as a place is east or west of the prime meridian. No place can have more than 180 degrees, or half the circumference of the globe. Hour circles are the same as meridians. The brass meridian and these circles always correspond.