as the parallels of terrestrial latitude extended to the Celestial Sphere. (c) The POINTS are the Celestial Poles and the Equinoxes. The Celestial Poles are the points where the axis of the earth extended pierces the Celestial Sphere, and are the extremities of the celestial axis, just as the poles of the earth are the extremities of the earth's axis. The North Point is marked very nearly by the North Star, and every direction from that is reckoned South, and every direction toward that is reckoned North, however it may conflict with our ideas of the points of the compass. The Equinoxes are the points where the Equinoctial and the Ecliptic (the sun's apparent path through the heavens) intersect. (d) The MEASUREMENTS are Right Ascension (R. A.), Declination, and Polar Distance. Right Ascension is distance from the Vernal Equinox, measured on the equinoctial eastward. R. A. corresponds to terrestrial longitude, and may extend to 360° East, instead of 180° as on the earth. R. A. is never measured westward. The starting point is the meridian passing through the vernal equinox; as the meridian passing through Greenwich is the point from which terrestrial longitude is measured. Declination is distance from the equinoctial, measured on any vertical circle or meridian North or South. It corresponds to terrestrial latitude. Polar distance (the complement of Declination) is the distance from the Pole, measured on a vertical circle. The Equinoctial System is largely used by modern astronomers, and accompanies the Equatorial Telescope, Sidereal Clock, and Chronographs of the best Observatories. III. THE ECLIPTIC SYSTEM. (a) The PRINCIPAL CIRCLE is the Ecliptic. This is the earth's orbit about the sun, or the apparent path of the sun in the heavens. It is inclined to the equinoctial 23° 28', which measures the inclination of the Earth's Equator to its orbit, and is called the obliquity of the ecliptic. (b) The SUBORDINATE CIRCLES are Circles of Celestial Longitude, the Colures, and Parallels of Celestial Latitude. The Circles of Celestial Longitude are now less employed. They are measured on the Ecliptic, as circles of Right Ascension (R. A.) are now measured on the Equinoctial. The Colures are two principal meridians; the Equinoctial Colure is the meridian passing through the equinoxes; the Solstitial Colure is the meridian passing through the solstitial points. The Parallels of Celestial Latitude are now little used, but are small circles drawn parallel to the ecliptic, as parallels of declination are now drawn parallel to the equinoctial. (c) The POINTS are the Poles of the Ecliptic, the Equinoxes, and the Solstices. The Poles of the Ecliptic are the points where the axis of the earth's orbit meets the Celestial Sphere. (Little used.) The Equinoxes are the points where the ecliptic intersects the equinoctial. The place where the sun crosses the equinoctial* in going North, which occurs about the 21st of March, is called the Vernal Equinox. The place where the sun crosses the equinoctial in going South, which occurs about the 21st of September, is called the Autumnal Equinox. The Solstices are the two points of the ecliptic most distant from the Equator; or they may be considered to mark the sun's furthest declination, North and South of the equinoctial. The Summer Solstice occurs about the 22d of June; the Winter Solstice occurs about the 22d of December. (d) The MEASUREMENTS are celestial longitude and latitude. Celestial longitude is distance from the Vernal Equinox measured on the ecliptic, eastward. Celestial latitude is distance from the ecliptic measured on a Subordinate circle, north or south. THE ZODIAC. A belt of the Celestial Sphere, 8° on each side of the ecliptic, is styled the Zodiac. This is of very "This is what is commonly called "crossing the line." high antiquity, having been in use among the ancient Hindoos and Egyptians. The Zodiac is divided into twelve equal parts-of 30° each--called Signs, to each of which a fanciful name is given. The following are the names of the "The first, , indicates the horns of the Ram; the second, 8, the head and horns of the Bull; the barb attached to a sort of letter m, designates the Scorpion; the arrow, 4, sufficiently points to Sagittarius; vs is formed from the Greek letters rp, the two first letters of rpayos, a goat. Finally, a balance, the flowing of water, and two fishes, tied by a string, may be imagined in,, and x, the signs of Libra, Aquarius, and Pisces." |