Precission of the equinoxes
The revolution of the earth's celestial pole around the pole of the ecliptic (see POLE, North, above) causes the equinoctial points (viz., the inter sections of equator and ecliptic, at hour XXIV, 360 degrees, and hour XII, 180 degrees) to shift their places among the stars, at the rate of I degree every 71% years, carrying with them all the circles of both the equator and the ecliptic systems, circles that during the "three score and ten years " of human life remain practically fixed and are so described and drawn in star-maps. Thus the equinoctial point called the "First point of Aries" (36oth degree) has advanced or "preceded" the stars of Aries far through the stars of Pisces (Map I, South). The Precession of the equinoxes with the signs of the zodiac is thus equivalent to the reces sion of the zodiacal constellations.Groups of fixed stars
Groups of stars. The constellations of the zodiac have been known from the remotest antiquity both by names and by signs, as Aries (the constellation) and V (the sign). (See SIGNS; also Table A.)Period of one revolution of the earth on its axis, divided into 24 hours. The civil day begins at midnight and counts 12 hours A. M. (before noon) and 12 hours p. M. (after noon). The astronomical day begins at noon (there fore 12 hours later than the civil day of the same date) and counts the hours from I to 24, from one noon to the next (mean solar time). Thus civic December 25th at 6 o'clock, A.M., is astronomic December 24th at 18 o'clock.
Angular distance from the celes tial equator. North declination is marked on all hour-circles, at intervals of 15 degrees from o (on the equator) to 90 degrees (Polaris). South declination (from the equator down) is indicated by adding S to the degree numbers. (Compare numbers above and below the equator on hour-circle XXIV, Map i, South.)
The 36oth part of the circumference of a circle.
The apparent track of the sun, moon and planets as seen from the earth; the actual track of earth and planets if viewed from the sun. The great circle on or near which all eclipses occur. The pole of the ecliptic circle ("E. P.") is shown in all north sections of the Road Map.
Terrestrial. A circle everywhere 90 degrees distant from the poles of the earth's axis. Celestial. A circle coinciding with the earth's equator, everywhere 90 degrees distant from the earth's pole extended to the star Polaris (shown in centre of all North maps, also in overlapping of all East and West maps). The celestial equator is often called the "equi noctial," because the sun crosses it at the equinoxes. But this "equinoctial" is at right angles with the equinoctial Colure.
The precise times when night and day are of equal length throughout the world, the vernal equinox (March 21), when the, sun (ascending) enters the sign Aries (stars of Pisces, Map I, South), or the beginning of Spring; (b) the autumnal equinox (September 21), when the sun (descending) enters the sign Libra (stars of Virgo, Map 7, South), or the beginning of Autumn. The point occu pied by the sun at noon March 21 is visible at midnight September 21 and vice versa. Table B shows other times when the equinoctial points (Maps I and 7) may be seen at night, though of course not on the meridian.
North sections of the Road Map
Thus THE ROAD MAP OF THE STARS will prove at once a key to the location and grouping of celestial objects, and an interesting introduction to the basic facts of the venerable system of the stars.In North sections of the Road Map show the direction of motion of "circum polar " stars; in East, the rising of stars; South, their course across the meridian; West, their setting. The arrow just over Polaris shows the direction in which the pole of the earth's axis describes a circle around the ecliptic pole. (See POLE, North.)
Axis. The straight line on which a body re volves. It points to the pole of the orbit of revolution.
CELESTIAL SPHERE. The apparent surface of the heavens which everywhere seems part of a sphere.
COLURES. Two hour-circles intersecting at right angles at the poles of the equator. The Equi noctial colure coincides with hour-circles XXIV (stars of Pisces, Map I, South), and XII (stars of Virgo, Map 7, South). The Solstitial colure coincides with hour-circles VI (between the stars of Gemini and Taurus, Map 4, South), and XVIII (between the stars of Sagittarius and Scorpio, Map 10, South).