BOOK II. OF THE CELESTIAL GLOBE. CHAPTER I. Definitions and terms belonging to the celestial globe. 1. The celestial globe, as has already been observed, is an artificial representation of the heavens, having all the stars of the first and second magnitude, and the most noted of the rest that are visible, truly represented on it, according to their proper angular distances in the concave surface of the heavens. 2. The rotation of this globe upon its axis from east to west, represents the apparent diurnal motion of the concave surface of the celestial sphere, on an axis passing through the poles of the world, completing its revolution in 23 hours, 56 minutes, and 4 seconds nearly, and carrying along with it the sun, moon, and stars. The axis of the celestial sphere, is usually called the axis of the heavens. This hypothesis illustrates and represents the apparent diurnal motion of the several celestial objects in parallel circles, with an equable motion, each completing its circular path in the same time. That the motion of each star is equable, and that they describe parallel circles on the concave surface, we reduce from observation and the computation of spherical trigonometry. See Dr. Brinkley's Astronomy. 3. The wooden horizon circumscribing the celestial globe, is divided exactly into the same concentric circles, as the wooden horizon of the terrestrial globe. See Book I. Chap. IV. The horizon of the celestial globe must be considered as continued to pass through the centre, where the eye is supposed situate viewing the hemisphere above the horizon, and the axis of the globe is to be placed at the same elevation as the axis of the concave surface of the spectator. In this way all the circles of the celestial sphere will be easily understood. Any consideration of the form or figure of the earth is entirely foreign to a knowledge of the circles of the sphere. They were originally invented without any reference to it. And in fact, the progress in astronomy was from the celestial circles to terrestrial, and not the contrary. 4. That imaginary great circle in the heavens, which the sun describes in his apparent diurnal revolution at the time of the equinoxes, or when the days and nights are equal all over the world, is called the equinoctial, and sometimes the celestial equator.. The circle in which the plane of the equinoctial cuts the surface of the earth, is usually called the equator or terrestrial equator, which has been already defined, (Art. 5, page 8.) 1 It is however proper to observe, that in treatises on astronomy and the globes, the terms equinoctial and equator are used indifferently for each other. 5. A great circle passing through the poles of the world and through the zenith of a place, is called the celestial meridian of that place. The celestial meridians are also called circles of declination. (See Art. 5, page 21.) The circle in which the plane of the celestial meridian intersects the surface of the earth, is called the terrestrial meridian. Those terms are used indifferently for each other. (See Art. 10, page 9.) There are no meridians drawn on the celestial globe; but they are supplied by the brazen meridian, which is graduated in the same manner as the brazen meridian belonging to the terrestrial globe. (Art. 12, page 10.) 6. The ecliptic, colures, equinoctial and solstitial points, are situated on the celestial globe just as on the terrestrial; and therefore, it is unnecessary to take any farther notice of them here, as they have already been sufficiently defined in the first book. It is also proper to observe that the tropics, polar circles, and parallels of declination, on the celestial sphere, correspond to the tropics, polar circles, and parallels of latitude on the terrestrial globe. 7. The poles of the ecliptic are situated on the celestial globe, at the distance of 23° 28' from the poles of the equinoctial. For the pole of any great circle on the surface of the sphere, is 90 degrees distant from every part of its circumference, and the angle which the ecliptic makes with the equinoctial is equal to 230 28'; consequently, the north pole of the ecliptic must be 230 28 distant from the north pole of the equinoctial, and the south poles must likewise be similarly situated. 8.. Secondaries to the ecliptic are called circles of celestial latitude, or circles of latitude; because the arc of the secondary, intercepted between any celestial object, and the ecliptic is called its latitude, north or south; according as the object is on the north or south side of the ecliptic. Every point on the surface of the celestial sphere is supposed to have a circle of celestial latitude passing through it, though, to prevent confusion, there are, in general, only twelve drawn on most of the celestial globes, the rest being supplied by the quadrant of altitude. 9. The longitude of a heavenly body is an arc of the ecliptic intercepted, in the order of the signs, between the equinoctial point Aries, and a circle of celestial latitude. passing through the body. Hence, the latitudes and longitudes of the heavenly bodies are ascertained by secondaries to the ecliptic, and the latitudes and longitudes of places upon the earth, are found by secondaries to the equator. 10. The right ascension of a heavenly body is an arc of the equinoctial intercepted, reckoning in the order of the signs, between the vernal equinoctial point and a circle of declination passing through the body. And the arc of the circle of declination intercepted between the celestial object and the equinoctial, is called the declination of the object. The definitions contained in this article agree exactly with those which are given in Art. 5, page 21, and Art. 12, page 36. In the practice of astronomy, the most general and convenient method of ascertaining the position of any celestial object on the concave surface, is to determine its position with respect to the equinoctial, or celestial equator, and the vernal equinoctial point, that is, to determine its declination and right ascension. The position of a celestial object, with respect to the equinoctial, being ascertained, it is very often necessary to determine its position with respect to the ecliptic, that is, to determine its latitude and longitude. See the foregoing two articles. 11. Diurnal arc is the arc described by the sun, moon, or stars, from their rising to their setting. The sun's semidiurnal arc is the arc described in half the length of the day. 12. Nocturnal arc is the arc described by the sun, moon, or stars, from their setting to their rising. 13. That parallel of declination, in an oblique sphere, which is as many degrees distant from the elevated pole of the heavens, as the place itself is distant in degrees from the equator, is called the circle of perpetual apparition; because all the stars included within this circle, are continually above the horizon of the place, and consequently never set. 14. The circle of perpetual occultation is another parallel of declination, opposite the former, and at a like distance from the depressed pole of the heavens. All the stars contained within this circle, never appear above our horizon, and consequently never rise. All the stars contained between these two circles, do alternately rise and set at certain moments of the diurnal rotation. QUESTIONS. How are the stars represented on the celestial globe? What does the rotation of this globe on its axis from east to west represent; and what is the axis of the celestial sphere called?. How is the wooden horizon of the celestial globe divided? What is the equinoctial or celestial equator? What are the celestial meridians, and what are they usually called ? At what distance in degrees is the north pole of the ecliptic from the north pole of the equinoctial? What are the circles of celestial latitude, and what is the latitude of a heavenly body ? What is the longitude of a heavenly body ? What is the right ascension and declination of a heavenly body?.. What are the diurnal and nocturnal arcs? What is the circle of perpetual apparition? What is the circle of perpetual occulation? |