It is measured on the equinoctial, eastward from a fixed point, to the meridian of the celestial body. Right ascension is measured from a point called the vernal equinox, or first point in Aries, which is indicated by the sign v. The meaning of these terms will be explained hereafter. (55.) 36. The place of a star.-We locate a place on the earth by giving its latitude and longitude; we locate a star on the sky by giving its declination and right ascension; we tell where it seems to be at any moment when it is above our horizon by giving its altitude and azimuth. We refer to the Equator, Latitude and longitude, terrestrial; Horizon, The declination and right ascension of a star are the same at all places on the earth; the altitude and azimuth vary with the position of the observer. His real ho 37. The sky as seen from the pole.-Let us suppose that a person is at the north pole, and that during the long night there, he sits in the same position for twentyfour hours. The earth rotates, and he turns with it. He is not conscious of motion, and therefore the stars seem to pass before him; they go toward his right hand, pass behind him, and re-appear upon his left. The pole of the heavens is over his head, in the zenith (8). rizon (6) coincides with the equinoctial. All heavenly bodies which have north declination, are above the horizon and are visible; all which have south declination are invisible. Celestial meridians coincide with vertical circles (9); the altitude (13) of a star is the same as its declination. The cardinal points can not be distinguished, for north is over his head; south is under his feet; westward is always toward his right hand, and eastward toward his left. Whatever route he takes is toward the south, that is, away from the north pole. zon. 38. The sky as seen from the equator.-An observer at the equator will find the poles of the sky in his horiThe equinoctial will pass through his zenith, and will coincide with the prime vertical (10). The sun and stars near the equinoctial will rise directly in the east, and will set directly in the west; other stars will seem to describe smaller circles whose planes are perpendicular to the horizon, and each star will be visible just twelve hours. THE LATITUDE OF THE OBSERVER. 39. The latitude of the observer is equal to the altitude of the pole. When the observer is at the equator, his horizon extends to the poles of the sky. If he goes ten degrees north of the equator, his zenith will be ten degrees north of the equinoctial, and his horizon will be removed ten degrees beyond the north pole; the pole will seem to have risen ten degrees above his horizon. At 20° north latitude the altitude of the pole will be 20°. At the pole, latitude, 90° (24), the pole of the heavens will be in altitude 90°, or in the zenith (37). Hence, the latitude of a place is found by finding the altitude of the nearest pole. 40. The length of a degree of latitude. We shall have gone one degree to the north whenever we shall have increased the altitude of the north pole one degree. The length of a degree of latitude differs slightly at different distances from the equator, being shortest near the equator, and longest near the pole. The average length is about 69 miles. Multiplying the length of a degree by the number of degrees in a circle, we find the circumference of the earth to be 24,930 miles; this gives a diameter of 7935 miles, nearly as found before (16). 41. The pole star.-The north pole of the heavens is near a rather bright star, called the North Star, Polaris, or the Pole Star. To the ordinary observer 'the pole star seems stationary, yet careful observation shows that it has a daily motion about the pole, like other stars in the sky. The sailor on the ocean, the Arab in the desert, the Indian in the forest, each considers this the only motionless star in the heavens, and guides himself by it. No similar star shows so nearly the place of the south pole of the heavens. 42. Circle of perpetual apparition. The largest circle about the pole, which does not pass below the horizon, is called the circle of perpetual apparition. The stars within it do not set, and vanish only because of the superior light of day. A similar circle about the opposite pole, which does not come above the horizon, is the circle of perpetual occultation. The stars within this circle never rise. The radii of the circles of perpetual apparition and occultation are equal to each other, to the altitude of the pole, and (39) to the latitude of the place of observation. 43. To a person south of the equator the circle of perpetual apparition is about the south pole; to one at either pole both circles coincide with the horizon and the equinoctial; to one at the equator, they are nothing. The heavenly bodies within the circle of perpetual apparition are called circumpolar bodies. THE TERRESTRIAL MERIDIAN. 45. The plane of the meridian.-A terrestrial meridian has been defined (22) as a circle of the earth which passes through the north and south poles. A line on the floor, a fence, or the foundation of a building, when placed precisely north and south, will mark a meridian line, and the posts of the fence, or the side of the building, if placed truly plumb, will be in the plane of the meridian. Fig. 13. 46. Hang a plummet by a fine strong line, so that the weight may dip into a vessel of water; this will prevent the line from swinging by the force of the wind. Fix a second line due north of the first, and the two lines will indicate the plane of the meridian quite accurately. While the eye is at one line, any object on the earth or the sky, seen to pass the other line may be said to cross the meridian. Fig. 14. As these lines can not be seen in the night, a still simpler way is to fix some stand-point fifty or one hundred feet north or south of a corner of a building which is usually plumb. A star crosses the meridian at the instant when an observer, standing at this point, sees it disappear behind the house. 47. Culminations.— The passage of a star across the meridian is called its culmination. As the plane of the meridian extends infinitely in both directions through the axis of the sky, each star passes the meridian twice in traversing its curve of daily motion; once above, and once below the axis. The upper passage is called the superior, the lower, the inferior, culmination. The inferior culmination is visible when the star is within the circle of perpetual apparition (42). THE MEASURE OF A YEAR. 48. The same stars pass the meridian at different hours of different nights. When the plane of the meridian has been found (46), let us watch the culmination of some star, and let us note the time; suppose it to be eight o'clock in the evening. The culmination of the same star on the next night will occur about four minutes earlier than it did the night before, and so on for the following |