The elements of astronomy; a textbook . ny ancient astronomer ignorant of the earths rota-tion, and it is still perfectly correct. But knowing, as we nowdo, that this apparent revolution of the celestial sphere is dueto the real rotation of the earth on its axis, we may also definethe poles as the points where the earths axis of rotation, pro-duced indefinitely, would pierce the celestial sphere. Since the two poles are dia-metrically opposite in the sky,only one of them is usually vis-ible from any given place. Ob-servers north of the earthsequator see only the north pole,and vice versa for o
The elements of astronomy; a textbook . ny ancient astronomer ignorant of the earths rota-tion, and it is still perfectly correct. But knowing, as we nowdo, that this apparent revolution of the celestial sphere is dueto the real rotation of the earth on its axis, we may also definethe poles as the points where the earths axis of rotation, pro-duced indefinitely, would pierce the celestial sphere. Since the two poles are dia-metrically opposite in the sky,only one of them is usually vis-ible from any given place. Ob-servers north of the earthsequator see only the north pole,and vice versa for observers inthe southern hemisphere. 27. The Celestial Equator, orEquinoctial.— This is a greatcircle of the celestial sphere,drawn half-way between the poles(therefore everywhere 90° fromeach of them), and is the great circle in ivhich the plane of theearths equator cuts the celestial sphere. It is often called theli Equinoctial. Pig. 6 shows how the plane of the equatorproduced far enough would mark out such a circle in Fig. 6. — The Plane of the Earths Equator produced to cut the Celestial Sphere. The equator cuts the horizon at the east and west points, but itdoes not cut it perpendicularly nor pass through the zenith unless the Pole-star (at present) is about \\° distant from the pole. If we draw animaginary line from the Pole-star to the star Mizar (Zeta Ursae Majoris,the one at the bend of the Dipper handle), it will pass almost exactlythrough the pole itself; the distance of the pole from the Pole-star beingvery nearly one-quarter of the distance between the two Pointers. § 28] HOUR-CIRCLES. 19 observer is at the earths equator; at its highest point it is just asfar below the zenith as the pole is above the horizon. 28. Parallels of Declination. — Small circles drawn parallelto the equinoctial, like the parallels of latitude on the earth,are known as Parallels of Declination. For any star situatedon one of these parallels, the parallel is obviously
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