The elements of astronomy; a textbook . ses this pair of co-ordinates ispractically less convenient than right ascension and declination ; but,as has been said, it came into use much earlier, and is not without itsadvantages in dealing with the planets and the moon. 40. Relation of the Place of the Celestial Pole to the Ob-servers Latitude. — If an observer were at the north pole ofthe earth,- it is clear that the Pole-star would be very near hiszenith, while it would be at the horizon if he were at theequator. The place of the pole in the sky, therefore, dependsevidently on the observers lati
The elements of astronomy; a textbook . ses this pair of co-ordinates ispractically less convenient than right ascension and declination ; but,as has been said, it came into use much earlier, and is not without itsadvantages in dealing with the planets and the moon. 40. Relation of the Place of the Celestial Pole to the Ob-servers Latitude. — If an observer were at the north pole ofthe earth,- it is clear that the Pole-star would be very near hiszenith, while it would be at the horizon if he were at theequator. The place of the pole in the sky, therefore, dependsevidently on the observers latitude, and in this very simpleway — The Altitude of the Pole (its height in degreesabove the horizon) is ALWAYS EQUAL TO THE Latitude of the Ob-server. This relationwill be clear from Fig. latitude (astronomi-cal) of a place may bedenned as the angle be-tween the direction ofgravity at that place andthe plane of the earthsequator,-—the angle OJVQin the figure. If, now,at 0 we draw HH perpendicular to ON, it will be a Relation of Latitude to the Elevationof the Pole. 26 THE EIGHT SPHERE. [§ 40 line, and will lie in the plane of the horizon. From 0 alsodraw OP parallel to CP, the earths axis. Both OP1 and CPbeing parallel, will be directed to the same vanishing-pointin the celestial sphere, (Art. 8), and this point is the celestialpole (Art. 26). The angle HOP is therefore the altitude of the poleas seen at O; and it obviously equals ONQ, since OH is per-pendicular to ON, and both CP1 and OP are perpendicularto QQ. This fundamental relation, that the altitude of the pole is iden-tical with the observers latitude, cannot be too strongly impressedon the mind. 41. The Right Sphere. —If the observer is situated at theearths equator (that is, in latitude zero), the pole will be in his hori-zon, and the celestial equator will be a vertical circle, coinciding withthe prime vertical (Art. 17). All heavenly bodies will rise and setvertically instead of obliquely, as
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