East and West on the Celestial Sphere

East and West on the Celestial Sphere - diurnal motion of...

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East and West on the Celestial Sphere It is useful to define east and west directions on the celestial sphere, as illustrated in the following figure. Thus, objects to the west of the Sun on the celestial sphere precede the Sun in the diurnal motion of the celestial sphere (they "rise" before the Sun and "set" before the Sun). Likewise, objects to the east of the Sun trail the Sun in the diurnal motion (they "rise" after the Sun and "set" after the Sun). Generally, one object is west of another object if it "rises" before the other object over the eastern horizon as the sky appears to turn, and east of the object if it "rises" after the other object. Motion in the Sky It is clear after only minimal observation that objects change their position in the sky over a period of time. This motion is conveniently separated into two parts: 1. The entire sky appears to turn around imaginary points in the northern and southern sky once in 24 hours. This is termed the daily or
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Unformatted text preview: diurnal motion of the celestial sphere, and is in reality a consequence of the daily rotation of the earth on its axis. The diurnal motion affects all objects in the sky and does not change their relative positions: the diurnal motion causes the sky to rotate as a whole once every 24 hours. 2. Superposed on the overall diurnal motion of the sky is "intrinsic" motion that causes certain objects on the celestial sphere to change their positions with respect to the other objects on the celestial sphere. These are the "wanderers" of the ancient astronomers: the planets, the Sun, and the Moon. Diurnal motion Diurnal motion at different latitudes Actually, all objects are slowly changing their relative positions on the celestial sphere, but for most the motion is so slow that it cannot be detected over timespans comparable to a human lifetime; only the "wanderers" have sufficiently fast motion for this change to be easily visible....
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