ASTRON 1101 Lecture Notes - Lecture 4: Celestial Equator, Optical Illusion, Apsis
1 Sep 2016
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Document Summary
We have no sense of depth looking out into the night sky. Thus, there"s a strong visual illusion that stars are attached to the celestial sphere. We need a coordinate system to navigate on the celestial sphere. The celestial equator is the projection of the earth"s equator onto the celestial sphere. Celestial objects rise in the east and set in the west. This apparent daily motion results from the earth"s rotation about its axis. Time per rotation = one sidereal day. The observed motion of the sun (rising in east, setting in west) provided humanity"s first clock. Time from noon to noon = 1 solar day = 24 hours. This is longer than the sidereal day, which is 23 hours 56 minutes. Seen from earth, the sun slips gradually eastward relative to the stars along the ecliptic. Motion of ~1 per day (~365 days for full circuit) The ecliptic is tilted by 23. 5 relative to the celestial equator.
