ASTR 3 Chapter Notes - Chapter 0-2: Celestial Equator, Solar Time, Northern Hemisphere
Document Summary
Apparent daily progress of sun and other stars across sky is known as diurnlal motion, consequence of earth"s rotation. However, each night the whold celestial sphere appears shifted a little compared with night before. Therefore, day measured by the stars (sidereal day) differs in length from solar day. This is because earth simultaneously rotates on its central axis and revolves around the sun. Because earth moves a small distance when it completes one orbit, it must rotate slightly more than 360 o in order for sun to return to same apparent location in the sky. In one year, earth must go 360 o around the sun in addition to rotating 360 o each day. Earth takes 3. 9 minutes to rotate this extra daily angle, so solar day is 3. 9 minutes longer than sidereal day. Because earth revolves around sun, earth"s darkened hemisphere faces different direction each night, about 1 degree difference per night.
