3 Earth’s Rotation23 hours and 56 minutes relative to background stars (sidereal period)Spin axis of Earth defines points on the celestial sphere (using north and south poles)North Celestial PoleSouth Celestial PoleCelestial EquatorSky appears to rotate east to west about the celestial poles because Earth rotates west to east.
7 DefinitionsCelestial Sphere: An imaginary sphere where celestial objects are projected on the basis of their direction from EarthCelestial Poles: The two points where the spin axis of the Earth’s spin axis intersects the celestial sphereCelestial Equator: The projection of the Earth’s equator onto the celestial sphereGreat Circle: A circle on the sphere's surface whose center is the same as the sphere’s center, and divides the sphere into two equal hemispheres
8 DefinitionsZenith: The point on the sky that is directly overhead of the observer.Horizon: The great circle on the celestial sphere that is 90 degrees from the zenithHour circle: The great circle through the position of a celestial body and the celestial polesMeridian: The hour circle that passes through the zenith and both celestial poles
9 Directions on the Local Sky Altitude: The minimum angular distance between the position of a celestial body and the horizonAzimuth: The angular bearing of an object, measured from North (0 degrees) through East (90 degrees), South (180 degrees), West (270 degrees), and back to North (360 degrees)Hour Angle: The angle between the meridian and an object’s hour circle (west is positive)
10 Celestial Coordinates Vernal Equinox: The position of the Sun when it crosses the celestial equator from south to northDeclination: The minimum angular distance from the position of a celestial body and the celestial equatorRight Ascension: The eastward angle from the vernal equinox to the intersection of an object’s hour circle with the equator1 hour of angle = 15 degrees
22 An Earth DaySidereal Day: 23 hr 56 min 4 sec Motion relative to background starsMean Solar Day: 24 hours The average time between meridian crossings of the SunApparent Solar Day: varies The actual time between the meridian crossings