Celestial Sphere

Earthly Sphere Latitude measures the number of degrees north or south of the equator. –DeKalb at 41° 55’ N Longitude measures degrees east or west of the prime meridian. –0° at Greenwich Observatory, London –DeKalb at 88° 45’ W

Projection The sky above us is measured as a projection outward from the earth. The celestial poles ( P, Q ) are directly out from the north and south poles. The celestial equator ( A-B ) is directly out from Earth’s equator. P Q BA O

Declination Declination is the celestial equivalent of latitude. –Angle  compared to equator –North positive –South negative P Q X  BA O

Rotation The earth rotates, so the stars appear to rotate. –Over same latitude but changing longitude –North and south celestial poles P, Q fixed East-west orientation is defined by the sun’s position ϒ at the vernal equinox. –Crosses equator from S to N –First day of spring, March 21

Right Ascension Right ascension is the celestial equivalent of longitude. –Angle  Stellar coordinates use right ascension and declination. –X( ,  ) P Q O X ϒ  E 

Heavenly Time Right ascension is not measured in degrees. Degrees are converted to time. –24 hours = 360° –1h = 15°1° = 4m –1m = 15'1' = 4s –1s = 15'' 1'' = 1/15 s

Local View From a point on the earth the stars appear to be fixed on a sphere that rotates. The point directly above the observer is the zenith. The point directly below through the earth is the nadir.

Horizon The horizon is the line of the ground for an observer. Altitude is the angle up from the horizon. Azimuth is the angle east from north.