Celestial Sphere Earthly Sphere Latitude measures the number

Celestial Sphere

Earthly Sphere • Latitude measures the number of degrees north or south of the equator. – De. Kalb at 41° 55’ N • Longitude measures degrees east or west of the prime meridian. – 0° at Greenwich Observatory, London – De. Kalb 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 (AB) is directly out from Earth’s equator. P A O Q B

Declination • Declination is the celestial equivalent of latitude. – Angle d compared to equator – North positive – South negative P A O Q X d B

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. P – Angle a X • Stellar coordinates use right ascension and declination. – X(a, d) O d ϒ a Q E

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