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Published byKelley Copeland Modified over 7 years ago

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Celestial Sphere

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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

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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

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

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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

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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

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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

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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.

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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.

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