Lecture 4Earth, Moon, Sun System

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Rising and Setting

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Circumpolar Constellations

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Motion of the Sun

Earth goes around the sun once a yearAppears that the sun moves through the skyalong a line called the ecliptic.Sun moves 1 degree eastward per day.Constellations along the ecliptic are the 12constellations of the zodiac

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Motion of Sun along Ecliptic

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Precession of the Earth’s Axis of RotationNorth Celestial Pole moves with a period of26,000 yearsIn Egyptian time Thuban was the North StarIn 12,000 years (14000 A.D.) Vega will be theNorth Star

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Precession

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Precession effect on pole star

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Motion of the Sun

Magnitudes again, 5 in 100. Know what thatmeans!Ecliptic: Path of the sun in the sky. Know thedifferent definitions for the ecliptic.Sun moves 1◦/day

360◦/year365.25day/year

≈ 1◦/day

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Seasons I

When it is summer in Oklahoma, whatseason is it in Chile?

(a) Summer(b) Fall(c) Winter(d) Spring

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Seasons II

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Seasons III

Are Seasons caused by a change in theEarth-Sun Distance?

(a) Yes(b) No

Earth’s Rotation axis tipped 23.5◦ to plane ofeclipticResponsible for the seasons.Planets confined to (nearly) the plane of theecliptic

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Celestial Sphere and Seasons

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Summer Light versus Winter Light

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Summer Solstice

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Winter Solstice

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Sun Position with Season

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Sun Simulation

Sun Simulator

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