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Apparent to Actual MotionsHow do celestial objects appear to move across the sky
and how do they actually move across the sky?
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Early Astronomy
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Apparent Motions
•Geocentric Universe - idea that Earth was at the center of the solar system
•Also called the Ptolemaic System
• Stars all rotate around the Earth on a single large sphere at 15º/hour
• Planets travel on smaller spheres around their own larger sphere in epicycles
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Geocentric Universe
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Apparent Motions
• Problems with the Geocentric Model:
• Locations of the planets could not accurately be predicted
•Changes in the apparent diameter of the Moon and Sun could not be explained
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Apparent Motions
•Apparent Motion - the way in which celestial objects appear to move across the sky
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Apparent Motions
•Celestial Sphere - the visible portion of the sky that celestial objects appear to travel on
•Celestial Object - any of the natural objects that can be seen in the sky
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Apparent Motions
•Horizon - the edge of the visible portion of the celestial sphere
•Zenith - highest point on the celestial sphere which is directly over the observer
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Apparent Motions
•All objects [except Polaris] appear to move across the celestial sphere from east to west at 15 º/hour or 360º/24 hours
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Apparent Motions
• Star Trails - long exposure photos of stars as they appear to move across the sky
•Circumpolar Stars - stars that move around a polar star
• Polar Star - star directly above the North or South Pole
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Apparent Motions
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Apparent Motions
• Locating positions on the celestial sphere:
•Altitude - angular distance above the horizon [0º to 90º]
•Azimuth - angular distance along the horizon measured from due north [0º to 360º]
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Altitude and Azimuth
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Apparent Motions
• The Sun’s path changes throughout the seasons
• The greater the Sun’s path the increased amount of daylight hours an area receives
• The shorter the Sun’s path the decreased amount of daylight hours an area received
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N
E
SW
June 21
December 21
September 21
& March 21
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Apparent Motions
•What’s Charlie’s approximate latitude if this photo was taken at noon on June 21?
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22º N 22º N
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Copernicus
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Actual Motions
•Heliocentric Model - current model of the solar system where the Sun is at the center
•Also called the Copernican Model
• Planet revolve around the Sun in circular paths
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Heliocentic Model Universe