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Geocentric/Heliocentric Models to Explain Motion • Geocentric Model – Earth centered. –Planets, stars, and other objects revolve around Earth. • This model was able to explain the apparent motion of the stars, sun, and moon. –Could not explain retrograde motion and foucault pendulum motion. Retrograde Motion • Retrograde motion is an apparent motion of planets (over weeks to months) when they appear to change position with respect to the background field of stars. Retrograde Motion Cont… • Retrograde motion is caused by Earth and other planets revolution around the Sun in different orbits at different speeds. Retrograde Motion of the Planets http://www.flex.com/~jai/astrology/retrograde.html Models to Explain Motion • Heliocentric Model – Sun centered. –Earth rotates on an imaginary axis and revolves around the Sun. • The Moon revolves around Earth. –Explained all of the observations made by the scientists including retrograde motion. Actual Earth Motion Rotation – Earth spins on its axis from west to east at 15 degrees per hour. • The Sun, Moon, stars appears to move across the sky at a rate of 15 degrees per hour. • The Foucault pendulum and the Coriolis Effect confirm Earth’s rotation. Foucault Pendulum • A pendulum is a freely swinging weight. • Over a day, the direction of motion of a pendulum will appear to change. Actually, Earth is rotating under the pendulum. Coriolis Effect • Coriolis Effect - the bending of winds and currents due to the rotation of Earth. • Northern Hemisphere, they bend to the right. • Southern Hemisphere, they bend to the left. Earth’s Revolution • Earth revolves around the Sun once a year. – It revolves approx. 1° per day. • Earth is closest to the Sun on January 3rd. This is called the perihelion. • Earth is farthest to the Sun on July 3rd. This is called the aphelion. • Evidence of revolution – Seasons – Different constellations throughout the year. http://www.onr.navy.mil/focus/spacesciences/observingsky/sun2.htm