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What Did We Used to Think About the Solar System? Observing the Solar System: A History The theory that the Earth is stationary was the basis of ancient Greek theories that prevailed for over 2000 years. The Movements of Planets and Stars Geocentric Model (or Earth-centered): _______________ ________________________ ________________________ ________________________. As long ago as 6000 years, astronomers recorded the movements of stars and noted that they appeared to move across the sky, but did not move in relation to each other. ________________________ ________________________ ________________________ ________________________________________________ ________________________________________________ ________________________________________________ ________________________________________________ ________________________________________________ ________________________________________________ ________________________________________________. ________________________________________________ ________________________________________________ ________________________________________________ ________________________________________________ ________________________________________________ ________________________________________________. Ptolemy’s Geocentric Model Greek astronomer that lived in Egypt in the second century A.D. was puzzled by retrograde motion (pattern of apparent backward motion of planet movement) developed the first model that could be used to predict locations of planets imagined planets on small circular orbits, called _______________ the center of each small orbit moved around Earth on a larger circular orbit called a ________________________ he believed that retrograde motion occurred when the planet moved along the part of the epicycle that an observer on Earth could see although his model did not work perfectly, it was the accepted model until the 16th century Copernicus’s Heliocentric Model Polish astronomer _________________________ _________________________ _________________________ suggested that Earth was a planet, that it rotated, and that Earth and the other planets revolved around the sun was one of the first astronomers to try to explain retrograde motion Retrograde Motion → ___________________________ ______________________________________________ ______________________________________________ ______________________________________________ ______________________________________________ ______________________________________________. Tycho, Kepler, and Planetary Motion Tycho Brahe was a 16th-centry Danish astronomer developed instruments to observe the moon, planets, and other celestial objects ______________________________________________ ______________________________________________ ______________________________________________ identified a number of unexpected occurrences (particularly in Mars’s orbit) his records were the most precise made before the invention of the telescope Kepler (Tycho’s assistant) explained the unexpected occurrences by suggesting that the planets’ orbits were elliptical Kepler’s 1st law of planetary motion → ______________ ______________________________________________ ______________________________________________ Kepler’s 2nd law of planetary motion (equal area law) → ______________________________________________ ______________________________________________ ______________________________________________ ______________________________________________ ______________________________________________ Kepler’s 3rd law of planetary motion (harmonic law) → ______________________________________________ ______________________________________________ ______________________________________________ ______________________________________________ Retrograde motion is observed because each planet orbits the sun from a different distance and at a different speed.