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Process of Science/ Concept Checks Chapter 2 Astronomy Today 7th Edition Chaisson/McMillan © © 2011 2011 Pearson Pearson Education, Education, Inc. Inc. © 2011 Pearson Education, Inc. In the geocentric view, retrograde motion is the real backward motion of a planet as it moves on its epicycle. In the heliocentric view, the backward motion is only apparent, caused by Earth “overtaking” the planet in its orbit. © 2011 Pearson Education, Inc. © 2011 Pearson Education, Inc. Mainly simplicity and elegance. Both theories made testable predictions, but until Newton developed his laws, neither could explain why the planets move as they do. However, Copernicus’s model was much simpler than the Ptolemaic version, which became more and more convoluted as observations improved. © 2011 Pearson Education, Inc. © 2011 Pearson Education, Inc. Because the laws were derived using only the orbits of the planets Mercury through Saturn, before the outermost planets were known. © 2011 Pearson Education, Inc. © 2011 Pearson Education, Inc. Because Kepler determined the overall geometry of the solar system by triangulation using Earth’s orbit as a baseline, so all distances were known only relative to the scale of Earth’s orbit—the astronomical unit. © 2011 Pearson Education, Inc. © 2011 Pearson Education, Inc. Kepler’s laws were accurate descriptions of planetary motion based on observations, but they contained no insight into why the planets orbited the Sun or why the orbits are as they are. Newtonian mechanics explained the orbits in terms of universal laws, and in addition made detailed predictions about the motion of other bodies in the cosmos— moons, comets, other stars—that were hitherto impossible to make. © 2011 Pearson Education, Inc. © 2011 Pearson Education, Inc. In the absence of any force, a planet would move in a straight line with constant velocity (Newton’s first law) and therefore tends to move along the tangent to its orbital path. The Sun’s gravity causes the planet to accelerate toward the Sun (Newton’s second law), bending its trajectory into the orbit we observe. © 2011 Pearson Education, Inc.