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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.