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Chapter 29
The Solar System
Ch. 29.1
Models of the Solar System
Geocentric model—early Greek belief that
the Earth was the center of the universe.
 The sun, planets, stars, etc. all revolved
around the earth!
 Could not explain the occasional backward
appearing motion (east to west) of some
planets…retrograde motion.
 Ptolemy proposed epicycles to explain
retrograde motion.
Copernicus’s Model
In 1500’s, Nicholas Copernicus proposed
that the sun was the center of the
universe…heliocentric model.
 Earth and other planets revolved around
the sun in the same direction, but at
different distances and speeds. This
explained retrograde motion.
Kepler’s Laws
Tycho Brache used a telescope to make
detailed observations of the stars and
 His assistant, Johannes Kepler, explained
Brache’s observations in precise
mathematical terms.
 Kepler’s three laws explain most aspects
of planetary motion.
Law of Ellipses
Each planet orbits the sun in a path called
an ellipse (oval shaped).
 Therefore, a planet is not always the same
distance from the sun.
 Perihelion—the point where a planet is
closest to the sun.
 Aphelion—the point where a planet is
farthest from the sun.
Astronomical Unit (AU)—The average
distance between the earth and the sun.
 Useful term for comparing other planets’
distances from the sun.
Law of Equal Areas
Describes the speed at which planets
travel at different points in their orbits.
 Kepler discovered that planets move the
fastest when they are closest to the sun.
 They move the slowest when they are
farthest from the sun.
 The triangular sections formed between
the sun and any two points in a planet’s
orbit are always equal in area for equal
time periods.
Law of Periods
Describes the relationship between the
average distance of a planet from the sun
and its orbit period.
 Orbit period—time it takes for a planet to
make one complete revolution around the
 Cube of average distance is always
proportional to the square of the period.
 K x r3 =p2
r = average distance from the sun.
 p= orbit period.
 K is a mathematical constant.
 Convenient to use astronomical unit for K,
so K = 1
 If K=1, then r3 =p2
If an asteroid is 4 astronomical units from
the earth, what is its orbit period?
 K x r3 =p2
 1 x 43=p2
 64=p2
 p= 8 years
Newton’s Application of Kepler’s
Newton explained why the planets move
the way they do.
 He developed the concept of inertia…a
moving body will change its direction only
if acted on by an outside force.
 Newton identified the force causing the
planets to move in curved paths (instead
of straight lines) as gravity!