Transcript Models of the Solar System

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.
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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.
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https://www.youtube.com/watch?v=1nVSz
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Kepler’s Laws
Tycho Brache used a telescope to make
detailed observations of the stars and
planets.
 His assistant, Johannes Kepler, explained
Brache’s observations in precise
mathematical terms.
 Kepler’s three laws explain most aspects
of planetary motion.
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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.
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Astronomical Unit (AU)—The average
distance between the earth and the sun.
 Useful term for comparing other planets’
distances from the sun.
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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.
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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
sun.
 Cube of average distance is always
proportional to the square of the period.
 K x r3 =p2
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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
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Practice
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
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Newton’s Application of Kepler’s
Laws
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!
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