Download How has the model of the solar system changed over time?

How has
the model
of the
solar
system
changed
over time?
The Basic Problem:
If observed from one
night to the next, a
planet appears to
move from East to
West against the
background stars
most of the time.
Occasionally,
however, the planet's
motion will appear to
reverse direction,
and the planet will,
for a short time,
move from West to
East against the
background
constellations.
How can we explain this?
Astronomers
have been trying
to design models
of our solar
system to explain
this backward
motion of planets
for over 2,000
years.
Plato’s Explanation
The ancient Greeks
believed the circle to
be the perfect form,
and since they
"knew" the heavens
were perfect, then the
Earth must stand still
and everything move
around it in perfect
circle orbits! This is
called the Geocentric
Model.
Cutaway of the Geocentric Model
Ptolemy’s Explanation
This model assumed that
the motions of the Sun,
Moon, and stars were
based on perfect circles.
To account for the
retrograde motion of the
planets, a system of
epicycles were “invented”
where the planets moved
around small circular
paths (Epicycles) that in
turn moved around larger
circular orbits around the
Earth (GEOCENTRIC.)
Copernican Explanation
Copernicus replaced the
geocentric universe of
Ptolemy with one that was
centered on the Sun
(heliocentric), with only
the Moon orbiting the
Earth. His explanation for
retrograde motion in this
system arises from the
fact that the planets
further from the sun are
moving more slowly in
their orbits than those
closer to the sun.
Kepler’s Explanation
Kepler became convinced the ellipse was the shape
of planet orbits, not the circle. This idea went against
the 2,000 years of belief! Kepler had a hard time
convincing other scientists of his time that planet orbits
are not circles. Even the great scientist, Galileo,
disagreed with Kepler.
The Current Model
The current model of the Solar System has identified nine
planets orbiting the Sun (HELIOCENTRIC MODEL). The orbit
of Pluto is different from the rest of the planets. Some
scientists are even beginning to think Pluto doesn’t meet
the definition of “planet.”
And we now know there are
solar systems other than our
own!
Recently, other
stars with orbiting
planets have been
discovered. These
planets (called
exoplanets) have
never been seen
by us, but they
have been
detected by the
very slight
"wobble" they
induce in their star.
First Exoplanet Image
Astronomers
confirm the
first Image of
a planet
outside of
our Solar
System on
April 30, 2005
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March 2005 Announcement
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The End