Early people
looked at the
stars and
wondered…
The ancient
Greeks studied
the movement
of the stars
and observed
patterns.
They
concluded the
patterns of
most of the
stars do not
change,
except…
5 points of light
that seem to
wander among
the stars.
The Greeks
named them
PLANETS,
which means
wandering
stars.
The Planets
were named
after the 5
Roman
Gods,
Mercury,
Venus,
Mars, Jupiter
and Saturn
Mercury,
Venus,
Mars, Jupiter
and Saturn
were the
only planets
that could be
seen at that
time.
The Greeks thought the Earth
was the center of a celestial
sphere with all stars and planets
revolving around the Earth.
This Earth centered theory is
called the geocentric system.
Observations of the planets did
not fit the simplest version of
the geocentric system.
.
Early astronomers tried to create a
model that would fit their observations
In A.D. 140 Ptolemy
suggested a
variation of the
geocentric model
that explained why
the planets move at
different speeds.
For 1,400 years people believed
Ptolemy was correct.
In the early 1500’s the
Polish astronomer
Nicolaus Copernicus
thought the sun was
the center of the
system of planets.
His sun-centered system was
called the helocentric system.
Helios is Greek for sun.
But people still
believed Ptolemy’s
geocentric system.
100 year’s later
Galileo agreed
with Copernicus’
heliocentric
system.
In 1609 Galileo
perfected the
telescope which
lead him to
discoveries that
supported the
heliocentric
system.
Galileo discovered the 4 moons
of Jupiter.
He thought if moons revolve
around Jupiter then all things do
not revolve around the Earth.
He discovered the phases of
Venus. This requires the sun to be
the center of the system of planets.
Galileo’s discoveries
were not accepted by
those that still
believed in the
geocentric system.
The geocentric view
had the support of
the church which was
in power at that time.
Galileo was put on trial for his beliefs in 1633.
He remained under house arrest until he died
in 1642.
This illustration was copied from Nick Strobel's Astronomy Notes. Go to his site at www.astronomynotes.com
for the updated and corrected version.
In the late 1500’s
a Danish
astronomer, Tyco
Brahe, carefully
observed and
recorded the
positions of the
planets for 20
years.
Brahe’s meticulous records of
planetary motion were passed
on and interpreted by a German
mathematician in the 1600’s.
Johannes
Kepler used
Brahe’s data
to figure out
the shape of
the planets
orbits.
Kepler concluded the planets orbit
the Sun in an ellipse.
An ellipse
can be
drawn as
shown here.
Notice how each discovery
overlaps with the next, many
times taking an entire lifetime to
convince others. Most of these
scientists did not live long
enough to see the results of
their life’s work.
-Brahe recorded the planets motion
for 20 years.
-Kelper interpreted the data
mathematically to describe elliptical
orbits.
-Next Isaac Newton hypothesized
why the planets stay in orbit around
the Sun.
In 1687 Isaac
Newton
concluded
inertia and
gravity
combined keep
the planets in
orbit.
The gravitational
pull of the sun
keeps the planet
from drifting into
space.
Inertia – the
tendency to remain
in motion,
keeps the planet
from being pulled
into the sun.
Timeline of Celestial Theories
Ptolomy
140 A.D.
Geocentric
System
Copernicus
1514
Heliocentric
System
Brahe
1580-1600
20 year record of
Planetary Motion
Kepler
1609
Elliptical orbits
Galileo
1633
House Arrest
Heliocentric
Newton
1687
Gravity & Inertia
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The Heliocentric System is now
accepted. We now refer to this
as our “SOLAR SYSTEM”.
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Today, scientists continue to
discover objects in space.