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Historic Models of Astronomy
Greek Ideas: Earth at the Center
• Thought they were inside a rotating dome
with stars pasted on it
• Called this dome the “celestial sphere”
• Believed Earth was stationed at the center
of the universe
Greek Ideas: Earth
at the Center
Geocentric systemEarth is at the
center of the
revolving planets
Ptolemy (140 AD)
• Believed the
geocentric model
• Thought the moon and
sun also revolved
around earth
Ptolemy (140 AD)
• Said planets
moved in little
circles that moved
in bigger circles
• This explained why
planets seemed to
wander
• Theory believed for
1400 years
Copernicus’ Idea: Sun at the Center
• Early 1500s
• Developed another
explanation for the motion of
the planets
• Thought the sun was at the
center of the system of
revolving planets
• Called this the “heliocentric
system”
• Did not publish his ideas
until after his death
Galileo
• Thought heliocentric
explanation was correct
• Used the telescope to
prove
• Saw four moons revolving
around Jupiter
• Saw Venus go through
phases
• Persecuted for his ideas
Kepler
Found that the orbit of
each planet is an ellipse
(oval)
Newton- Inertia
• Newton explained that
inertia and gravity
combine to keep the
planets in orbit
• The tendency of a
moving object to continue
in a straight line or a
stationary object to
remain in place is called
inertia
Newton- Gravity
• Gravity is the force that attracts all objects
to one another
• Strength of gravity depends on
– Masses of the objects
– Distance between them
• Newton figured out that the planets stay in
orbit around the sun because the sun’s
gravity pulls on them while their inertia
keeps them moving ahead