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Transcript
Understanding
Planetary Motion
Seven Key Players
NOTE – none of the links in this presentation are LIVE – you
must copy the link into your address-bar on the web.
1. Aristotle (384  322? BCE)
• Greek Philosopher and Scientist
• Believed that motion was caused by a body's desire to
move and that in order to change the motion of an object
some violent outside cause was required.
– For example – He would observe a tree that remained at rest for
years – the tree was at rest because it didn’t want to move. A
strong storm however, was a violent cause that could make the
tree move (i.e fall down)
• Devised the theory of a Geocentric Solar System
Geo (Earth) + Centric (Centered) = Earth Centered System
Meaning that the Earth is the center of the universe and all
bodies revolve about the Earth.
2. Ptolemy (125  160? CE)
• Greek astronomer and geographer
• Supported Aristotle’s Geocentric Theory
and offered proof by acknowledging the
phenomena of retrograde motion.
– Retrograde motion is the apparent reversing
of an object’s motion. This is apparent when
charting the motion of planetary bodies.
– Although retrograde motion is an actual
phenomena, Ptolemy explained it incorrectly
(in a way that supports a Geocentric theory)
Ptolemy’s Retrograde Motion
• Ptolemy’s explanation for retrograde motion was
that a planet will move “forward” in its orbit then
suddenly circle back in the opposite direction.
– He explained that planets move in two paths in their
orbits. The deferent (the larger orbit) and smaller
epicycles (in which they circle back)
• Visit the following link to view an animation of
Ptolemy’s retrograde motion.
http://abansil.physics.neu.edu/~gross/Animations/Ptolemy/epicycle.html
3. Copernicus (1473  1543)
•
•
Polish Monk and astronomer
Challenged the Ptolemaic (Geocentric) Theory
by devising the Copernican Heliocentric
System
–
•
Helios (Sun) + Centric (Centered) = Sun-Centered System
Copernicus has four proofs for his new
(controversial) theory
1)
2)
3)
4)
Retrograde Motion (new explanation)
Parallax of Stars
Seasons
Aberration of Starlight
Copernicus’s Retrograde Motion
• Copernicus agreed that retrograde motion
was a real phenomena, but he said that it
occurred when planets passed each other
in their orbits.
– If Earth passes a planet the other planet
seems to move backward
• This confirmed that the planets must be moving in
the same direction just at different speeds. It is
just like passing a slow moving car on the highway.
Visit the following
link to view an
animation of
retrograde motion.
Follow the
instructions on the
page.
http://www.astro.utoronto.ca/~zh
u/ast210/helicentric.html
Parallax of Stars
• Parallax is the apparent shifting of position
of an object when viewed from different
locations.
– If you watch any object it will seem to zig-zag
back and forth across the background as you
move relative to it.
– For star as we move in our orbit, the stars
seem to shift their position with respect to
more distant stars.
As the Earth revolves
around the Sun a near
star will seem to shift
back and forth across a
backdrop of distant stars.
For example in July the near
star seems to align with
the right-most distant star
while in December it is
aligned with the left-most
star.
July
Dec
Visit the following link to view an animation of parallax of stars.
http://www.astro.washington.edu/labs/parallax/solar.html
The Seasons
• Copernicus knew that the Earth was tilted on its
axis at 23.5-degrees*. If the Earth was
stationary, then the amount of light that a given
part of the Earth receives would not vary.
– We know that this does not occur as the amount of
daylight varies according to location and time of year.
How did they know 23.5-degrees? We owe this to Eratosthenes
who first determined it around 200 BCE. There is more to come
about his discoveries later.
Aberration of Starlight
• The true path of light from a star to a planet is in
a straight line, however, due to the motion of a
planet it appears as if starlight bends into a
planet. This makes it seem as if the light is
originating from a different location.
– This is similar to driving in the snow. The snow may
actually be falling straight down but as we drive we
“crash into the snow”. It seems as if the snow if
coming toward the car diagonally from a point ahead
of the vehicle.
4. Tycho Brahe (1546  1601)
• Danish Mathematician and astronomer
• Tycho kept meticulous records of the motion of
planetary bodies.
– He tried to mathematically analyze the motion to
derive patterns, but was unsuccessful.
– Reluctantly provided Johannes Kepler with his
records of planetary motion.
– Interestingly, Tycho was NOT a believer of the heliocentric theory. His
planetary data however, helped to solidify this theory.
5. Galileo Galilei (1564  1642)
• Italian astronomer, mathematician and physicist.
• First to use a telescope to observe planetary
bodies.
– He discovered the moons of Jupiter.
– Devised the theories of motion that are part of the
foundation for classical mechanics
• 1. Neglecting air resistance, all objects accelerate toward
Earth at the same rate.
• 2. Derived the relationships for uniform linear motion (i.e. the
Big Five)
• 3. Defined Inertia
The other players:
6) Johannes Kepler (1571  1630)
7) Isaac Newton (1642  1727)
We will discuss the contributions of these
gentlemen in class.
Honorable Mentions
Eratosthenes – His early discoveries about the
geometry, tilt and mass of the Earth laid the
foundation for future scientists. For more
information visit:
www.plu.edu/~dornerbc/teaching/math203/eratosthenes.pdf
http://www.utc.edu/Faculty/JonathanMies/wkshop/wkshop_prod/eratosthenes.pdf#search='Eratosthenes%20%2C%20tilt
%20of%20earth‘
Albert Einstein – In trying to explain gravity, he
developed theories that forever changed
physics. For more information visit:
http://www.aip.org/history/einstein/