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 SystemGeo (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) Retrograde Motion (new explanation)

2) Parallax of Stars

3) Seasons

4) 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/~zhu/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/Jonathan-Mies/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/