1. Understanding Planetary Motion
Seven Key Players

2. The picture of the solar system that we accept today was not devised overnight, nor by one individual.
MANY people contributed to our understanding, the following individuals made discoveries that forever changed the way we view the solar system.

3. 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 – he concluded that 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).

4. 1. Aristotle’s System
Devised the theory of a Geocentric Solar System Geo (Earth) + Centric (Centered) = Earth Centered System
This means that the Earth is the center of the universe and that all bodies revolve about the Earth.

5. 2. Ptolemy (125  160? CE) Greek astronomer and geographer
Supported Aristotle’s Geocentric Theory and offered proof by acknowledging the phenomena of retrograde motion.

6. 2. Ptolemy’s Retrograde Motion
Retrograde motion is the apparent reversing of an object’s motion. This is apparent when charting the motion of planetary bodies.
For example, if you watch how Mars moves cross the night sky it will typically advance from East to West. However, there is a period of time in which Mars seems to go backward –from West to East. This “backwards” motion is called the retrograde motion.
Although retrograde motion is an actual phenomena, Ptolemy explained it incorrectly (in a way that supports a Geocentric theory). Retrograde is real! Ptolomy got it wrong!

7. 2. Ptolemy’s Retrograde Motion
Ptolemy’s (incorrect) 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 around the Earth) and smaller epicycles (small secondary orbits).

8. 2. Ptolemy’s Retrograde Motion
Consider if Mars orbited around the Earth… (by the way…it does NOT)
Ptolemy’s (incorrect) explanation of retrograde motion is the combination of these two orbits.
…the deferent is the main orbit around the earth.
…the epicycle is a secondary orbit

9. 3. Copernicus (1473  1543) Polish Monk and astronomer
Challenged Ptolomy’s (Geocentric) Theory by devising the Heliocentric System
= Helio (Sun) + Centric (Centered) = Sun Centered System
This means that the Sun is the center of the system and that all bodies revolve about the Sun.

10. 3. Copernicus’s Heliocentric Theory
Copernicus has four proofs for his new (very controversial) theory:
1. Retrograde Motion (a new, accurate explanation)
2. Parallax of Stars
3. Changing Seasons
4. Aberration of Starlight

11. 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 for a period of time.
It is just like passing a slow moving car on the highway.

12. Copernicus’s Retrograde Motion
The Copernican description of retrograde motion confirmed that the planets must be moving in the same direction just at different speeds.
Planets closer to the Sun move faster and will “lap” the ones further away.
Click here to check it out.

13. Copernicus’s Retrograde Motion
When a distant planet gets lapped by Earth, we view the other planet as turning around and going the other way.
From our perspective, we can’t detect the motion of the Earth so it seems as if the other (distant) planets slow down, stop then move in the opposite direction.

14. Copernicus’s 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 a star, as we move in our orbit the stars seem to shift their position with respect to more distant stars.

15. Copernicus’s parallax of 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. E
July Dec
Click here to check it out.

16. Copernicus’s explanation of the seasons
Copernicus knew that the Earth was tilted on its axis at 23.4-degrees*.
IF the Earth was stationary, then the amount of light that a given region of the Earth receives would NOT vary.
*How did Copernicus know that the Earth was tilted?
Eratosthenes did the leg work on that (you can read more here if you are interested.)

17. Copernicus’s explanation of the seasons
The Earth, tilted on its axis,* moves around the sun.
Consider living in Ohio…
In the summer, we receive the most direct sunlight – thus we have more hours of sunlight and warmer weather.
In the winter, we receive less direct sunlight – thus we have fewer hours of sunlight and colder weather.

18. Copernicus’s 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 bending of the light makes it seem as if the light is originating from a location other than the star’s actual position.
Seems like the light came from the right…but the star is actually above.

19. Copernicus’s aberration of starlight
This is similar to driving (or walking) in the snow or rain. 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.
From YOUR point of view (you are holding the umbrella) – the rain seems to come in at an angle. In reality YOU are moving horizontally and the rain is coming straight down.

20. 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.
As a side note – Tycho led a very interesting life…we will discuss some of the details in class.

21. 5. Galileo Galilei (1546  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

22. 6. Johannes Kepler (1571  1630)
3 Laws of planetary motion Kepler didn’t understand WHY the planets moved as they do, but he did discover the manner in which they move. He described their motion in 3 laws…click on the links (use your ear buds) to learn about (and take notes on)…
1. First Law - Planets travel around the Sun in elliptical paths with the Sun at one foci.
Foci – the two points that define the shape of the ellipse. One is the Sun…what’s the other??? (Kind of a trick question)

23. 6. Johannes Kepler (1571  1630)
3 Laws of planetary motion Kepler didn’t understand WHY the planets moved as they do, but he did discover the manner in which they move. He described their motion in 3 laws…click on the links (use your ear buds) to learn about (and take notes on)…
1. First Law - Planets travel around the Sun in elliptical paths with the Sun at one foci. 2. Second Law - A line from the sun to a planet will sweep out equal areas in equal time periods.

24. 6. Johannes Kepler (1571  1630)
3 Laws of planetary motion Kepler didn’t understand WHY the planets moved as they do, but he did discover the manner in which they move. He described their motion in 3 laws…click on the links (use your ear buds) to learn about (and take notes on)…
1. First Law link: Planets travel around the Sun in elliptical paths with the Sun at one foci. 2. Second Law link: - A line from the sun to a planet will sweep out equal areas in equal time periods. 3. Third law link: - The larger the orbit of a planet, the longer it takes to complete a revolution.
The videos are on YouTube… if you can’t watch them at school then watch them at home.

25. 7. Issac Newton ( )
Among other things, he devised his law of Universal Gravitation – this is HUGE! We will take notes on this in class this week. In the meantime…
…explore the following virtual lab to see if you can discover the main idea of the law. Work with your lab partner to answer the questions on your lab worksheet as you explore the virtual lab.

26. Thank you.