Understanding Understanding Planetary MotionPlanetary Motion

Seven Key Players

MANY people contributed to our understanding, the following individuals

made discoveries that forever changed the way we view the solar system.

The picture of the solar system that we accept today was not devised overnight, nor by one

individual.

1. Aristotle1. Aristotle (384 (384 322? BCE) 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).

1. Aristotle’s System1. 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.

2. Ptolemy2. Ptolemy (125 (125 160? CE) 160? CE)

• Greek astronomer and geographer

• Supported Aristotle’s Geocentric Theory and offered proof by acknowledging the phenomena of retrograde motion.

2. Ptolemy’s 2. Ptolemy’s Retrograde MotionRetrograde 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)

2. Ptolemy’s 2. Ptolemy’s Retrograde MotionRetrograde 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).

2. Ptolemy’s 2. Ptolemy’s Retrograde MotionRetrograde Motion

Consider if Venus orbited around the Earth…

…the deferent is the main orbit around the earth.

…the epicycle is a secondary orbit

Ptolemy’s (incorrect)

explanation of

retrograde motion is

the combination of these two

orbits.

This means that the Sun is the center of the system and that all bodies revolve about the Sun.

3. Copernicus3. Copernicus (1473 (1473 1543) 1543)

• Polish Monk and astronomer

• Challenged the Ptolemaic (Geocentric) Theory by devising the Copernican Heliocentric System

= Helio (Sun) + Centric (Centered) = Sun Centered System

3. Copernicus’s 3. Copernicus’s Heliocentric TheoryHeliocentric 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

Copernicus’s Copernicus’s Retrograde MotionRetrograde 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.

The Copernican description of

retrograde motion confirmed that the planets must be

moving in the same direction just at

different speeds.

Click here to check it out.

Copernicus’sCopernicus’s Retrograde MotionRetrograde Motion

Copernicus’s Copernicus’s parallax of starsparallax 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.

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

Click here to check it out.

Copernicus’s Copernicus’s parallax of starsparallax of stars

E

• 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.

Copernicus’s Copernicus’s explanation of the seasonsexplanation of the seasons

*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.)

• The Earth, tilted on its axis,* moves around the sun.

Copernicus’s Copernicus’s explanation of the seasonsexplanation of the seasons

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.

• 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.

Copernicus’s Copernicus’s aberration of starlightaberration of starlight

Seems like the light came from the right…but the star is actually above.

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.

Copernicus’s Copernicus’s aberration of starlightaberration of starlight

• 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.

4. Tycho Brahe4. Tycho Brahe (1546 (1546 1601) 1601)

• 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

5. Galileo Galilei5. Galileo Galilei (1546 (1546 1642) 1642)

6. Johannes Kepler 6. Johannes Kepler (1571 (1571 1630) 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 to learn about (and take notes on)…

1. First Law2. Second Law (part 1 and part 2)3. Third law

First Law - Planets travel around the Sun in elliptical paths with the Sun at one foci.

Second Law - A line from the sun to a planet will sweep out equal areas in equal time periods.

Third Law - The larger the orbit of a planet, the longer it takes to complete a revolution. 32 RT

7. Issac Newton 7. Issac Newton (1642- 1727)(1642- 1727)

Among other things, he devised his law of Universal Gravitation – this is HUGE!

Explore the following virtual lab to see if you can discover the main idea of the law. Answer the questions on your outline as you explore the virtual lab.

We will discuss the contributions of these gentlemen in class.

Plus an Honorable Mention to Cavendish

Thank you.Thank you.

• You will be responsible for knowing (and understanding) the concepts covered in this power point.

• There is an outline on the webpage to help you organize your thoughts and information.