Transcript Understanding Planetary Motion

Understanding
Planetary Motion
Seven Key Players
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.
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).
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.
2. Ptolemy (125  160? CE)
• Greek astronomer and geographer
• Supported Aristotle’s Geocentric Theory
and offered proof by acknowledging the
phenomena of retrograde motion.
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!
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).
2. Ptolemy’s Retrograde Motion
Consider if Mars
orbited around
the Earth…
(by the way…it
does NOT)
…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.
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.
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
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.
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.
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.
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.
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.
Click here to check it out.
E
July
Dec
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.)
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.
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.
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.
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.
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
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)
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.
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: http://www.youtube.com/watch?v=tw5MvHNw0Co
- Planets travel around the Sun in elliptical paths with the Sun at
one foci.
2. Second Law link: http://www.youtube.com/watch?v=NiWK5z7z_Oc
- A line from the sun to a planet will sweep out equal areas in equal
time periods.
3. Third law link: http://www.youtube.com/watch?v=acrLrlApvy8safe=active
- The larger the orbit of a planet, the longer it takes to complete a
revolution.
T 2  R3
The videos are
on YouTube…
if you can’t watch
them at school then
watch them at
home.
7. Issac Newton (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. Work with your lab
partner to answer the questions on your lab worksheet
as you explore the virtual lab.