Motions of the Planets
This presentation will introduce these
terms: Geocentric, Heliocentric,
Retrograde, Rotation, Revolution.
The Celestial Sphere
The ancient astronomers believed that the Universe was made up of
spheres (orbs). The sphere that contained all of the objects seen in
space was the Celestial Sphere. The sphere rotated around the
earth making it appear as if all celestial objects were revolving
around the earth. In this image you can see the stars and the Sun
look as if they’re attached to the sphere Where is the person in the
image trying to get to?
The Geocentric Model of the Universe
Historically, the ancient Greek
astronomer Ptolemy (87 to
around 170 AD) made many
observations of the motions of the
Sun, Moon, and planets. Watch
both movies. Describe the
observations made by Ptolemy
and other ancient astronomers
that caused them to come up with
this geocentric model.
Based on his observations Ptolemy designed
a Geocentric model of the universe.
Based on his observations, he placed all the planets, stars, Sun, and Moon on
separate spheres with the earth placed at the center. Click the Green “Play” button in
the animation to see Ptolemy’s idea in action. What do you suppose might’ve been a
problem with this model? See then next slide for the answer.
Click here to learn more about
the Geocentric Model
Epicycle
The movie explains how Ptolemy saw the universe and how he explained
the motions of the planets.
Mars’ Orbit
Start with the movie player at the upper left. This will give you an idea of the
problem Ptolemy had with his geocentric idea. The “problem” is illustrated in
the lower left and his solution for this problem is shown in the upper right.
Ptolemy used “epicycles” to solve the problem of explaining retrograde motion
but these epicycles were cumbersome and really not natural though his
geocentric model was the model of choice for centuries. Why do you suppose
people chose to believe this model rather than a Sun-centered model?
This animation shows the retrograde motion of Mars
over several months. The image to the right shows
Mars’ retrograde path. What causes this retrograde
motion of the planets?
Development of
The Heliocentric Model of the Universe
Watch this short movie. More than a millennia after
Ptolemy proposed his geocentric idea, what did Brahe
observe that radically changed the view of the
Universe? Based on his observation, how did his
model differ from the very popular Ptolemy model?
Tycho Brahe.
Copernicus and the Heliocentric Model
Click here to learn more about
the Heliocentric Model
How did the Copernicus model
differ from the Brahe and
Ptolemy models?
Geocentricvs.
vs. Heliocentric
Geocentric
Heliocentric
Both models were riddled with mistakes.
The major problems were:
Geocentric – could not explain retrograde motion.
Heliocentric – circular orbits were not consistent with
the changing apparent diameter of Mars and the Sun (this
apparent changing diameter of the sun was a problem
really with both ideas). See next slide for demo 
Definitions of
Perihelion and Aphelion
This slide shows the sun on two different days of the year. Each day is 6 months apart (January 3 and July 4).
What do you notice about the Sun’s diameter from January to July?
What would cause the sun to appear to change its diameter cyclically over the course of each year?
Could the planets be orbiting the sun in a perfect circle or was it some other shape?
See the next slide to learn who was responsible for this new radical way of thinking.
Be sure you check out the definitions of these two new words. (http://epod.usra.edu/archive/images/animated.gif)
Along Comes Kepler
Johannes Kepler
Kepler’s Geometry of Orbits
designed by Johannes Kepler early1600
1. LAW OF ORBITS:
Planets orbits are in the shape of an ellipse
with the sun at one foci. See next slide for
demonstration.
Earth’s orbit
An Ellipse
L
X X
F2
F1
The Sun
See next slide for demo
Law of Orbits Comprehensive Diagram
Not drawn to scale.
F = Foci
L = Major Axis
L
X
X
This diagram shows the Earth in orbit around the Sun. Notice the distance from the Sun at both perihelion and
aphelion. Is it the same or different?
F2
F1
Notice the placement of the Sun with respect to the other focus point and the earth’s orbital path..
The Sun is place at one of the two focus points (foci = plural). The Foci are fix points used to create the ellipse.
How many foci are needed to create a perfect circle?
What is the difference between a circle and an ellipse?
What are Foci?
What’s the Major Axis?
Eccentricity = a measurement of the “flatness” of an ellipse.
What’s a Circle?
What’s an Ellipse?
Eccentricity of Orbits
• Determine the eccentricity of the following ellipse. F1 and F2 are
the foci and the dotted line is the major axis.
• Measure the distance between the foci and divide that by the length
of the major axis to determine an ellipse’s eccentricity (e).
• Look at the ellipses on the slide.
F1 What
F2 happens to the eccentricity
of an ellipse (value of e) when you move the foci further apart?
Closer together?
• When calculating eccentricity (e), what do you call an ellipse that
has an eccentricity of 1.0? What about 0.0?
• Can an ellipse be more elliptical than 1.0? Less elliptical than 0.0?
• What do you think the eccentricity of a straight line (completely
flattened out ellipse) is? What about the eccentricity of a perfect
circle?
F1
F2
Earth Science Reference Tables
Solar System Data (page 15)
• Using your ESRT Solar System Data (page 15)
determine, if any, relationships between a
planet’s mean distance from the Sun and the
planet’s eccentricity.
• Which planet has the most eccentric (flattened
out) ellipse? Which planet has the least
eccentric ellipse?
• Is Earth’s orbital eccentricity closer to that of a
perfect circle or a straight line?
• HINT!!! – round all the eccentricities to the
nearest tenth!!
Eccentricity of Orbits
• Look carefully at this chart. What can you
determine from the data regarding the
eccentricities of the planets.
• Try to answer the same questions from the
previous slide by using this chart.
• You can now begin Lab 8-3
• Watch the movie to get an idea of how to do this
lab.
• Doing this lab notice the differences in
eccentricity when you create your ellipses and
change your foci.
• How does the eccentricity change as the ellipse
gets more flattened?
• How does the eccentricity change as the ellipse
gets less flattened?