1. The Solar System
and Planetary Motion

2. Aristotle
Earth Centered
The Geocentric Model

3. The planets periodically seemed to make a loop against the background stars
Lets watch one more time

4. Are the planets really making this strange looping motion?
What is going on here?

5. This looping motion is called retrograde motion
Mars looping near the “teapot” a few years ago
This looping motion is called retrograde motion

6. Ptolemy’s Model

7. Ptolemy’s Model

8. Ptolemy had all the known planets of the day on
Epicycle orbits around the earth. The model lasted almost 1500
Years and was adopted by the catholic church

9. The Copernican model was
much simpler than Ptolemy’s
and it placed the sun rather
than the earth in the center
of the solar system

10. Copernican model
Heliocentric
Model
This is where I belong

12. To an observer on the train going faster it appears as if the other train is going backwards
70mph
100mph
See Ya

14. Kepler’s 1st Law of Planet Motion
1. The planets go around the sun in an ellipse with the sun at one focus of the ellipse

15. . Calculating Eccentricity Eccentricity = focal distance major axis
Eccentricity ranges
between 0 and 1 .
focus 1
focus 2
asteroid
focal distance
major axis
Eccentricity = focal distance
major axis
Notice the units cancel out

16. E = 0 (circle)
E = 0.1
E = 0.4
E = 0.9
E = 0.7
The higher the eccentricity
the more elongated the orbit

17. Earth Orbit = 0.017 (very slightly elliptical)
Like the earth’s shape, the
human eye probably can not
distinguish earth’s orbit from
a circle

18. Energy of an asteroid in orbit
downhill
Low kinetic energy
Low
potential
energy
perihelion
aphelion
High kinetic energy
high potential
energy
uphill
KE + PE = constant

19. Watch the comet change speed as it gets further and closer to the sun
Low Velocity
High Velocity

20. As this planet orbits the sun, where will it be going fastest? Slowest?

21. Gravity The force of attraction between
any two objects in the universe
The MORE MASS the object has, the stronger its
force gravity
The CLOSER the objects the stronger the
attraction between them

22. Gravity Stronger gravitational attraction
Weaker gravitational attraction

23. Gravity Stronger gravitational attraction
Weaker gravitational attraction

24. Would you weigh the most
North Pole
Where on earth
here
equator
Would you weigh the most
South pole

25. If the sun has so much mass (& strong gravity), why don’t the planets fall into the sun?

26. Because of Inertia
An object at rest will tend to remain
at rest
An object in motion will maintain
its direction and speed unless a
an opposing force affects is
(like gravity)

27. Newton first explained gravity using the moon’s orbit around earth
Solid arrow = Earth’s gravity
Dashed arrow = moon’s inertia

28. Inertia + gravity give all objects in the solar system “orbits”
around the sun

29. Apparent Diameter The size an objects appears to
be depends on how far away it is.
Objects far away appear small
Objects nearby appear big

30. How large something appears in our
What does this suggest about the sun’s distance?
How large something appears in our
Sky is called the apparent or angular
diameter
Yes, the sun is almost exactly 400 times further away
than the moon
The sun is actually 400 times larger than the moon
SUN
By shear coincidence the sun and moon
both appear to have the same apparent
diameter as viewed from Earth
MOON

31. What could cause the moon to appear slightly
larger than the sun?
When the moon does appear larger than the
Sun, a total eclipse can occur

32. As a result of the earth’s slightly
elliptical orbit, the sun will appear slightly
larger in January compared with July
Which Sun appears slightly larger?
Photo taken in January
Photo taken in July
In what season would the sun look larger?

33. Which photo was taken when Venus was closest to Earth?
crescent
quarter phase
nearly full phase