1. Chapter 25.1: Models of our Solar System
Objectives:
Compare & Contrast geocentric and heliocentric models of the solar sytem.
Describe the orbits of planets explain how gravity and inertia keep the planets in orbit.
Name the components of the solar system.

2. Answer these questions on the top of your Guided Reading Worksheet.
What is the difference between the geocentric and heliocentric models of the solar system? Which one is accurate?
Saturn is 10 x farther from the sun than Earth. What is the distance between Saturn and the sun in AU? In kilometers or miles? (show your work)
The Andromeda galaxy is 2.5 million light years away. How long does it take the light from Andromeda to reach us ?

3. 1. Models of the Solar System
Ancient people thought …
E. is stationary; Sun, planets, & stars move around E.
Observed 5 planets: Mercury; Venus; Mars; Jupiter; & Saturn

4. A. Geocentric Model = Earth is the center of the universe (a sphere)
A. Geocentric Model = Earth is the center of the universe (a sphere). Stars & planets orbit E. each day.
Who: Most ancient Greeks … later Ptolemy (Egyptian – 140 AD) maps out position of planets with this model (Accepted for ~1400 yrs)
Earth is stationary.
fixed # of stars.

5. A. Geocentric Model: Ptolemy’s Epicycles
When E. passes planets – they look like they are moving backwards relative to us for a while. This apparent (not actual) backward motion in known as “retrograde motion”.
Ptolemy didn’t believe E. moved (it was the center of the universe) – so he couldn’t explain this “backward” motion of planets. That’s where Epicycles come in – but they are not real.

6. Problems with Geocentric Model
Geocentric model cont.
Problems with Geocentric Model
It’s WRONG! But also …
2. Earth is not stationary (It rotates)
3. It could not explain the motion of planets … so Ptolemy created “EPICYCLES” to explain planetary motion.(Especially RETROGRADE motion)
See Epicycles: zebu.uoregon.edu/.../121/lecture-3/ptolemy.html

7. B. Heliocentric Model = Sun is the center of universe
B. Heliocentric Model = Sun is the center of universe. Earth orbits the sun.
Aristarchus – Ancient Greece Proposes theory. Not widely accepted.
Nicolaus Copernicus –1543 AD Thinks Aristarchus had the right idea and develops it into a publication.
Problem w/Copernicus’s Model: Orbits are not perfect circles.
Retrograde Motion of Mars

8. B. Heliocentric Model: Galileo
Galileo – 1616, 1632 Publications
Explains how Inertia & Gravity keep planets in motion.
Finds Evidence to Support Copernicus’s Heliocentric Theory
Venus has phases (new & full can’t be seen b/c sun blocks … Venus must move around the sun, not Earth).
Jupiter’s moon’s: Galileo discovers 4 moons orbiting Jupiter … NOT EARTH (Earth must not be at the center of the UNIVERSE).
Reward: House arrest for life.

9. 2. Planetary Orbits: Inertia & Gravity keep planets moving in a curved path.
1. INERTIA: Newton 1st Law of Motion tells us that planets want to keep moving in a straight line forever (in motion … stay in motion)
GRAVITY: The Sun’s gravitational pull keeps planets fr. Moving in a straight line. This tugging creates the curved path.
Gravity is an attractional force between two masses.
Gravity: pulls on Earth
Inertia: Earth want to keep moving in a straight line (moving keep moving, in same direction). The force that prevents this: Gravitational force from sun.

10. Johannes Kepler (1600) revises Copernicus’s model
2. Planetary Orbits
Johannes Kepler (1600) revises Copernicus’s model
Orbits are not circles … they’re ELLIPSES (OVAL)
Planets orbit the sun in a plane called the:
ECLIPITIC

11. 3. Components of the Solar System
Solar System = Sun, planets, their moons (satellites),asteroids, kuiper belt objects, & comets (All orbit sun)
Sun Mercury Venus Earth Mars Asteroid Belt Jupiter Saturn Uranus Neptune * Pluto Kuiper Belt Objects Comets

12. 3. Components of the Solar System
A.U. (Astronomical Units) =
average distance between Earth & sun (1 AU)
93 million miles (149.6 million km)
Used to measure distances w.in S.system
Jupiter is 5 AU from sun … its 5 x farther fr. Sun than E.

13. Distance from Sun in kms
A.U.s
Distances Within the Solar System
Planet
Distance from Sun in kms
Distance in A.U.s
Mercury
57,900,000
0.387
Venus
108,200,000
0.723
Earth
149,600,,000
1.000
Mars
227,900,000
1.524
Jupiter
778,400,000
5.203
Saturn
1,427,000,000
9.539
Uranus
2,896,600,000
19.182
Neptune
4,520,000,000
30.214
Pluto
5,899,900,000
39.439

14. 4. Exploring the Solar System
Telescopes; spacecraft; space probes
Manned Missions to the Moon
July : Apollo 11 touches down on moon (Neil Armstrong)
Apollo 17 – last mission