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.

Answer these questions on the top of your Guided Reading Worksheet.

1. What is the difference between the geocentric and heliocentric models of the solar system? Which one is accurate?

2. 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)

3. The Andromeda galaxy is 2.5 million light years away. How long does it take the light from Andromeda to reach us ?

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

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.

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.

A. Geocentric Model: Ptolemy’s Epicycles

• Problems with Geocentric Model

1. 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

Geocentric model cont.

B. Heliocentric Model = Sun is the center of

universe. Earth orbits the sun.

• Aristarchus – Ancient GreeceProposes theory. Not

widely accepted.

• Nicolaus Copernicus –1543 ADThinks 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 http://www.sciencemaster.com/space/item/helio_2.phphttp://www.astro.utoronto.ca/~zhu/ast210/both.html

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). http://astronomyonline.org/SolarSystem/GalileanMoons.asp?Cate=SolarSystem&SubCate=Jupiter&SubCate2=JT02

– Reward: House arrest for life.

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)

2. 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.

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.

Gravity: pulls on Earth

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

3. Components of the Solar System

Solar System = Sun, planets, their moons (satellites),asteroids, kuiper belt objects, & comets (All orbit sun)

Sun MercuryVenus EarthMars

Asteroid BeltJupiterSaturnUranus

Neptune * Pluto

Kuiper Belt ObjectsComets

A.U. (Astronomical Units) =

•average distance between Earth & sun (1 AU)

•93 million miles (149.6 km)

•Used to measure distances w.in S.system

•Jupiter is 5 AU from sun … its 5 x farther fr. Sun than E.

3. Components of 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

Distances Within the Solar System

4. Exploring the Solar System

A. Telescopes; spacecraft; space probes

B. Manned Missions to the MoonJuly 20 1969: Apollo 11 touches down on moon (Neil

Armstrong)

Apollo 17 – last mission