The center of the Solar System:

Heliocentric Model vs. Geocentric

Model

Aristotle (384-322 B.C.)• Established the Geocentric view

– States that the Earth is the center of all things and the Sun, moon and planets revolve around it

– Works for the Sun and moon,

– Sucks for the planets • Can’t explain:

– Their changing brightness and – Retrograde motion:

» Appear to stop and move backwards in the sky during the year

A Geocentric Solar System

Retrograde Motion of the Planets as seen from Earth

Claudius Ptolemaeus (Ptolemy) – around 140 A.D.

• Greek, 400 yrs. after Aristotle

• Made the best Geocentric Model

• “The Ptolemaic Model”

• Problem: very complex

• Had 80 circles to explain the sun, moon and 5 planets known then!

Ptolemy made “epicycles” to explain retrograde motion of planets

Nicholas Copernicus (1473-1543)

• 2,000 years after Aristotle and Aristarchus

• Copernican Revolution –The change from a belief in a Geocentric model to a Heliocentric model

• Copernicus liked Heliocentric model because it’s simpler:– “…it is more pleasing to the mind”, – It also explained some small inconsistencies in

the Geocentric theory and states that the Sun is the center of the solar system, planets revolve around the Sun

A Drawing of the Heliocentric model made by Copernicus

Nicholas Copernicus• The only major flaw in his version of the

heliocentric model was that he used circular orbits when we now know the planets travel in elliptical orbits

• But that might have been because the orbit of Mars is very close to a perfect circle around the Sun.

• His ideas were never accepted in his lifetime either,

• But with the observations of some later astronomers, the world eventually did “revolve around” to the correct understanding.

Galileo Galilei (1564-1642) • Italian born 21 years after Copernicus died

• A key to his success:

– He embraced cutting edge technology and techniques of his day like • Experimentation over logical reasoning and• The telescope

– Invented in Holland around 1600, Galileo was already around 36 years old

– He heard about it (never had seen one!) and built one for himself.

Galileo’s discoveries of Jupiter’s moons with his telescope showed that Earth was not the center of all orbits strongly supported a heliocentric model even though Copernicus’ model was no more accurate than Ptolemy’s.

Galileo also observed ‘blemishes’ on the Sun (sunspots): This contradicted ‘perfect spheres’ idea of Aristotle.

Galileo’s discoveries of Venus’ phases with his telescope showed that Venus must orbit the Sun strongly supported a heliocentric model even though Copernicus’ model was no more accurate than Ptolemy’s.

Galileo’s discoveries of Venus’ phases with his telescope showed that Venus must orbit the Sun strongly supported a heliocentric model

Venus can only go through phases if it orbits the Sun.

Galileo Galilei• Some key observations he made in that

trusty telescope of his: 1. Mountains, valleys and craters on the Moon, 2. The sun has imperfections - sunspots, which

he watched rotating3. 4 moons of Jupiter (if other planets have

moons, earth is not the center of all things)4. Venus goes through phases like the moon,

which can only be explained by the heliocentric theory (pic on pg.30)

Johannes Kepler (1571-1630) • German, 7 years younger

than Galileo

• Poor eyesight– So he based his laws on Tycho

Brahe’s observations

• Discovered 3 simple laws that accurately described the planets’ motion in the heliocentric theory

Johannes Kepler • Called: “The 3 Laws of Planetary Motion”• Kepler’s Laws pretty much sealed the lid on

the Geocentric model’s coffin and allowed most scientists to accept the Heliocentric model. – Especially because they followed the Occam’s

razor principle (simple)

• They work for every planet in our solar system, even the ones that Kepler never knew about!

Kepler’s Kepler’s model for model for the Solar the Solar System: A System: A Concentric Concentric Series of Series of Crystalline Crystalline SpheresSpheres

Kepler’s Three lawsKepler’s Three laws

11stst Law: All orbits are Law: All orbits are Elliptical with Sun at Elliptical with Sun at one focus one focus (circular (circular orbits are special case of orbits are special case of elliptical with e = 0)elliptical with e = 0)

22ndnd Law: Planets Law: Planets sweep out equal sweep out equal areas in equal timeareas in equal time

33rdrd Law: P Law: P22 = a = a33 (P in (P in yrs, a in AU)yrs, a in AU)

The Copernican RevolutionThe Copernican Revolution

9/13/2005

Isaac Newton• (1642-1727) British • Born the same year that Galileo died• Finally explained WHY the earth orbits the

Sun. • Before him Galileo, Brahe and Kepler

explained WHAT the solar system looked like, but couldn’t say WHY it looked that way.

• Newton came up with…– The 3 laws of motion– The law of Universal Gravitation – and helped invent Calculus to explain WHY.

Isaac Newton

• The law of gravity says that all masses attract each other and that the more mass an object has the greater the attraction it has. – Explains why the earth and planets are pulled

towards the Sun, but why don’t they get pulled INTO the Sun??

Isaac Newton• The first law of motion is the law of inertia

– An object tends to maintain its current state of motion

• In both speed and direction

– When the solar system formed, the Sun “caught” giant masses in space, which are now the planets.

– Those masses moving through space also wanted to maintain their forward motion through space because of their inertia, so they ended up moving around the Sun instead of stopping and sitting still in space.

Isaac Newton– The planets continue to move because there

is no friction in space to slow them down – If gravity was suddenly “turned off”, they

would fly off in a straight line pointing out from their orbit because of their inertia.

• ball on a string example

• So to sum upSo to sum up: –GravityGravity keeps keeps

the planets the planets pulled inwardpulled inward• towards the towards the SunSun

–InertiaInertia keeps keeps them moving them moving outward, outward, • results in a results in a circular pathcircular path

Isaac Newton Quote

• ““If I have been able to see farther than If I have been able to see farther than others it is because I stood on the others it is because I stood on the shoulders of giants.”shoulders of giants.”

--- Newton’s letter to Robert --- Newton’s letter to Robert Hooke,Hooke,

• probably referring to Galileo and Keplerprobably referring to Galileo and Kepler

Lesson Review Time!

#1. (T/F) Aristotle was the first to propose that all planets revolve around the Sun.

Lesson Review Time!

#2. (T/F) During retrograde motion, planets actually stop and move backwards in space.

Lesson Review Time!

#3. (T/F) The heliocentric model of the universe holds that the earth is the center of the universe.

Lesson Review Time!

#4. (T/F) Copernicus’ theories gained widespread acceptance during his lifetime.

Lesson Review Time!

#5. (T/F) Galileo’s observations of the sky were made with the naked eye

Lesson Review Time!

#7. (T/F) Kepler’s laws hold true only for the 6 planets known in his time.

Lesson Review Time!

#9. A major flaw in Copernicus’ model was that it still had:

A) The Sun at the center

B) Earth at the center

C) retrograde loops for the planets

D) circular orbits for the planets

Lesson Review Time!

#10. A accurate sketch of Mar’s orbit around the Sun would show:

A) The Sun far off centerB) An oval twice as long as it is wideC) A nearly perfect circleD) Phases