Origins of Modern Astronomy

Earth-Centered (Geocentric) vs.

Sun-Centered (Heliocentric)

OR ?

Why a Geocentric Universe?

• The Earth does not feel like it is moving.

• All objects in the sky appear to revolve around us.

• It’s comforting to think we are so important.

A Strong Point in Favor of the Geocentric Model…

there is no observed parallax.

June Dec.

A Major Problem for

the Geocentric

Model…Retrograde

Motion of the Planets

The Path of MarsMars across the night sky…

Mars Retrograde

This was explained by Ptolemy, using epicycles,

orbits on top of orbits:

Animation

Copernicus: The Sun-Centered Universe

The Polish astronomer, Copernicus, believed that Ptolemy’s model was too complicated, so he proposed the heliocentric model of the universe….

By having planets,

including the Earth, revolving around the Sun,

the epicycles are eliminated.

In the heliocentric model, retrograde motion is explained by the Earth passing a slower planet.

Animation

Jupiter’s Retrograde Motion

• It takes Jupiter 12 years to revolve around the sun.

• Every Earth-year it goes through one Zodiac constellation.

• When the Earth passes it, Jupiter “falls backward,” and then resumes its pro-grade motion. June 2007July 2007

August 2007Sept. 2007

Oct. 2007

Nov. 2007

Dec. 2007February 2008

May 2008Sept. 2008Nov. 2008

Jupiter 2006-2018

• Each year Jupiter moves slowly to a new constellation in the zodiac.

• During this time is goes through a retrograde stage when the Earth passes it.

2006 2007 2008 2009

Enter Galileo with the first physical evidence of a heliocentric universe!

THE TELESCOPE

The telescope greatly improved our ability to observe the universe

There are too many stars to be “pasted on” one sphere.

Galileo discovered that Jupiter had moons of its own -a mini-Solar System, independent of the Earth

Jupiter 4 moons

More significantly, he discovered that

Venus goes through phases like the

Moon.

This is difficult to explain with an earth-centered universe.

Sketch made by Galileo:

The crescent Venus is much larger, since it is closer to Earth

Tycho’sTycho’s Compromise

• Danish astronomer Tycho Brahe was a superb observer of the heavens.

• He didn’t observe any parallax, so he kept the Earth in the center.

• Other planets revolved around the Sun, as it revolved around us.

Mathematical Evidence:Johannes Kepler

Kepler studied the records of planetary positions and came

up with three laws…

Law #1: All the planets travel around the Sun in

elliptical orbits.

Law #2: Planets move faster as they get near the Sun, slower when farther away

A1A2

A1 = A2

Animation

Equal Area Ani.

Law #3: The distance of the planet to the sun cubed is equal

to the period of revolution squared: D3 = P2

For Mars: D = 1.5237 AU and

P = 1.881 years

(1.5237)3 =

3.538

(1.881)2 =

3.538

Finally, there was Sir Isaac Newton, whose

Laws of Gravitation proved the heliocentric model.

Newton was able to show how the forces of gravity and the

inertia of moving planets produces a Solar System.

Orbits in a cone

Quiz!1. Why did the geocentric view hold on

so long?

2. Who plotted out the epicycles for the known planets?

3. How did Copernicus, Galileo, and Kepler help promote the heliocentric model of the universe?

4. What was Newton’s contribution?