Chapter 2:

The Rise of Astronomy

Ancient Roots: Early Homo-Sapiens

Ancient Roots: Egyptian

Ancient Roots: England

Ancient Roots: Mayan

Ancient Roots: Aztec

Ancient Roots: Incan

Ancient Roots: Cambodia

Ancient Roots: Anasazi

Early Ideas: Pythagoras

• Pythagoras in 500 B.C. taught that the Earth was round based on the belief that the sphere is the perfect shape used by the gods

Early Ideas: Aristotle

• By 300 B.C., Aristotle presented naked-eye observations for the Earth’s spherical shape:– Shape of Earth’s

shadow on the Moon during an eclipse

The Hot Debate!

• A geo-centric cosmology is a theory that proposes Earth to be at the center of the universe.

versus

• A helio-centric cosmology is a theory that proposes the Sun to be at the center of the universe.

Aristotle’s model of a geo-centric solar system

But model couldn’t account fully for retrograde motion of the planet!!!

(384-322 BC)

Retrograde motion is the apparent “backward” motion of a planet

Ptolemy’s Geo-centric Solar System

Tried to accountfor retrogrademotion, but couldnot quite match observations

Copernicus devised the first comprehensive helio-centric cosmology to successfully explain retrograde motion

Tycho Brahe’s Golden Nose

Mathematician Johannes

Kepler created laws of

planetary motion

Kepler’s First Law: The orbit of a planet about the Sun is an ellipse with the Sun at one focus.

Kepler’s Second Law: A line joining a planet and the Sun sweeps out equal areas in equal intervals of time.

In other words, the closer a planet is in its orbit around the Sun, the faster its speed is.

Kepler’s Third Law

In other words, the planets that are

closest to the Sun orbit the Sun at a faster

speed than planets that are

farther away .

Summary of Kepler’s Laws of Planetary Motion

The orbit of a planet about the Sun is an ellipse with the Sun at one focus

1st Law

The closer the planet is in its orbit around the Sun, the faster its speed is.

2nd

Law

The planets that are closest to the Sun orbit the Sun at a faster speed than planets that are farther away.

3rd

Law

Italian scientist Galileo made

discoveries that strongly supported a helio-centric cosmology

“I do not feel obliged to believe that the samegod who has endowed us with sense, reason andintellect has intended us to forgo their use.” - Galileo

Galileo’s telescope revealed that

Jupiter had moons which orbited

Jupiter instead of Earth.

Galileo’s telescope revealed all phases of Venus which could only occur IF Venus

orbits the Sun.

Geo-centric or Helio-centric?

Galileo Ptolemy

Kepler Copernicus

Aristotle

What goes up, must come down!

Sir Isaac Newton

Newton’s Universal Law of Gravitation

• Every body that has mass has gravity

• The gravity between the two bodies increase with the mass

• The gravity between the two bodies increase if they are closer together.

Mass vs. Weight

• Mass is a measure of the total amount of material in the object

remains the same everywhere

• Weight is the force with which an object is

pulled down while on the ground (due to gravity’s attraction)

changes depending on the body you are

standing on

Foucault proves that the Earth rotates!