The Copernican revolution

Discussion

What is the simplest universe imaginable: one where making predictions about the future would be the easiest thing to do?

Second Simplest Universe

• Stars unchanging and fixed to the celestial sphere

• Celestial sphere rotates about Earth moving east to west once a day carrying the Sun, Moon and planets with it.

Third Simplest Universe

• Sun, Moon, and planets are carried east to west along with rotation of celestial sphere

• Sun, Moon and planets move more slowly west to east against celestial sphere each at a different rate on a sphere of their own

Ptolemy’s explanation for retrograde motion

• Each planet moves on a small circle called and epicycle.

• The center of each epicycle moves along a larger circle centered near the Earth called a deferent.

Ptolemaic system

• Very successful at predicting positions of the planets but was not perfect

• Offered no explanation of why the planets moved on deferents and epicycles

• There was no relationship between period of revolution and epicycle size

Heliocentric model

Aristarchus – 3rd century BC

Reintroduced by Copernicus in 16th century

Heliocentric model

• Sun at the center• Diurnal motion explained by rotation of the

Earth • All the planets including Earth revolve

about the Sun in circular orbits with different speeds

Advantages of heliocentric model

• Provides natural explanation of retrograde motion.

• Provides natural explanation of motion of Mercury and Venus as inferior planets, i.e. their orbits are interior to that of the Earth.

• Provided a relationship between distance from Sun and orbital period.

Discussion

Why do you think the heliocentric model rejected?

Disadvantages of the heliocentric model

• Still required epicycles• Was no better at predicting planetary

positions• No stellar parallax observed

Tycho Brahe’s Epilepsy Medicine:

The basic substance is the head of a person who has been hanged or otherwise executed. The head should be dried and crushed together with peony seeds to a powder. This medicine should not be taken at the full moon.

Tycho Brahe showed that the celestial sphere could change

• Tycho’s supernova of 1572 – showed that this new star had no parallax and thus was more distant than the Moon

• Comet of 1577 – showed that it too was beyond the distance of the Moon

Kepler’s first law of planetary motion

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

Kepler’s Second law of planetary motion

A line drawn from the planet to the Sun sweeps out equal areas in equal intervals of time.

Kepler’s third law of planetary motion

The square of the sidereal period is equal to the cube of the semimajor axis of the orbit.

Advantages of Kepler’s model

Allowed near perfect ability to predict planetary positions

Disadvantages of Kepler’s model

No explanation as to why planets moved according to his three laws

Still no parallax

Galileo

Galileo’s Observations

• The Sun had spots which were considered imperfections

• The Moon had mountains and valleys• The Milky Way resolved into countless stars• Jupiter had four moons that clearly orbited

it and not the Earth• Venus had phases

Discussion

Why is the discovery of Jupiter’s Moons important to the Copernican model?

Jupiter acted like a smaller version of the Solar System

• Jupiter is bigger than its four moons and the moons orbit it

• Jupiter’s moons orbit with periods that are longer for those moons that are furthest from the planet

Explain why the observations of the phases of Venus prove that Venus must orbit the Sun. Why is this different than the Moon, which also has phases but orbits the Earth?

Discussion

Newton

Newton’s Universal Law of Gravity

• Every mass attracts every other mass through a force called gravity

• The force is directly proportional to the product of their masses

• The force is inversely proportional to the square of the distance between them

221

d

MMGF

Argument by analogy

If Earth is a planet orbiting the Sun just like the other planets, than the other planets are just like the Earth.

All planets have life and humans living on them.