Summary

Models of our Solar System (Ch. 4)

• Geocentric Model• Heliocentric Model

Lunar Eclipse Yesterday morning!

Strange Motions in the Sky

The motions of planets are not easy to explain Planets may take years to return to the same part of the sky. Some planets reverse their direction of motion This is called retrograde motion.

Ancient people struggled to explain these motions

Retrograde Motion of Mars

Astronomical Models of Planetary Motion To understand this motion, we need to make a model.

Aristotle devised one of the earliest models of the Solar System (c. 350 BC)

Earth was at the center of the Solar System So the model was Geocentric

The Sun and planets moved around Earth, each on a different sphere

A “Prime Mover” controlled the stars’ motion.

Astronomy of Ancient Greece

Greek scientist Ptolemy worked at the Great Library of Alexandria in Egypt (~140 AD)

His research included astronomy, astrology, geography & music. Ptolemy came up with an improved geocentric model.

In Ptolemy’s model, the planets traveled along small circles that move on larger circles.

This can explain retrograde motion.

Ptolomy’s Geocentric model

Ptolemy’s Model A planet orbits the Earth on a small circle, called an epicycle, The epicycle moves along a large circle, called a deferent Since circles were thought to be “perfect” shapes, it seemed

natural that planets should follow circles in their motion.

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The Geocentric Model

Ptolemy’s views were published in a book known as the Almagest

Because it could explain retrograde motion, his theory was very influential in the study of astronomy for a long time.

The Geocentric Model was not challenged for 1,600 years!

Copernicus’ Heliocentric theory

We credit Nicolaus Copernicus, a 16th-century Polish astronomer, with our modern view of the Solar System

Copernicus suggested a heliocentric theory He proposed the Sun was at the center of the Solar System. His theory also explained retrograde motion.

“I began to be annoyed that the movements ofthe world machine … were notunderstood with greater certainty by the philosophers, who otherwise examined so precisely the most insignificant trifles of the world.”

Copernicus, Book on the Revolutions, 1543

“… it is just like someone taking from various places hands, feet, a head, and other pieces, eachvery well depicted, but not for the representationof a single person. In forcing these fragmentstogether, they created a monster rather thana man.”

Copernicus, Book on the Revolutions, 1543

The Heliocentric System of Copernicus

heliocentric systemneeds no epicycles!

retrograde motion is anatural consequence of the relative speeds of the different planets

(e.g., Earth orbiting faster than Mars)

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Copernicus’ Heliocentric Theory

Copernicus’ new theory allowed him to calculate: The relative distances to the planets. The length of time the planets take to orbit the

Sun

This let him predict the next appearances of planets in the sky.

These predictions proved correct, lending support to Copernicus Heliocentric Model. However, Copernicus’ model was no more accurate

than the Ptolemaic model.

A Heretical Idea?

Copernicus’ Heliocentric theory caused problems with the Church. The Church had accepted Aristotle & Ptolemy’s geocentric universe. It agreed with the Christian notion of heaven as a perfect place.

To question this model was to question the Church’s authority.

Copernicus died before the full effects of his model were felt However, astronomers continued to challenge the Church, using their

observations.

Galileo Galilei (1564-1642) was an Italian scientist who studied many fields.

He is famous for dropping cannonballs from the Leaning Tower of Pisa

He obtained the first observations toconfirm the Heliocentric Model

Galileo

More Evidence for the Copernican Model

Galileo was the first person to use a telescope for astronomy.

Several of his observations supported the Heliocentric model of Copernicus:

He discovered moons orbiting Jupiter (not the Earth)

Galileo Galilei

Galileo also reported seeing crescent and gibbous phases on Venus

Gibbous phases are only possible if Venus is on the other side of the Sun from us.

This could not happen in the Geocentric model of the Universe.

Geocentric model

Of Ptolemy

Theory: Venus is always between the Sun and Earth

Prediction: Venus will always be seen in crescent phase

Theory: Venus sometimes goes behind the Sun…

Prediction: Venus will sometimes be seen in gibbous phase.

Observation: Venus does show gibbous phase when seen with a telescope

Galileo Challenges the Geocentric Model

Heliocentric model challenged beliefs held for 2000 years. The Church felt threatened by this. Galileo was arrested and tried. He was not allowed to advocate the Heliocentric Model. He had reason to fear: Giordano Bruno, another advocate of

the Heliocentric Model, was tried by The Inquisition. Bruno was burned at the stake for heresy!

However word of Galileo’s discoveries spread….

Tycho Brahe

Tycho Brahe (1546-1601) was a Danish nobleman who built his own observatory on an island.

Has a child, he had witnessed a Solar Eclipse

He was so impressed that eclipses could be predicted, that he dedicated his life to astronomy.

Tycho Brahe

Tycho made the most precise observations of the planets to date …without using a telescope.

During his life, Tycho was reluctant to share his precise data.

He also had a wild lifestyle, and lost his nose in a duel. He attended many royal parties

Perhaps one too many…. He died after eating and drinking too much at a party….

Johannes Kepler

After Tycho died, his assistant, Johannes Kepler, took his place.

Kepler studied Tycho’s precise observations of the planets

He noticed patterns in their motion never seen before.

His first model for the planets involved geometric shapes

At first, Kepler thought the orbits of the planets had the same proportions as the “5 Perfect Solids” of geometry.

He worked on this idea for years, but finally admitted it was a failure.

Johannes Kepler

However, Kepler did not give up.

He found that the planets’ motions could be explained if he gave up the idea of circular orbits.

In time he discovered that all planets follow the same three rules…..

Kepler’s Laws of Planetary Motion

Ellipse Shapes

Circle

How to draw an Ellipse

Kepler’s First Law The major axis of an ellipse is the line within the ellipse

that passes through the two foci. The minor axis is perpendicular to the major axis. The semimajor axis is half the length of the major

axis, and the semiminor axis is half the length of the minor axis.

In astronomy, the semimajor axis is designated: “a”

a

Kepler’s 2nd Law: Equal Areas in Equal times

Suppose a planet moves from points A to B in a fixed time.

In the same amount of time it moves from A’ to B’

These two wedges have the Same Area!