Early people looked at the

stars and wondered…

The ancient Greeks studied the movement of the stars and

observed patterns.

They concluded the patterns of

most of the stars do not

change, except…

5 points of light that seem to

wander among the stars.

The Greeks named them

PLANETS, which means

wandering stars.

The Planets were named after the 5

Roman Gods, Mercury,

Venus, Mars, Jupiter and

Saturn

Mercury, Venus, Mars, Jupiter and Saturn were

the only planets that

could be seen at that time.

The Greeks thought the Earth

was the center of a celestial

sphere with all stars and planets

revolving around the Earth.

This Earth centered theory is called the geocentric system.

Observations of the planets did not fit the simplest version of the geocentric system.

.

Early astronomers tried to create a model that would fit their observations

In A.D. 140 Ptolemy suggested a variation

of the geocentric model that explained

why the planets move at different speeds. Bodies

revolved in small circles as they orbited

Earth

For 1,400 years people believed Ptolemy was correct.

In the early 1500’s the Polish astronomer

Nicolaus Copernicus thought the sun was the center of the system of

planets.

His sun-centered system was called the heliocentric system.

Helios is Greek for sun.

But people still believed Ptolemy’s geocentric system.

100 year’s later Galileo agreed with

Copernicus’ heliocentric system.

In 1609 Galileo perfected the

telescope which lead him to

discoveries that supported the

heliocentric system.

Galileo discovered the 4 moons of Jupiter.

He thought if moons revolve around Jupiter then all things do not revolve

around the Earth.

He discovered the phases of Venus. This requires the sun to be the center

of the system of planets.

Galileo’s discoveries were not accepted by those that still believed in the geocentric system. The geocentric view had the support of the church which was in power at that time.

This illustration was copied from Nick Strobel's Astronomy Notes. Go to his site at www.astronomynotes.com for the updated and corrected version.

Galileo was put on trial for his beliefs in 1633. He remained under house arrest until he died in 1642.

In the late 1500’s a Danish

astronomer, Tyco Brahe, carefully observed and recorded the

positions of the planets for 20

years.

• He believed in a modified version of geo-centric model.

• Every planet EXCEPT EARTH revolved around the sun.

• The sun with all the planets REVOLVED around Earth.

Brahe’s meticulous records of planetary motion were passed on

and interpreted by a German mathematician in the 1600’s.

Johannes Kepler used

Brahe’s data to figure out the shape of the

planets’ orbits.

Kepler concluded the planets orbit the Sun in an ellipse.