1. Observing the Solar System
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2. Ancient Observations
The ancient Greeks observed the sky and noticed that the moon, sun, and stars seemed to move in a circle around the Earth.
It seemed that the Earth was not moving and everything in the heavens revolved around the Earth.
As it turned out, it was very difficult to prove that the planets did not revolve around the Earth without leaving the planet.

3. Wandering Stars
The Greeks also noticed that while the stars move across the sky, their patterns do not change.
But five points of light did move among the stars.
The Greeks called these objects planets, which means “wandering star.”
They made careful observations of these planets, which we call Mercury, Venus, Mars, Jupiter, and Saturn.

4. Geocentric Theory
Ancient Greeks such as Aristotle believed that the universe was perfect and finite, with the Earth at the exact center.
This is the geocentric theory, which has the planets, moon, sun, and stars revolve around the Earth.

5. Ptolemy
In 140 CE the Greek astronomer Ptolemy ( CE) revised the geocentric model to explain all the planetary motions.
This was necessary as at times the planets seem to move backwards in the sky.
His model had the planets move in little circles that also moved in bigger circles. Very complex, but a good model.
This belief persisted for about 1400 years.

7. Ptolemaic Discrepancies
Retrograde motion: At times the planets would stop their movement in the sky, turn backwards for a few days, and then resume their forward progress.
Jupiter's Moons: Galileo first saw evidence that objects revolved around something besides the Earth.
Galileo also saw that Mercury and Venus went through phases, just like our moon, but the outer planets did not.

8. Copernicus ( )
In the early 1500’s the polish astronomer Copernicus developed the heliocentric model.
Helios means “sun” in Greek.
He placed the sun at the center of the solar system and had the planets revolve around the sun.
This was a very simple and elegant theory that explained all the planetary motions without the complexities of the Ptolemaic theory.
Scientists prefer simple explanations over complex ones.
Occam's Razor:
The simplest explanation is usually the best.

9. Galileo Galilei Italian physicist
considered to be first true scientist
Used telescope to show that Copernicus was correct
Wrote in the vernacular instead of Greek/Latin. This upset the Church mightily!
They felt threatened if the common people could read this as it might cast doubt on other church teachings.

10. Retrograde Motion The apparent temporary backward motion of a planet.
This is caused when a faster moving inner planet, say the Earth, bypasses a slower moving out planet, say Mars.

11. Planetary Phases
Only planets that are always closer to the sun will show phases when viewed from the Earth.
Planets orbiting further from the sun than the Earth will always appear gibbous or full.

13. Heliocentric Theory Prevails
Since Galileo’s theory clashed with the religious views of the time, he underwent many trials and tribulations, eventually being sentenced to house arrest for his remaining years.
But his view has withstood the test of time.
Today we talk about our solar system, not our Earth system

14. Theory Has a very different meaning in science than in everyday life.
“The detective has a theory about who robbed the bank.” This is a guess.
When scientists use the word theory it is not used as a guess.

15. Theory defined
An explanation based on many observations during repeated experiments that is valid only if it is consistent with observations, makes predictions that can be tested, and is the simplest explanation.
A logical, time tested explanation for events that occur in nature.

16. So the theory of gravity, theory of electricity, the germ theory of disease, and the theory of evolution are tested, accepted explanations for events that occur in nature.
Theories can really never be completely proven, only disproved. When new evidence comes along, we must modify our theory or at times even get rid of it and start over again.

17. Ptolemy’s earth centered theory of the solar system is an example of what can happen when new evidence comes along. When Copernicus showed that putting the Sun in the center made it much easier to predict the planets motions, the old earth centered theory was discarded and a new one developed.