1. Observing the Solar System: A History
Chapter 26.2 P

2. Galileo’s Observations:
Copernicus’s Idea
Inertia:
Greek Observations:
Wandering Stars:
Brahe:
Kepler:
Newton:
Gravity:

3. Wandering Stars
Greeks noticed 5 points of light that appeared to wander amongst the stars.
They called them Planets
Greek for “wandering star”

4. Greek Observations
Saw that these objects appeared to move and they thought that the Earth remained stationary.
They believed they were inside a rotating dome- called the celestial sphere.
Aristotle believed that the universe was perfect and finite and that the Earth was stationary at the center.
That theory is known as:
Geocentric (Earth Centered) Model

5. Greek Observations Ptolemy: Puzzled by retrograde motion
He created the Geocentric Model
Planets orbit the Earth and move in smaller circles.

6. Copernicus's Idea Early 1500’s Polish Astronomer
Thought the Sun was at the center.
Created the Heliocentric Model
Helios- Greek for Sun
The Earth and all the other planets move around the sun.
Included the six planets.

7. Galileo’s Observations
Made two discoveries that supported the heliocentric model.
Saw 4 moons revolving around Jupiter
Showed that not everything in the sky revolves around the Earth.
Venus goes through phases
Venus could not go through phases if Earth was at the center.

8. Heliocentric Model

9. Tycho Brahe A Danish Astronomer
Made very accurate observations of the positions of the planets for 20 years.
He died before he could analyze his data.

10. Keppler 1600’s A German mathematician Analyzed Brahe’s data.
Kepler discovered that the orbits of each planet was an ellipse.
An elongated circle or oval shape.
His calculations fit Brahe’s observations.

11. Issac Newton An English scientist.
Provided the answer to the question of what keeps the planets in orbit.
Concluded that gravity and inertia combine to keep the planets in orbit.

12. Inertia
An object’s tendency to move in a straight line or to remain stationary.
The more mass, the more inertia it has.

13. Gravity The force that pulls objects towards one another.
The strength depends on the masses and the distances between them.