Topic: Models of the Universe

Key Terms:Geocentric Theory Heliocentric Theory

Geocentric Theory

Ancient Greece

Geocentric Model

• ___• ___

Geocentric Model: Ptolemy (87-150 AD) Used epicycles to explain planetary

movements

Retrograde Motion

Motion of the other planets

Mars Retrograde Motion

Ptolemy’s system provided the first framework for all discussion of the universe for nearly 1500 years!!

But… there was a problem…

Problems with Geocentric Theory

1. This model does explain apparent motions of celestial (space) objects

2. Cannot explain Foucault’s Pendulum or the Coriolis Effect

Heliocentric Theory

Heliocentric Model

• ___• ___

Copernicus (1473-1543) • Suggests a model that the

Sun was the center of the universe.

• Argued that the motion of the Sun & planets could be equally described by a Sun centered (heliocentric) system.

Earth1

Sun

1

2

3

4

5

6

6

Outer Planet

5

4

3

2

Fixed Stars

Planet's apparent path around sky.

Earth overtakes slow outer planet so the outer planet appears to slow down, move in reverse, and then move forward again with respect to the fixed stars.

Retrograde Motion

http://faculty.salisbury.edu/~jwhoward/astro108/html/GeoHelio.ppt#274,10,Slide 10

Tycho Brahe (1546-1601): the greatest of the pre-telescope observers in Europe.20+ years measuring the positions of the Sun, Moon and planets with great accuracy.

Johannes Kepler (1609)• Johannes Kepler was an assistant to

Tycho Brahe. He used Brahe’s observations to study the orbit of Mars

• Discovered three laws of planetary motion: predict with accuracy the motions of the planets

• Galileo (1564-1642) first scientist to use a telescope to observe the sky. He observed the phases of Venus and the moons of Jupiter. Both observations supported the heliocentric model.

Modern Universe

Is the Sun the center of the Universe?

Final Question

• Final Question