Lesson 2 Models of the Universe

Geocentric Theory Ancient Greece

Geocentric Model Earth Centered Universe All objects move around Earth, a fixed point.

Problems with Geocentric Theory This model does explain apparent motions of celestial (space) objects

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

But… there was a problem… Ptolemy’s system provided the first framework for all discussion of the universe for nearly 1500 years!! But… there was a problem…

Heliocentric Theory

Heliocentric Model Sun Centered Universe All celestial objects in the Solar system move around the Sun

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.

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 Copernicus (1473- 1543)

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.

Kepler’s Laws

Each planet’s orbit is an ellipse with the Sun at one focus. 1st Law Each planet’s orbit is an ellipse with the Sun at one focus.

Eccentricity A number indicating the roundness of an ellipse. Eccentricity = Distance Between Foci Length of Major Axis e=d/L

Perfect circle Straight line e = 1 e = 0

ESRT pg 15

Eccentricity Practice

2nd Law Planets change orbital speed as they revolve around the Sun.

Caused by the Sun’s gravitational pull

3rd Law As distance from the Sun increases, a planet’s period of revolution increases.

ESRT pg 15