Models of the Solar System

Some early mythology described the Earth as flat.

If you sailed to the end you would fall off!

Ptolomy

*Ptolomy (90–168) lived in the time of the ancient greeks.

*Described a geocentric solar system

*Supported by Aristotle

*Two common observations supported the idea that the Earth is in the center of the Universe

*the stars, sun, and planets appear to revolve around the Earth each day, with the stars circling around the pole and those stars nearer the equator rising and setting each day and circling back to their rising point

*Earth is solid and stable it is not moving—but is at rest

Ptolomy

Astronomers observed that the planets moved a little differently than stars

Occasionally a planet, particularly Mars, would reverse direction for a while and then resume motion in it’s original direction. We call this back and forth motion “retrograde” motion.

Mars in retrograde

Ptolomy

How can this be explained?!? Epicycles

Ptolomy

The Ptolemaic order of spheres from Earth outward is:Moon Mercury Venus Sun Mars Jupiter Saturn

Copernicus

• Nicolaus Copernicus (1473 –1543) was the first astronomer to formulate a comprehensive heliocentric cosmology

• Heliocentric means sun-centered

Carl Sagan on Ptolomy and Copernicus

’S LAWS

Tycho Brahe

His observations of planetary motion, particularly that of Mars, provided the crucial data for astronomers like Kepler (1571-1630) to construct our present model of the solar system.

KEPLER’S 1st LAW

All planets orbit the sun in elliptical paths with the sun at one focus.

Ellipses can have different shapes

Eccentricity

• The more “flattened” an ellipse is, the larger its eccentricity.

KEPLER’S 2nd LAW

A planet sweeps out an equal area of

space in an equal amount of time.

Faster at perihelion or aphelion?

Perihelion: Faster

Aphelion: Slower

KEPLER’S 3rd LAW

P2 = # of solar massesa3

For our sun p2/a3 = 1 Where P stands for the period, or time to go around once,

and a stands for the semi-major axis

P must be in years, while a must be in AUs

Carl Sagan on Ptolomy and KeplerTycho Brahe, Johannes Kepler and Planetary Motion (1 of 2)