Solar System Models

Geocentric Model

Earth Centered• Moon, Sun, Planets, and Stars revolve around the

Earth• Feels right• No observed parallax of stars• Later adopted as we are the center of the universe

and everything is perfectly made for us.

Aristotle Model

Greeks knew that the Earth was round

Ships sink as they sail awayShape of Earth’s shadow on the Moon during a lunar eclipse

Eratosthenes (200 BC)

Used position of the sun from two locations to determine the circumference of the Earth

α = 1/50 of a circleFigure it out

Problems with Geocentric Model

Wanders: – Planets vary brightness and speed as they move along the ecliptic

Apparent Retrograde Motion – backward motion of planets at opposition– Video

Ptolemy Proposal

Epicycles

– small cycle on orbit– allow more accurate predictions

demo

Ptolemy Model

Earth in the center– Moon, sun, planets and stars

revolve around us

– The stars are far away, therefore they must be going real fast

Heliocentric Model

Sun-centered System

Copernicus Model

Was able to use simple math to determine relative distances to planets.

Not perfect but simpler; he added some small epicycles.

Motion of sun and planets are caused by movement of the Earth

Space is really big, stars are very far away

Tycho

Very careful observations with calibrated sighting tube, no telescope

Proposed variation – Earth-centered/Sun-centered system

Kepler

The orbits of planets are ellipses with the sun at one focus

Equal area in equal time– They go faster when they are closer to the sun

• The period 2 is proportional to the distance 3

Galileo

Used small telescope to look at planetsEvidence that things were not as they thought

– Moon has features– Jupiter has moons– Saturn has appendages– Sun has spots– Milky Way is lots of stars– Venus has phases and

changes size

Evidence for the Geocentric Model

Feels right– Do you feel like

the Earth is spinning?

– Do you feel like we are traveling through space as we revolve around the sun?

What about Apparent Retrograde Motion?

Do the planets back up?

Video

Evidence for the Geocentric

Model

No parallax was observed– If we are traveling around the

sun then we should be able to see it.

Evidence against the Perfect Universe model

The moon was not a perfect object, it had craters, highlands and seas.

Evidence against the Heliocentric model

Jupiter has moons, therefore not everything revolves around the earth.

Evidence against the Perfect Universe model

Saturn had moons or arms on the side that sometimes disappeared.

Evidence against the Perfect Universe model

Sun has spots– It was not perfect– It rotated

Evidence against the Perfect Universe model

The Milky Way is really a lot of stars

Evidence for the Heliocentric model

Venus has phases and it changes size.

Which is correct?

The End