Download Geocentric Model of the Solar System

Chapter 26.2: Observing the
Solar System
Early views of the organization of
Space were much different than ours.
Early astronomers believed that the Earth was the
center of our Universe, and that the Earth was
surrounded by a ball, called a Celestial Sphere, on
which were fixed most of the objects of the heavens.
This interpretation of our solar system is called a
Geocentric Model, meaning Earth-centered.
Early astronomers noticed groups of stars that were visible
at the same time every year. These were named
constellations, and were used as a basis for calendars.
The Orion
Constellation
Astronomers noticed that some celestial bodies
did change position relative to the constellations.
They called these “wandering stars” planets.
Notice the planet Mars moving across the constellations Gemini and
Leo over the course of 11 months.
Astronomers also noted that,
periodically, these planets which
normally moved eastward,
moved backward for a few
weeks, then resumed their path
eastward.
In 200 A.D., the Greek astronomer Ptolemy explained
this “retrograde’ motion by stating that the planets
orbited the Earth in a circle, but also orbited another
point in a circle, what he called an epicycle.
In the late 1400’s, the astronomer Copernicus
proposed what was to become known as the
Heliocentric Model.
Copernicus stated that the Sun was the center of
the Solar System, the Earth was a planet, and
that it orbited the Sun.
And, here is how he explained retrograde
motion.
Mars retrograde motion
Venus retrograde motion
In the 16th –century, the astronomer Johannes
Kepler developed three laws to describe the way in
which planets move through Space.
Kepler’s 1st Law: Planets move through Space in an
elliptical orbit, not a circular one. This causes the
distance from a planet to the Sun to vary.
Kepler’s 2nd Law: When a planet is closer
to the Sun, it moves faster. (centrifugal
force and gravity at work!)
Kepler’s 3rd Law states that the further a
planet is from the Sun the longer its period
of revolution (its year) will be.
In the 1600’s, Isaac Newton stated that a force
called gravity was causing the planets to behave
as Kepler had observed.
He stated that all objects with mass exerted a force of
attraction on other objects with mass, and that the
strength of that force is proportional to the mass of the
objects and their distances from one another.