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Early Views of the Cosmos
Models of Planetary Motion
2000 years ago the Greek philosopher Aristotle proposed the
Geocentric model of the universe to explain the movement of stars and
planets. In this model the earth is the center of the universe and
everything else moves around the Earth
In 1530 Nicholas Copernicus proposed the Heliocentric model of the
universe. He suggested that the sun was at the center and that the nine
planets revolve around it.
This is the model we use today
Elliptical orbits
Tycho Brahe - Made a ton of observation and had Kepler as an
assistant. (He had a silver nose)
Johannes Kepler - Discovered that the planets have an
elliptical orbit. (He was nearly blind)
See activity:
1. grab a piece of cardboard, a piece of string and 2 thumbtacks.
Make sure your string is not longer than your hand.
2. Tie your string together to make a circle.
3. Your thumb tacks represent a planet and the sun.
Place your thumb tacks in your cardboard about
1 to 2cm apart. And place your string around the top of the
tacks.
4. Put your pencil in the string and trace
around.
5. Make a note of the shape.
6. Move the tacks further and further apart
and keep making note of the shape.
As the tacks moved further and further apart what happened to
the shape of the planets orbits?
This is what Kepler discovered. This was a huge break through.
Solstice
These are 2 very important events that occur in the year.
The summer solstice occurs near June 21. It is the longest day of the
year and marks the start of summer.
The winter solstice occurs around December 21. It is the shortest day of
the year and marks the start of winter.
Equinox
Two other very important events that happen every year are the
EQUINOX.
The spring equinox occurs usually around March 21 and the fall
equinox occurs around September 21. These days have equal amounts
of daylight and night.
Questions:
1. What model has the earth at the center of the solar system?
2. What model has the sun at the center of the solar system?
3. What shape do planets travel in? As you get closer to the sun
what shape do they start becoming more and more like?
4. What did Copernicus discover?
5. What are the solstice events?
6. What are the equinox events?
7. Describe the solstice and equinox in terms of the Earth's
Orbit.