2. The First Astronomers It is likely that the first astronomers were simply curious people who: –imagined how big this dirt place we stand on is and what shape it would be. –looked up to noticed patterns in the night sky. –even noticed seasonal trends to where and when these patterns appeared.

3. Modeling the Universe Early Greeks made more advanced observations, noting that some of the objects in the sky moved in relation to the stars. These were called “planets” from the root Greek word for wanderer. Mars motion plotted against the stars…

4. Modeling the Universe The Greeks made the first recorded attempt at a “model” of what they saw. All our senses tell us that the Earth is not moving… so the first model placed Earth at the center with planets and sun rotating around it… the “geocentric” model (earth – centered).

5. The Earth is a Sphere A man named Eratosthenes (~ 200B.C.) notices a strange phenomena with shadows in different cities and uses it to calculate the radius of the Earth. No Shadow in well Shadow behind post

6. Geocentric Model Challenged Observations of the planets (mostly Mars) reveal irregular movement of some planets pointing to a problem in the earth-centered (geocentric) model of the solar system. As long ago as ~ 250 B.C. some began to think that the Sun may be at the center of the solar system.

7. Geocentric Model Fights Mainly based on a lack of evidence for Earth’s movement, the idea that the Earth is the center of the universe remains accepted for about 1500 years… until the 1500’s.

8. Evidence Wins the Fight More and more problems develop when the earth-centered (geocentric) model is studied. In the late 1400’s, Nicholas Copernicus proposes the Sun-centered (heliocentric) model of the universe… essentially the current model. This model, though not quickly accepted by people, explained the erratic motions of the planets in a simple easy way. If a new model is better supported by the evidence, the old must be thrown out.

9. Supporting the Model In addition to his explanations for moving objects; In the late 1500’s Galileo builds (not invents) a telescope to look at the planets and stars. His observations support the heliocentric model.

10. Heliocentric Model is Refined In the early 1600’s, A man named Johannes Kepler, studies the observations of another man (Tycho Brahe) and comes up with 3 laws of planetary motion. The easiest to understand being that the planets orbit the sun (heliocentric)… but they orbit in ellipses… not perfect circles. An ellipse can be drawn with this method

11. Explaining the Forces in the Heavens In the late 1600’s, Isaac Newton describes why planets move in elliptical paths around the Sun. GRAVITY… a force that causes all masses to attract other masses pulls the moving bodies inward. Opposing gravity is the moving object’s tendency to move in a straight line (inertia). The two forces balance and the planets move in near circular (elliptical) paths around the most massive body… the Sun.

12. Beyond that… Modern technology such as the Hubble space telescope, radio telescopes, and the work of scientists like Einstein, Hubble, etc… have lead to a wealth of understanding. Currently, the model of the solar system is essentially the heliocentric model already explained. However, the universe consists of countless “suns” (stars) organized in a countless number of galaxies… Makes your head hurt to think about infinity or forever… ouch.