1. Intro to Astronomy Through the early history of civilization, people made observations about the night sky, sun, and moon. They tried to explain what they were observing with stories and folklore. They believed that the sun and stars revolved around the Earth.

2. The Greeks came up with the idea of a geocentric universe. Meaning they thought everything revolved around the Earth. A Greek astronomer named Ptolemy (1400 CE) designed a model of the universe which put the Earth at the center and had the planets moving on small circles which moved on larger circles.

3. Copernicus Copernicus (1543)- Made observations which led him to believe that the geocentric model was not accurate. His observations put the sun at the center and the planets revolved around it in perfect circles. In a heliocentric model the sun is placed in the center and the planets revolve around it.

4. Planetary Motion Galileo (1610) Galileo used the newly invented “spyglass” to make observations of the night sky. He discovered that Jupiter had moons that orbited it. This led him to the conclusion that not all objects revolved around earth. While looking at Venus he noted that it always stayed near the sun. He discovered that Venus goes through a series of phases similar to the Earth’s moon. Venus would not have a full set of phases if it circled the Earth. Therefore the geocentric model is incorrect.

5. Tycho Brahe Copernicus and Galileo correctly placed the sun at the center of the planets but incorrectly stated that the planets moved in perfect circles. Brahe continued his observations for another 20 years accurately recording the positions of the planets with out a telescope! He died in 1601 His assistant named Johannes Kepler continued TB’s work by analyzing the planets’ orbits and mapped Mar’s orbit. He thought the orbit was round but his calculations did not match up. He discovered that the orbit must not be circular but an ellipse.

6. The Birth of Modern Cosmology Astronomers had been debating for over a century about the nature of the universe that we live in. They new that the Sun, is just one star in a larger collection of stars in the Milky Way. They saw faint objects outside our galaxy, but could not determine the distance of objects beyond 100,000 light years away. A light year is the distance it would take a beam of light to travel in one year. The Milky Way galaxy is 100,000 light years across.

7. In 1919 Edwin Hubble was working as an astronomer in California. He was using a telescope that was nearly 3 meters across and discovered that there were objects millions of light years away- and that these objects are other galaxies!!

8. Hubble’s Law Perhaps the greatest contribution that Edwin Hubble made was that all the distant objects he was observing were moving! Not only were they moving, but they were all moving away from one another as though they were all on the surface of a balloon that was inflating. He made this discovery using something called the Doppler effect also known as “red shift.”

9. The Big Bang Theory Hubble’s discovery led to our understanding that the universe is expanding. Like the leading edge of an explosion going farther and farther out. If you think about what would happen if you let the air out of the balloon- or ran the “explosion” in reverse, it all would have started at a single point. This concept led to the theory of the “Big Bang.” The idea that the universe began at a specific point in the past, and it has been expanding ever since!

10. This explosion formed sub atomic particles which led to the formation of simple atoms like Hydrogen. Large clouds of hydrogen formed and within them, stars began to form. Stars began to form star systems and galaxies. The stars within the galaxies formed planetary systems

11. Since Galileo's time our knowledge of the solar system has increased dramatically. In Galileo’s time only the planets Mercury, Venus, Earth, Mars, Jupiter and Saturn were known. Since then we have discovered three more planets as well as asteroids and comets. We are now discovering planets orbiting distant stars and discovered black holes.