1. Models of the Universe

2. Throughout history we have looked at the stars and wondered about the universe

3. Scientific theories change when scientists discover new information.

4. Observational evidence caused the model of the solar system to be changed from one in which the sun and planets orbit the Earth to one in which the Earth and planets orbit the sun.

5. Prior to the 1500's it was believed that the earth was the center of the planets. This was known as the geocentric model.

6. Aristotle’s and Ptolemy’s Universe  The Geocentric Theory is credited to the Greek astronomers Aristotle and Ptolemy.

7. Geocentric ideas  The Earth is still, motionless, and at the center of the Universe  Celestial bodies move in perfect circles at uniform speeds  Stars were set in a rotating sphere that turned E to W once a day  Planets, moon, sun also set in separate spheres that moved slower

8. The Copernican Model: A Sun-Centered Solar System

9. Sun Centered  The distinction between the Solar System and the Universe was not clear until modern times  Polish priest – astronomer Nicolaus Copernicus (1473 – 1543) eventually decided to reject the geocentric model

10. Heliocentric  Heliocentric – Sun centered universe Copernicus

11. Galileo Galilei Proves Copernicus’ Hypothesis  Galileo Galilei (1564-1642) was a pivotal figure in the development of modern astronomy. He proved the Copernican hypothesis.  He also invented the telescope.

12. Galileo  The sun is the center (heliocentric) of our solar system  Our solar system is a part of a larger universe  Using a new invention, the telescope, Galileo was able telescope, Galileo was able to view parts of our Solar to view parts of our Solar System in motion System in motion

13. What could Galileo see?  New stars (Milky Way made up of stars)  Mountains and valleys on the moon  Four moons orbiting Jupiter (now called Galilean moons)  Phases of Venus  Sunspots (rotating around the sun about once a month)  The rings of Saturn  Planets are disks, not pinpoints of light like the stars

14. Johannes Kepler (1571-1630)  Johannes Kepler, a German astronomer who lived at about the same time as Galileo.  He showed mathematically that Copernicus' idea of a sun-centered system worked well if uniform circular motion was replaced with uneven (but predictable) motion along off- center ellipses.

15. Modern View  Like any experiment, as we learn more, we change our hypothesis about the universe  Geocentric – Ptolemy's model Earth-Centered Universe  Heliocentric – Copernicus & Galileo Sun- Centered Solar System and larger Universe  Big Bang Theory – the Universe started from one big explosion

16. Modern thought begins to take hold  Other scientists began to look and explain how the universe worked  Isaac Newton (1642-1727) proposed that gravity is the main source that holds our solar system together.  The sun's gravitational pull holds the Earth and other planets in their orbits.  Things on or near the Earth are pulled toward it by the Earth's gravity.

17. So what have we learned? Before modern thought, the universe was seen in 2 ways 1. Geocentric: Ptolemy's Earth-centered Universe 2. Heliocentric: Copernicus’ and Galileo’s Sun Centered Universe 3. Kepler: explained the orbits of planets 4. Newton: explained gravity is the glue that hold the solar system together 5. 20 th century: newer ideas would change the way we thought about gravity, space, and time