1. The (brief) History of Astronomy

2. Ancient Ideas vs. Modern Science
There are many things that were believed in the past that we all think are quite silly today.
We take these things for granted as fact but in the past they were unknown and/or other explanations were accepted.
For example:

3. Lots of people have believed this.
Is the Earth flat?
Lots of people have believed this.
How do we now know the Earth isn’t flat?
Lots of ways.

4. Does the Sun move across the Sky? What do we believe today?
The ancient Greeks believed the god Helios drove a chariot that moved the Sun each day.
What do we believe today?

5. Does the Moon move across the Sky?
Yep. Just not once a day.

6. Is the Earth at the center of the Universe?
We're not at the center of anything

7. Geocentric Model “Earth Centered”
This model places the Earth at the center of the Universe.
Common in ancient Greece, it was believed by both Aristotle ( BC) and Ptolemy (90-168).
They believed the stars, planets, the moon, and the Sun went around the Earth in perfect circles

8. Why did they hold this belief?
Celestial objects (seem to) move across the sky
We do not feel ourselves moving
Believed God created the world with Earth at the center

9. Heliocentric Model “Sun Centered”
Proposed by Nicholas Copernicus ( )
Improved upon by Galileo Galilei ( ) and Johannes Kepler ( ).
Places Sun at the center of the solar system
Planets go around in circles

10. Why the new model? Geocentric model was very complicated
Astronomical events could not be predicted accurately
Calendars which were based on celestial events were getting out of sync

11. Johannes Kepler
Developed three mathematical laws to describe planetary motion during the 1600’s.
Planets travel in ellipses
Each planet moves faster when it is closer to the Sun
Closer planets have shorter years than further planets

12. Planets follow elliptical, not circular, orbits
Planets follow elliptical, not circular, orbits. They do appear circular. Comets follow much flatter paths.

13. Law of universal gravitation
Isaac Newton’s (1642 – 1727) law states that every object in the universe attracts every other object with a force that is proportional to mass and the distance between them.
The closer and/or larger they are, the stronger the gravitational attraction.
Helps explain Kepler’s laws of motion.

14. Hubble Law
Edwin Hubble ( ) discovered that the Universe is expanding
It expands at a rate described as the Hubble constant.
Doppler red shift is evidence for expansion
Used to infer the age of the Universe at 13.7 byo