1. Developing the Science of Astronomy
(Power Point 03)

2. Student Learning Objectives
Examine the contributions of ancient and modern astronomy to the understanding of gravity and planetary motion.
Identify contributions from the first Astronomers and compare the models
Recall the contributions of Galileo and analyze the significance of these contributions

3. How did early Greek philosophers describe motions in the sky?
The Greeks had 3 basic theories for the sky
All heavenly bodies are spheres that move in circles
Heavens are unchangeable
Earth is stationary at center of universe
Most Greeks in B.C. times believed in the geocentric model.

4. Aristotle (300 BC) Eratosthenes (200BC)
Placed Earth at the center of the universe by logic
(persisted for 2000 years)
Eratosthenes (200BC)
Measured circumference of Earth using observation and simple geometry
Image Credit: sciencebuddies.org

5. Practice
Explain why Aristotle would have assumed Earth is at the center of the system. How does this support the Celestial Sphere Model? Why is this model still useful to us today?

6. Hipparchus & Magnitudes
Hipparchus (150 BC)
Divided the stars into apparent magnitudes by comparing them according to their brightness
Magnitudes are a measure of brightness.
Sun
−26.7
Full Moon
−12.6
Venus
−4.4
Vega
Naked Eye Limit +6
Binoculars Limit
+10

7. Practice
1) Which would appear fainter to us when viewed from Earth with our eyes?
−4 magnitude star
+4 magnitude star
0 magnitude star
2) If two stars have the same energy output, what would make one star appear fainter than the other, when viewed from Earth?

8. The Epicylce Model Ptolemy (150 AD)
Developed the epicycle model to explain Mars “backwards”, or retrograde motion
Today’s Astrology is based on principles developed by Ptolemy (fun fact)

9. Ptolemy’s Epicycle Model

10. Copernicus (mid-1500’s) Heliocentric Model
Used mathematics to determine the Sun was at the center of the solar system
Heliocentric Model
Image Credit: slideshare.net

11. The Early Models
Aristotle
Ptolemy
Copernicus
Stationary Earth
Rotating Earth
Earth at Center
Sun at Center
Circular Motion
Circular motion
Orbital speeds
same for all
depend on distance
Based on
Observation of
Apparent Motion
Mathematics
Practice Write a compare and contrast sentence which relates these models.

12. The Transition to Modern Science
Galileo (1600’s)
Used observations - Venus goes through phases like the Moon - to provide visual proof of Copernicus’s Sun centered system
First to use telescope to view sky

13. Galileo’s Telescope Observations
Venus goes through phases as it orbits the Sun
Planet positions confirmed
Moon & Sun not perfect spheres
Objects orbiting Jupiter (not Sun or Earth)
Image Credit: commons.wikimedia.org

14. Image Credit: wordpress.com

15. Practice
Analyze and describe the significance of the following discoveries made by Galileo in terms of confirming or refuting earlier models.
Venus goes through phases like the Moon
Visually confirmed relative locations of planets
Observed objects orbiting Jupiter