1. History of Astronomy Renaissance Astronomy (1400 A.D. to 1640 A.D.)
Lesson 1-4
History of Astronomy Renaissance Astronomy (1400 A.D. to 1640 A.D.)

2. Lesson overview Copernicus (heliocentric theory)
Brahe (observations via instruments)
Kepler (3 laws of motion)
Galileo (telescopes to observe the solar system)

3. Introduction
Renaissance period—four people established the way we see our solar system:
Nicolaus Copernicus—challenged earlier theories
Tycho Brahe—made amazing observations
Johannes Kepler—built on Brahe’s observations; created three basic laws
Galileo Galilei—used a telescope to observe the Moon, Sun, and three planets
His observations and Keplar’s math destroyed the earlier geocentric theories

4. Nicolaus Copernicus (1473-1543)
Polish physician and lawyer, renaissance humanist and Catholic cleric
Had trouble with Ptolemy’s geocentric (earth-centered) model
Believed in Aristarchus’ heliocentric (sun-centered) model
Asserted Earth rotates on its axis once per day

6. Heliocentric and retrograde motion?
Ptolemy used epicycles to explain retrograde motion (but got very complicated)
Heliocentric models explain retrograde motion as a natural result of two planets passing each other as they revolve counterclockwise around the Sun
Animation: The retrograde motion of Mars according to the heliocentric model

7. More Copernicus Wrote “On Revolutions of Celestial Spheres”
Calculated with accuracy the relative distances of the planets to the sun using geometry
Could not explain lack of “stellar parallax” when viewing stars
Copernicus surmised the distances were too great to have any parallax effects
People at the time couldn’t imagine such vast distances
Theory rejected as a model of reality but embraced as useful for calculations
Animation: The retrograde motion of Mars according to the heliocentric model

8. Tycho Brahe (15461601)
Build island observatory -- “Uraniborg” Invented “modern” instruments to observe planets Accurately measured the locations of the planets Gathered other data – Mars in particular
Brahe’s Quadrant

9. For measuring altitude or angular distances
Azimuthal Semicircle

10. Armillary Sphere

11. Tycho Brahe (15461601)
Observed things that suggested heavens were changeable and complex
Observed comet traveling through the orbits of several planets
Shattered Aristotle’s crystalline-sphere theory
Proposed different, but still incorrect, geocentric model
Geo-heliocentric model
The Sun and the Moon revolved around the Earth
Everything else revolved around the Sun
Lacked mathematical know-how to explain the motion of the planets
Hired Johannes Kepler

13. Johannes Kepler ( )
Challenged by Brahe to explain mathematically Mars’ orbit around the sun
Used Brahe’s data on Mars to show the orbit was elliptical
Put the sun at one focus resulting in 3 laws of motion

14. Kepler’s First Law
The orbits of the planets are ellipses with the sun at one focus of the ellipse.

15. Kepler’s Second Law
The line joining the planet to the sun sweeps out equal area in equal times.
What does this mean? When a planet in its orbit is closer to the sun, it must speed up to sweep out an equal area…

16. Kepler’s Third Law
The square of the orbital period (time it takes to complete one orbit) is directly proportional to the cube of the mean (average) distance from the sun to the planet
What does this mean? A larger orbit requires more time to make one complete revolution.
Kepler predicted EXACTLY how much time each planet’s orbit would take.

17. Johannes Kepler ( )
Brought new emphasis on finding and quantifying causes of motion
United geometric or kinematic description of orbits with their physical, dynamic force that causes it
Authored fictional account of a space journey, the Somnium
Described trip to the Moon
Inspired other fiction writers, including Jules Verne and H. G. Wells

18. Galileo Galilei ( )
Refined telescope design to a state-of-the-art 20X
Observed several things about the physics of motion
Ushered in new era of space exploration

19. Galileo’s Discoveries
Saw imperfections in “perfect bodies” of Aristotle’s model of the universe
Mountains and valleys of the Moon
Sun spots (noticed the sun rotated like the Earth)
Discovered four moons orbiting Jupiter
Disproved Aristotle’s assessment that everything revolved about the Earth
Noticed the phases of Venus indicating it too revolved around the sun
Observed the Milky Way and noted its vast size
Telescope’s inability to magnify stars confirmed Copernicus’s guess about their vast distances

20. Galileo and Gravitation
Measured time for spheres to reach bottom of an incline
Dropped stones from the Leaning Tower of Pisa
Weight didn’t affect time to reach the bottom –both balls had the same acceleration
Previously, people thought heavier objects fall faster
Theory proven during Apollo 15 using a hammer and feather

21. Galileo vs the Pope His “theories” were at odds with Church “law”
In 1610, published “Starry Messenger”
Non-technical style—readable for common class
Presented as dialogue of the Copernican system between a wise teacher (him) and an unbeliever he named Simplicio (the Pope)
In 1632, published “Dialogue Concerning the Two Chief World Systems” with the approval of the Inquisition
Supposed to be neutral, but wasn’t
In 1633, placed under house arrest for the rest of his life (church admitted its mistake in 1992)

22. Lesson review What idea did Copernicus develop and promote?
Used planetary parallax to argue for heliocentric (Sun-centered) solar system
Measured relative distances of planets from the Sun
Surmised the vast distances to stars

23. Lesson review (cont’d)
How did Tycho Brahe contribute to our understanding of space?
Invented new instruments
Observed and accurately recorded the locations of the planets over time
Had a wacky theory of a geo-heliocentric solar system

24. Lesson review (cont’d)
What are Kepler’s three laws?
Planets move in elliptical orbits around the sun
The closer to the Sun, the faster they move
The larger orbit requires longer time to complete
What did Galileo do?
Telescope observations led to the downfall of Aristotle’s geocentric model
Confirmed vast distances to stars
One of the first to study gravity