The document discusses the history of astronomy from ancient times through the 16th century. It describes how early civilizations like the Egyptians studied the movements of celestial objects and how the Greeks developed the first models of the universe, with the geocentric model of Aristotle and Ptolemy placing Earth at the center. The document then outlines the shift to the heliocentric model of Copernicus and the laws of planetary motion discovered by Kepler, based on Tycho Brahe's accurate observations. It concludes by discussing Galileo's astronomical discoveries using the telescope, which provided support for the heliocentric model.

1. Ancient Astronomy
• Objects in the Sky move in cycles
– Stars, Sun, Moon, eclipses, etc.
• Why did most ancient people care?
– Agriculture
– Religion
• Egyptians aligned pyramids with cardinal
directions (2700 - 2100 B.C.)
• Natural philosophy of Greece strongly
influenced astronomy (500 B.C - 150 A.D)

3. Music of the Spheres
• Plato (428-347 B.C) argued that the sphere
and the circle were perfect shapes because
of their symmetry
• The heavens being the creation of the god
would have to be “perfect”
• The heavens were assumed to contain
spheres in uniform (constant speed)
circular motions
• Earth was the center of motion

4. Geocentric Universe
• Aristotle (384-322 B.C.) expanded on the
ideas of Plato and Eudoxus of Cnidus
• Aristotle placed the Earth at the center of 56
concentric spheres
• Spheres rotated as to explain the observed
motions of the Moon, Sun, planets, and
stars in the sky
• Stars were on the celestial sphere

5. Aristotle’s Model

6. Earth the Central Sphere
• The Earth always cast a round shadow on
the Moon when there was an eclipse
• Eratosthenes measured circumference of the
Earth (c. 240 B.C)
• 250,000 stadia is
about 41,700 km
• Modern value is
40,000 km

7. Aristarchus (310-230 B.C)
• Hypothesized the Sun was the center of
motion for the planets and Earth
• Retrograde motion of Mars, Jupiter, Saturn
• Believed Sun was bigger than Earth
• Heracleides (388-315 B.C) suggested that
Mercury and Venus orbited the sun
• Heliocentrism was dismissed for 1800 years

8. Ptolemaic Model
• Ptolemy (100-170 AD.) explained
retrograde motion with epicycles

10. Heliocentrism
• Copernicus (1473-1543) developed a theory that
explained retrograde motion without epicycles
• De Revolutionibus Orbium Caelestium was
published near his death
• Completed in 1530 not published until his death
– fear of Church
– inaccurate predictions for planets required epicycles
• Copernicus put the Sun at the center of motion
• The Copernican Model was inaccurate for planet
positions, but heliocentrism is correct

11. Tycho’s Data
• Tycho Brahe (1546-1601) was a Danish nobleman
who studied Astronomy
• As student, noticed inaccurate planet positions of
earlier models
• In 1572, observed a Supernovae and deduced that
celestial sphere was not constant
• Tycho performed accurate measurements of
positions for 777 stars and all the planets, the
Moon, and the Sun (better than arcminutes)
• Before his death, named Johannes Kepler to be his
successor

12. Kelper’s Laws
• Kepler was brilliant at mathematics
• Kepler supported heliocentrism
• Noticed that predictions for Mars were off
by up to 8 arcminutes using circular orbits
• Believed Kepler’s data was accurate
• Only other explanation was elliptical orbits
• Discovered 3 laws of planetary Motion

13. Circles vs. Ellipses

14. Kepler’s First Law of Planetary Motion

15. Kepler’s Second Law of Planetary Motion

16. Kepler’s Third Law of Planetary Motion

17. Galileo and the Telescope
• The telescope was invented by Hans
Lippershy in 1608 (Holland)
• Galileo (1564-1642) was the first to use the
telescope for astronomical observations
• Galileo supported heliocentrism but not
publicly in his early years
• The first 3 of his 5 biggest discoveries were
published in the Sidereal Messenger (1610)

18. Galileo's Telescopic Discoveries
• The Moon was not a perfect sphere because of
shadows from mountains and valleys
• Milky Way was composed of many stars that are
too faint to see individually without a telescope
• Observed 4 "planets" orbiting Jupiter
– These are now known as the 4 Galilean moons (Io,
Europa, Ganymede, Callisto)
– Proved there could be other centers of motion
• Observed sunspots on Sun
• Venus had phases much like the Moon

19. Phases of Venus
Supports heliocentrism
(Copernicus & Kelper)