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)