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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)
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
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
Aristotle’s Model
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 isabout 41,700 km
• Modern value is 40,000 km
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
Ptolemaic Model
• Ptolemy (100-170 AD.) explained retrograde motion with epicycles
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
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
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
Circles vs. Ellipses
Kepler’s First Law of Planetary Motion
Kepler’s Second Law of Planetary Motion
Kepler’s Third Law of Planetary Motion
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)
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
Phases of Venus
Supports heliocentrism(Copernicus & Kelper)