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)