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History of Astronomy
How have ideas about the solar system
and our place in it changed?
Saw patterns in the Sun, moon, and Venus
 Called the sun “Ra” (Sun god) – rode in
his boat across the sky daily
 Developed 365 day year solar calendar
Ra (on right)
Eudoxus, born 400 B.C.
saw objects that moved in sky
 called planets, sun, and moon “wanderers”
 Earth motionless and at the center of universe
Ptolemy, AD 140
Leading astronomer
 “geocentric” theory – earth is center of
universe. Lasted for 1400 years.
Caliph Harun-al-Rashid, ruler
 Scholars translated Greek texts into Arabic
and preserved in “House of Wisdom”
library – in Baghdad
 Measured positions of stars and planets
with fine instruments, astrolabe, can
perform calculations
 Named the red giant Betelgeuse, in Orion
325 light years away
Betelgeuse – “shoulder”
Orion constellation
Copernicus – 1540
First to suggest “heliocentric model” – sun is
center of universe
 Objects orbit sun in perfect circles
 Still not a perfect model
from book De Revolutionibus
The English version…
What do you think?
Why do you think it was difficult for people to
accept a heliocentric model (sun centered) over
the geocentric model (Earth centered)?
Geocentric model
Heliocentric model
Tycho Brahe – 1576
Collected data of position of planets for 20
 Able to make accurate predictions of positions
without telescopes
 Had own “Tychonic Universe” – combination
of Ptolemy and Copernicus
 Earth is stationary
 Believed in circular orbits
Tychonic Universe
Why the Circles?
Why do you think these astronomers
believed in circular orbits?
Works with Tycho – after Tycho’s death,
Tycho’s family sued to recover instruments
and books of observations
 Why?
Kepler was a Copernican, and they knew he
wouldn’t follow the Tychonic system
Kepler kept the books! Began to study
motion of Mars …
Elliptical orbits
Kepler’s First Law
of Planetary Motion
Orbits are elliptical (oval shaped),
sun is at the focus
Kepler’s Second Law
of Planetary Motion
A line from a planet to the sun sweeps
over equal areas in equal times
 Planets travel at different speeds – due to
pull of gravity
Closer to Sun,
moves faster…
Same time,
moves slower
Kepler’s Unanswered Question
What keeps planets in orbit?
Galileo Galilei
Born in 1564
 Supports Copericus’ theory of heliocentric
 Used telescope to observe:
surface of Moon – was not perfect (like
Ptolemaic model)
 Phases of Venus
 Milky Way was made up of many stars
 Moons of Jupiter
Moon surface
Phases of Venus
Isaac Newton
Born in 1654 (year Galileo dies)
 Three Laws of Motion
Review from exam! 
These laws help us understand orbital
 1st law –
 2nd law –
 3rd law –
Newton answers
Kepler’s Question…
What keeps planets in orbit?
Tying it all up…
 See patterns
 Ptolemy
 Copernicus
 Kepler
 Galileo
 Newton
Timeline – list at least four models of
relationship between sun and planets.
Include: dates, astronomer, model
Read – Chapter 16.1: pp. 526 – 530.