History of Astronomy
How have ideas about the solar system
and our place in it changed?
Egyptians
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)
Greece

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.

Ptolemy
“epicycles”
Arabs
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
Poland

Copernicus – 1540
First to suggest “heliocentric model” – sun is
center of universe
 Objects orbit sun in perfect circles
 Still not a perfect model

Copernicus
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
Denmark

Tycho Brahe – 1576
Collected data of position of planets for 20
years
 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?
Kepler
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 …
Kepler
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…
Why?
Same time,
moves slower
Kepler’s Unanswered Question

What keeps planets in orbit?
Galileo Galilei
Born in 1564
 Supports Copericus’ theory of heliocentric
universe
 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

Galileo
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
motion
 1st law –
 2nd law –
 3rd law –

Newton answers
Kepler’s Question…
What keeps planets in orbit?
Tying it all up…
Myths
 See patterns
 Ptolemy
 Copernicus
 Kepler
 Galileo
 Newton

Homework

Timeline – list at least four models of
relationship between sun and planets.


Include: dates, astronomer, model
Read – Chapter 16.1: pp. 526 – 530.