How to we Observe?
Getting our Bearings
The Moon
The moon is about 5° out of the plane of Earth’s orbit
Brief History of Astronomy
And why you should care
What was Astronomy used for?
•To tell time
•To determine the
seasons
•Calendars
•Navigation
•Predict the future
(eclipses & rainfall)
Greek Science
•Did not rely on
supernatural
explanations
•Used mathematics
•Used logic and
reasoning
•Developed the idea of
scientific models
Arabic Science
•Much of the Greek
knowledge was lost with
the burning of the
Library at Alexandria
•Europe feel into Dark
Ages, Baghdad became
center
•Influence from China,
India, and Greece
•Continued to develop
math and astronomy
•When Constantinople
fell, scholars headed
west, leading to
European Renaissance.
Greeks and Planets
Aristotelian Model
•Geocentric
•Concentric crystalline
spheres
Retrograde Motion
Greeks and Planets
Ptolemaic Model
•Still geocentric
•Explained Retrograde
motion of planets
•Used smaller circles called
epicycles moving around
bigger circles called the
deferent
•Very successful, used for
1500 years
Epicycle
Equant
Deferent
Copernican Revolution
•Nicholas Copernicus
born in Poland, 1473
•Adopted idea from
Aristarchus
•Heliocentrism –
planets orbit the sun
•Not really accepted
quickly because
It didn’t work
Tycho Brahe
•Lived 1546-1601
Tycho Brahe
•Noticed Copernicus’s
system didn’t work
•Set out to collect better
data
•Once observed a
supernova in 1572
•Lost his nose in a sword
fight over who was the
better mathematician
•Had a tame pet elk
Tycho Brahe
Johannes Kepler
•Lived 1571-1630
•Used Tycho’s data to find a
model of planetary motion
Three Laws:
•Orbits are ellipses
•Equal area in equal time
•p2=k a3
Johannes Kepler
VS
Tycho Brahe
Johannes Kepler
Galileo
Objections to Copernicus
Galileo’s Responses
•If Earth were moving, birds,
clouds, etc. would fall off as
Earth moved
•Physics experiments
leading to Newton’s first law
•Heavens are perfect
•Sunspots  Imperfection
 circular orbits
•If Earth orbits, should
observe parallax
•Observed parallax
•Moons around Jupiter
Not everything orbits
Earth
Galileo
•Lived 1564-1642
•Did not invent the
telescope, improved it
•His championing of the
Copernican model and
his evidence to support
it is why it became
accepted