The History of Astronomy

Theories, People, and Discoveries of

the Past
 Prehistory
• Early man recorded very
little history.
• Left some clues in the
form of petrographs.
• Stone drawings that show
eclipses, comets,
supernovae.
• The petrograph to the
right represents a
supernova.
– Found in a pueblo in
southwest America.
 Prehistory
• Early man was frightened
by the sky.
• Believed that the heavens
held power over earthly
existence.
– Psychology of the
Unknown
• Astrology born as an
attempt to understand,
predict, and influence
events.
History
• First written records
were astronomical
observations.
• Early Chinese, Central
American (Mayans),
and Northern Europe.
– Stonehenge in England
marks solstices.
1600 B.C.
• Babylonians
• First to write stuff
down.
• This early culture
recorded the positions
of the planets, the
times of eclipses, and
other celestial
phenomena.
~500 B.C.
• Hellenistic Culture
• Greeks acquired the
records of the
Babylonians.
• Conquest of Alexander
the Great.
• Greeks applied data to
construct a cosmological
framework.
– Basically means – this is
the way heaven is
arranged.
~500 B.C.

• The Greeks used the star

charts for more than just
practical uses.
– Navigation on the seas.
• Began to develop new
ideas about the universe.
• Devised new experiments
to try to prove their
theories.
 ~ 480 B.C.
• Thales of Miletus
• Proposed that the
universe was rational and
could be understood by
humans.
• Used the data of the
Babylonians to predict
the occurrence of
eclipses.
 400 B.C.
• Plato
• Made what seems like a
minor contribution.
• But his statement
dominated astronomical
thinking for centuries.
• Heaven is perfect and a
circle is the perfect form.
 330 B.C.
• Heraclides
• Proposed the first model of the solar system.
• Geocentric – Earth at the center of the universe.
 330 B.C.
• Orbits were perfect circles (Plato).
• Was the first round in the debate over geocentric
vs. heliocentric solar system.
 270 B.C.
• Round 2
• Aristarchus of Samos
• Made several important
contributions.
• He was the first person to
propose a heliocentric
model of the solar system.
• Mathematically proved
the Sun is farther away
from the Earth than the
Moon.
 270 B.C.
• This model put the Sun at the center (helios is
Greek for Sun, thus heliocentric model).
• Still used circles for the orbits (Plato).
 270 B.C.
• Used trigonometry to prove that the Sun is farther
away from the Earth than the Moon.
• Idea was correct, just made mathematical errors.
 270 B.C.
• Problems with Aristarchus’
model:
1. If the Earth is moving, why
couldn’t people feel it?
2. No parallax seen in the stars.
a. As Earth moves in orbit, the
apparent position of nearby stars
move in relation to stars farther
away.
3. Geocentric = Egocentric
Parallax
a. More natural.
b. Highlights importance of man!
 100 B.C.

• Hipparchus
• Produced the first star
catalog showing
positions and
magnitude
(brightness).
• Recorded names of
constellations.
200 A.D.
• Ptolemy
• Librarian in the ancient
city of Alexandria.
• Resurrected Heraclides
geocentric model.
• Used the library
resources containing
centuries of data to
formulate a complete
description of the solar
system.
200 A.D.
• Ptolemy explained and
predicted the apparent
motions of celestial
bodies (planets).
• He used perfect circular
orbits for the planets,
drawing on Plato’s idea
that all things in heaven
were perfect.
• Major problem was
retrograde motion.
 Retrograde Motion
• Retrograde motion is movement
in the opposite direction from the
expected motion.
• Some planets would move
“backwards” during their motion
across the nighttime sky over the
year.
• This became very difficult to
explain in a geocentric model
because the orbits were supposed Epicycle Motion
to be perfect circles around the
Earth.
 Ptolemaic Model
• Ptolemy developed a very complex system using circles
to explain the retrograde motion of the planets.
• He used circles on circles as seen below.
• The Sun and Moon did not need them because they did
not exhibit retrograde motion.
 Ptolemaic Model
• The main circle around the Earth was called a deferent.
• The smaller circle on which the planet traveled around
the deferent was called an epicycle.
• To explain all the known motions, Ptolemy was forced
to use 28 circles.

Epicycle Motion
 Ptolemaic Model
• As the planet moves
around the epicycle, it is
also traveling along the
deferent.
• This causes the retrograde
motion of the planets
(points 3 to 5 in the
diagram to the right).
• This is a way to diagram
how Ptolemy ‘s theory
would work.
200 A.D.
• Ptolemy realized that the
planets were a lot closer to
Earth than the stars, but he
still believed in the
existence of celestial
spheres.
• Stars and planets were
attached to these spheres.
• He also realized there might
be other objects outside the
sphere of fixed stars.
 200 A.D.
• So, Ptolemy proposed there were other spheres outside
those visible.
• Ended with the “Primum Mobile” (God), which provided
the motions of the other spheres.
• This was the first mathematical attempt to explain nature.
 ~ 410 A.D.
• Alexandria burns and is
destroyed along with most
of the records in the library.
• Loss of most knowledge
acquired by the Greeks and
Romans.
• Roman culture collapses
leading to the Dark Ages.
• Roman Catholic Church
absorbs Aristotle’s scientific
method and Ptolemy’s
model of the universe. Ancient Lighthouse at
Alexandria
~ 1500’s
• The Renaissance
• People returned to scientific
values where new ideas
were more important than
religious beliefs.
• Art and science began to
flourish.
• Da Vinci, Copernicus,
Raphael, and others served
the church, but they began
to rebel (secretly).
~ 1500’s
• Nicholas Copernicus
• Reinvented the heliocentric
model.
• More than just an attempt to
solve retrograde motion.
• It had both social and
political consequences by
challenging the power of
the Catholic Church.
• Psalm 93 – “Thou hast
fixed the Earth immovable
and firm.”
~ 1500’s
• Copernicus’ model also
questioned the authority of
the most revered wise men
of the ancient world
(Ptolemy, Plato,
Heraclides).
• He forced a change in
humanity’s view of the
world and our importance
in it.
• Yet, even his model had
problems.
~ 1500’s
• Copernicus still used
circular orbits.
• As a result, he still was
forced to use epicycles and
deferents.
• His model was even more
complicated than
Ptolemy’s.
• Would have failed our
modern criteria that models
be as simple as possible
(Occam’s Razor).
 1580
• Tycho Brahe
• 1st true observer.
• Built the Danish
Observatory.
• Measured the positions of
planets and the stars to a
very high degree of accuracy
for his time.
• He also used trigonometry to
measure the distance to the
Sun.
1600’s
• Johannes Kepler
• A student of Tycho
Brahe with access to
all of Brahe’s data.
• Used this data to
formulate the Laws of
Planetary Motion.
• He was the first to
realize that orbits were
elliptical.
– Theory fit the data, not
the other way around.
 1600’s
• Solved one problem of heliocentric model.
• Eliminated the need for epicycles and deferents.
 1600’s
• This highly accurate system determining all the
motions of the planets marked the beginning of the
“clockwork universe” concept.
 1620’s
• Galileo
• Finished off the idea of a
geocentric universe.
• Used a telescope to make
the following discoveries:
1. Sunspots
2. Mountains on the Moon
3. Milky Way with Lots of
Stars
4. Venus had Phases Like Our
Moon
5. Jupiter’s Four Main Moons
 1620’s
• Most damaging discovery
was the four moons of
Jupiter.
• Those moons were orbiting
Jupiter instead of the Earth.
• This did not fit the idea of a
geocentric model.
• At this point, the Catholic
Church was forced to give
in and accept a heliocentric
model.
• Began the decline of
religion.
 1680’s

• Isaac Newton
• One of the most
intelligent men in
history.
• Two main applications
to Astronomy.
1. Law of Universal
Gravitation
2. Laws of Motion