Download What Did The Ancient Know About Astronomy?

Lecture 4
100
Ancient to Renaissance Astronomy
Announcements
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Homework 3 – Due Monday, February 12
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Unit 9: Problem 3, Test Yourself 3
Unit 11: Test Yourself 1
Unit 12: Problem 1, Test Yourself 1, 3
Test 1 is one week from today.
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Will cover the first five lectures (units 1 – 12,
more or less)
Details next time
What Is A Cosmology?
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View of the universe and our place in it
Always has:
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Layout of the universe
Often has:
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Associated creation story
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Fundamental human question: is there a meaning to our
existence?
Method to explain celestial motion (movement of the
planets and stars)
Earliest Cosmologies (3000+ BC)
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Earth is flat
Universe centered on individual
Size of universe limited to local environment
Sun and moon important?
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Cave paintings feature disk with rays (sun?) and
“spots and crescents” (phases of the moon?)
Hunter-gatherer: value in understanding motions
of stars and planets (useful in predicting
winter)?
No evidence of concept of “celestial sphere”
Creation myths virtually unknown
Stonehenge (2800 BC – 1100 BC)
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Horizon astronomy
(position of sun
during solstices and
equinoxes)
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Star/planet position
not important
Calendar
Associated
cosmology?
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Probably not:
utilitarian tool
Stonehenge (2800 BC – 1100 BC)
A303
A344
Caracol (Maya culture, ~A.D. 1000)
Paint Rock, Texas
Archeoastronomy
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Knowledge of motions
of sun and moon
allows prediction of
seasons.
Important for
agriculture
Agriculture important
for rise of civilization.
Archeoastronomy
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Astronomical
knowledge important
to religious/elite
classes.
Perceived power:
ability to “control” the
objects in the
heavens.
Babylonian Astronomy
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Advanced calendar
Ability to predict planetary motion
Subtle shift in cosmology:
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World is flat, but much larger than local
environment.
Sky is a “vault” containing the stars.
Sun and moon moves through the sky.
Fundamentally different from planets/stars.
Babylonian Astronomy
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Precise understanding of
solar/lunar motion
necessary for lunar
calendar.
Great interest in
understanding planetary
motion – associated with
predictions of fate
(astrology).
Babylon and Mathematical
Astronomy
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Early records are
observationally based.
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Records found from 1500
BC and before.
Highly accurate
mathematics developed
to predict future motion.
Math ultimately supplants
observation.
No geometrical model –
highly abstract.
Egyptian Astronomy
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Cosmology similar to
Babylonian.
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Sky is body of the
goddess Nut. Sun is
divine.
Sky “attached” to
land. The Earth is
synonymous with the
universe.
Afterlife is
“elsewhere”
Egyptian Astronomy
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Little interest in
mathematical astronomy,
or the sky:
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Only five named
constellations (records
from 1100 BC).
Partial reason: Ancient
Egypt lacks the
mathematical
sophistication of Babylon.
Egyptian Astronomy
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Limited to the practical:
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36 decans (“star groups”)
used to tell time at night.
First appearance of Sirius
heralds annual flood of
Nile.
Orientation of major
monuments based on
sun worship and
significant constellations
(like Orion – Osiris in
Egyptian myth)
Greek Astronomy
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“Classical Revolution”
begins c.700 BC.
Astronomy used for
timekeeping. Techniques
imported from Babylon
and Egypt.
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Example from Hesiod –
Pleiades star cluster used
to track time for use in
agriculture (Pleiades =
harvest time).
Greek Astronomy
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Pythagoras (c.500 BC):
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Earth is a sphere
Morning star and evening star (associated
with Venus) are the same object
Greek Astronomy
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Plato (c. 360 B.C.)
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Established a philosophy
based on the teachings of
Pythagoras that favored
mental reasoning power
over observational science
Taught that what is seen in
the natural world is an
imperfect representation of
ideal creation
Teachings dominated much
of Western philosophy and
science for about 2,000
years
Greek Astronomy
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Models based on the philosophy of Plato were
generally wrong because they were based on
wrong “first principles”, believed to be
“obvious” and not questioned:
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Geocentric Universe: Earth at the Center of the
Universe and stationary.
“Perfect Heavens”: Motions of all celestial bodies
described by motions involving objects of “perfect”
shape, i.e., spheres or circles.
Greek Astronomy
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Eudoxus (c.360 BC)
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Complex motions of
planets through the
sky can be explained
through simple circular
motion.
Planets located on
giant “crystal” spheres
with Earth at the
center.
Model of 27 nested
spheres
Greek Astronomy
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Greeks are first to devise a “model” that
tries to explain (not just document and
predict) these motions!
Circles / Spheres important in Greek
philosophy (“the most perfect shapes”) –
so model is based on spheres.
Aristotle (384 – 322 BC)
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Eudoxus’ ideas refined by
Aristotle (c.350 BC):
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Earth is spherical at center
of universe.
“Edge” of universe is a
literal celestial sphere
holding the fixed stars.
Sphere rotates around
earth.
Sun moon and planets on
concentric spheres that
rotate around the earth.
Parallax
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Aristotle could
not measure
stellar parallax.
He concluded the
earth must not
be moving.
Distances to stars
are too great to
measure parallax.
Aristotle’s Universe
Sun, Moon and five
known planets
attached to 7
spherical “heavens”
Would dominate
astronomical thought
for the next 2,000
years!
Aside – Days of the Week
These seven heavenly bodies were
associated with the days of the week.
In fact, we still use their names:
Planet
Saturn
Sun
Moon
English
Saturday
Sunday
Monday
French
Samedi
Dimacnche
Lundi
Mardi
Mercredi
Jeudi
Vendredi
Spanish
Sabado
Domingo
Lunes
Martes
Miercoles
Jueves
Viernes
Swedish
lordag
sondag
mandag
tistag
onsdag
torsdag
fredag
Mars
Mercury
Jupiter
Tuesday Wednesday Thursday
Venus
Friday
Aristarchus (310 BC – 230 BC)
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Measured relative
distances/sizes of sun
and moon:
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Moon is about 1/3rd the
size of Earth.
Sun 20 times farther than
moon and 7 times larger
than Earth.
Since sun is larger, reasons
it must be at the center of
universe.
First known heliocentric
cosmology.
Some Incredible Insights
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Aristarchus Develops Heliocentric Idea:
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Moon shines because it reflects light from the sun.
Stars show no parallax:
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Must be extremely far away
If viewed close up, must be as large and bright as the sun
Must be “distant suns”
Universe must therefore be MUCH bigger than
indicated by Aristotle’s geocentric model.
Aristotle Hijacks Science
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Aristarchus’ hypothesis fails in the face of
Aristotle’s philosophy.
Why no perception of motion:
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Earth rotates? Why don’t dropped objects fly off to
the west?
Earth circles the sun? Why don’t we fly off its
surface?
Ironically: Why no parallax?
Geocentric model wins the day!
Calculation of the Earth’s radius
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Eratosthenes (~ 200 B.C.)
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Angular distance between
Syene and Alexandria:
~ 7°
Linear distance between
Syene and Alexandria:
~ 5,000 stadia
 Earth Radius ~ 40,000
stadia (probably ~ 14 % too
large) – better than any
previous radius estimate.
Hipparchus (c. 180 B.C.)
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Used naked-eye
sighting instruments
to aid observations
Introduced the use of
celestial coordinates,
made star maps (850
stars)
Invented the
magnitude scale
Hipparchus
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Estimated and compared stellar
brightness's
Used (possibly invented) trigonometry in
astronomy
Discovered the precession of the Earth's
axis
Maintained the idea of the Earth-centered
(geocentric) universe, because he could
not detect any hint of stellar parallax.
Later Refinements
Epicycles Introduced to explain
retrograde (westward) motion of
planets
Deferent (off-center rotation)
used to help fix deviations from
the observed positions
Ptolemy (c. 85 - 165 A.D.)
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Summarized all
previous Greek
astronomy in his
book the Almagest.
Further refined the
deferent/epicycle
concept using the
equant.
The point of uniform
motion of the planet
is off-center.
Ptolemy’s Geocentric Model Of
The Cosmos
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His model was the
most accurate
predictor of
planetary positions
in the ancient
world and was
used as the model
of the Cosmos up
to the time of the
Renaissance.
Other Greek Contributions
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Names and mythological identities for
nearly all of the northern hemisphere and
many southern hemisphere constellations.
Names of the major planets (carried over
by Romans, using Roman names).
The Dark Ages
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Fall of Rome, Rise of Christianity
Scriptural Basis for Geocentric Cosmology
Slight philosophical alterations to
Ptolemy’s model
Earth is center of universe – “crown jewel”
of God’s creation.
The Dark Ages
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Sin is centered on Earth: “Flawed” surface
Taint reaches to moon – mostly perfect,
except for some dark splotches.
Other celestial bodies are perfect,
luminous orbs (perfection of Heaven).
The Dark Ages
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Problems creeping up with geocentric
cosmology.
The number of “tweaks” needed getting
out of hand.
Support is waning among some of the
educated in Europe.
The Renaissance
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Nicholas Copernicus
(1473 – 1543)
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Influenced by ideas of
Aristarchus.
Sun near center of
universe. Planets orbit
sun, including Earth.
Naturally explains
retrograde motion.
Copernicus’ new explanation
for retrograde motion of the planets
Retrograde
(westward)
motion of a
planet occurs
when the
Earth passes
the planet.
Heliocentric Model
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Copernicus did not have strong observational
evidence to support his theory (stellar
parallax), nor did he propose new laws of
motion to explain the motion of the Earth.
Copernicus was rather conservative and
harkened back to the "pure" ideas of Plato
and Aristotle such as "perfect" circular motion
in the heavens and the platonic idea of the
Sun/God deity.
He greatly disapproved of the more recent
ideas of the eccentric and equant used to
modify the geocentric model.
Problems With Copernicus
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Assumes circular motion:
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Needs epicycles to match observed motion as well as
geocentric model.
Makes predictions that cannot be verified with
technology of the time (e.g. phases of Venus).
Why no stellar parallax (Brahe)?
“Philosophical” argument for next 50 years.
Tycho Brahe (1546-1601)
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Was the last and greatest of
the naked eye observational
astronomers
Built an observatory with large
sighting devices to make
accurate records of planet and
star positions for over 20 years
Observed and reported on the
supernova of 1572, and he
observed and estimated the
distance to a comet in 1577
Both observations helped to
discredit the Aristotelian view
of the Cosmos.
Parallax Shift
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Did not accept the Copernican (heliocentric)
model, because he could not measure an annual
parallax shift in the positions of the stars.
Tycho’s Model
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Proposed his
own modified
geocentric
model of the
Cosmos that
was
mathematically
nearly identical
to the
Copernican
model.
For Next Time
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Read Unit 12.
Homework 2 due at the beginning of next class
period!
Lab Report 1 due now (by beginning of lab)
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Please be next door ready for lab in about 5 minutes.