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A Short History of Astronomy
• Ancient (before 500 BC)
• Egyptians, Babylonians, Mayans, Incas, Chinese
• Classical Antiquity (500 BC-500 AD)
• Greeks, Romans: Plato, Aristotle, Ptolemy
• Middle Ages (500-1450 AD)
• Arabic astronomers
• Renaissance (1450-1550 AD)
• Copernicus
• Brahe, Kepler, Galilei, Newton
Ancient Astronomy
Stonehenge,
England
Newgrange,
Ireland
Pyramids,
Egypt
The Babylonians
• Made systematic measurements as early as
~2000 BC
• By ~ 800–400 BC
– State support for the calendar and astrology
– Compiled the first star catalogs and began longterm records of planetary motions
– Were able to predict lunar and solar eclipses
• May also have invented astrology
The Greeks
Plato (428 BC)
• Introduces the celestial
sphere. The stars are
fixed to a sphere that
rotates around the Earth
• introduces prejudice in
favor of circles
• values theory over
observation
Alexander the Great
(356-323 BC)
• Much of the knowledge of the world came
together as the rule of Alexander spread
across Europe, Asia, and Northern Africa.
• Founded the city of Alexandria
• Library of Alexandria contained the written
works and inventions of many great
thinkers and scientists.
The Greeks
• Aristotle (384–322 BC)
– Argued that the planets move on spheres around the
Earth (“geocentric” model)
– Argues that the earth is spherical based on the shape of
its shadow on the moon during lunar eclipses
• Aristarchus (310–230 BC)
– Attempts to measure relative distance and sizes of sun
and moon
– Proposes, nearly 2000 years before Copernicus, that all
planets orbit the Sun, including the Earth
(“heliocentric” model)
Using the distance between the Earth and the Moon as a baseline
The Greeks
• Eratosthenes (ca. 276 BC)
– Measures the radius of the earth to about 20%
Eratosthenes (~ 200 B.C.):
Calculation of the Earth’s radius
Angular distance between
Syene and Alexandria:
~ 70
Linear distance between
Syene and Alexandria:
~ 5,000 stadia
 Earth
Radius ~ 40,000
stadia (probably ~ 14 %
too large) – better than
any previous radius
estimate.
The Greeks
Hipparchus (~190 BC)
– His star catalog a standard
reference for sixteen
centuries!
– Introduces coordinates for
the celestial sphere:
• Declination (dec)
• Right Ascension (RA)
(analogous to latitude and
longitude, respectively)
Ptolemy (~140 AD)
• Puts forth a complete
geocentric model
• dominates scientific
thought during the
Middle Ages
• Longest lasting (wrong)
theory ever: 1000 yrs
Major Work: Almagest
Retrograde Motion
Epicycles
• Ptolemy’s
explanation of
retrograde motion
• About 40(!) epicycles
necessary to explain
all observations
complicated theory
Hypatia of Alexandria
(355 or 370 - 415/416 A.D.) Exact
dates are unknown. Many records
were destroyed.
• Hypatia was, simply, the last great
Alexandrian mathematician and
philosopher.
• Edited the work On the Conics of
Apollonius
The Medieval Setting
•
•
•
•
Dominant Church
1000 years of relative stagnation
Experimental research greatly reduced
To answer a question:
“Study the Bible or Aristotle!”
The Renaissance Setting
• Invention of the print (1450) by Gutenberg
Books widely available!
(Think: Manuscripts vs Amazon.com)
•
•
•
•
End of Middle Age Church Domination
Back to the roots (renaissance=rebirth)
Study of Arabic astronomers
Intellectual movement
Nicolas Copernicus (1473–1543)
• Rediscovers the heliocentric
model of Aristarchus
• Planets on circles
needs 48(!!) epicycles to explain
different speeds of planets
• Not more accurate than Ptolemy
Major Work : De
Revolutionibus
Orbium Celestium
(published posthumously)
The heliocentric Explanation of
retrograde planetary motion
See also: SkyGaze
The Scientific Method
• Systematized by Francis
Bacon, Descartes and
Galileo in the 17th
century
• Not the only way of
knowing, but a very
successful one
• A method to yield
conclusions that are
independent of the
individual
• Conclusions are based
on observation
Tycho Brahe – The Data Taker
• Key question:
Where are things?
• Catalogued positions of planets
in Uraniborg and Prague
• Working without telescope
• Data ten times as accurate as
before
• Died at banquet binge drinking
Tycho Brahe (1546–1601)
Tycho Brahe
• collects detailed and accurate (1-2’
accuracy) observations of stellar and
planetary positions over a period of
20 years
• His research costed 5-10% of Danish
GNP
• shows that comets and novas are
extralunar contrary to Aristotle
• Shows that stars can change
(Supernova of 1572)
Tycho Brahe observing
Johannes Kepler–The Phenomenologist
• Key question:
How are things happening?
Major Works:
• Harmonices Mundi (1619)
• Rudolphian Tables (1612)
• Astronomia Nova
• Dioptrice
Johannes Kepler (1571–1630)
Kepler’s Beginnings
• Astrologer and Mystic
• Tried to find “music in
the skies”
• Tried to explain
distances of the
5 known planets by
spheres resting on the
5 mathematical bodies
 pre-scientific
Johannes Kepler
Manuscript: trying to disentangle
The mystery of Mars’ orbit 
Kepler’s First Law
The orbits of the planets are ellipses, with
the Sun at one focus
Ellipses
a = “semimajor axis”; e = “eccentricity”
Conic Sections
From Halley’s book (1710)
Kepler’s Second Law
An imaginary line connecting the Sun to any planet sweeps
out equal areas of the ellipse in equal times
Kepler’s Third Law
The square of a planet’s orbital period is proportional to the cube of its
orbital semi-major axis:
P 2  a3
a
P
Planet Orbital Semi-Major Axis Orbital Period
Mercury
0.387
0.241
Venus
0.723
0.615
Earth
1.000
1.000
Mars
1.524
1.881
Jupiter
5.203
11.86
Saturn
9.539
29.46
Uranus
19.19
84.01
Neptune
30.06
164.8
Pluto
39.53
248.6
(A.U.)
(Earth years)
Eccentricity
0.206
0.007
0.017
0.093
0.048
0.056
0.046
0.010
0.248
P2/a3
1.002
1.001
1.000
1.000
0.999
1.000
0.999
1.000
1.001
Galileo Galilei – The Experimentalist
Did experiments (falling bodies) rather
than studying Aristotle
Major Works
• Siderius Nuntius (1610)
• Dialogue concerning the Two Chief
World Systems (1632)
The latter discusses Copernicus vs
Ptolemy ban by Church (1633)
– revoked by pope 1992
(1564–1642)
Siderius Nuntius (1610)
Dialogo (1632)
Galileo Galilei (1564–1642)
• Astronomical observations that
contradict Aristotle:
– Observed mountains on the Moon,
suggesting that the Earth is not unique
– Sunspots; suggests that celestial bodies
are not perfect and can change
– Observed four moons of Jupiter; showed
that not all bodies orbit Earth
– Observed phases of Venus (and correlation of apparent
size and phase); evidence that Venus orbits the Sun
• Also observed
– the rings of Saturn
– that the Milky Way is made of stars
Phases of
Venus
Heliocentric
(observed)
Geocentric
(not observed)
Isaac Newton – The Theorist
• Key question:
Why are things happening?
• Invented calculus and physics
while on vacation from college
• His three Laws of Motion,
together with the Law of
Universal Gravitation, explain all
of Kepler’s Laws (and more!)
Isaac Newton (1642–1727)
Isaac Newton (1642–1727)
Major Works:
• Principia (1687)
[Full title: Philosophiae naturalis
principia mathematica]
• Opticks [sic!](1704)
• Major findings:
– Three axioms of motion
– Universal gravity
Law of Universal Gravitation
Mman
MEarth
R
Force = G Mearth Mman / R2
Orbital Motion
Cannon “Thought Experiment”
• http://www.phys.virginia.edu/classes/109N/more_stuff/Appl
ets/newt/newtmtn.html