A Short History of Astronomy

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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

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Ancient Astronomy
Newgrange, Ireland
Stonehenge, England

• Pyramids,
• Egypt

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The Babylonians

• Made systematic measurements as early as 2000 BC
• By 800400 BC
• State support for the calendar and astrology
• Compiled the first star catalogs and began
  long-term records of planetary motions
• Were able to predict lunar and solar eclipses
• May also have invented astrology

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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

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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.

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The Greeks

• Aristotle (384322 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 (310230 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)

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Using the distance between the Earth and the Moon
as a baseline
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The Greeks

• Eratosthenes (ca. 276 BC)
• Measures the radius of the earth to about 20

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Eratosthenes ( 200 B.C.)Calculation of the
Earths 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.
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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)

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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

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Retrograde Motion
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Epicycles

• Ptolemys explanation of retrograde motion
• About 40(!) epicycles necessary to explain all
  observations ?complicated theory

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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

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The Medieval Setting

• Dominant Church
• 1000 years of relative stagnation
• Experimental research greatly reduced
• To answer a question
• Study the Bible or Aristotle!

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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 (renaissancerebirth)
• Study of Arabic astronomers
• Intellectual movement

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Nicolas Copernicus (14731543)

• 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)

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The heliocentric Explanation of retrograde
planetary motion

• See also SkyGaze

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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

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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 (15461601)
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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
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Johannes KeplerThe Phenomenologist

• Key question
• How are things happening?
• Major Works
• Harmonices Mundi (1619)
• Rudolphian Tables (1612)
• Astronomia Nova
• Dioptrice

Johannes Kepler (15711630)
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Keplers 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

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Johannes Kepler

• Manuscript trying to disentangle
• The mystery of Mars orbit ?

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Keplers First Law

• The orbits of the planets are ellipses, with the
  Sun at one focus

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Ellipses

• a semimajor axis e eccentricity

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Conic Sections

• From Halleys book (1710)

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Keplers Second Law
An imaginary line connecting the Sun to any
planet sweeps out equal areas of the ellipse in
equal times
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Keplers Third Law

• The square of a planets 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
  Eccentricity P2/a3
• Mercury 0.387 0.241 0.206 1.002
• Venus 0.723 0.615 0.007 1.001
• Earth 1.000 1.000 0.017 1.000
• Mars 1.524 1.881 0.093 1.000
• Jupiter 5.203 11.86 0.048 0.999
• Saturn 9.539 29.46 0.056 1.000
• Uranus 19.19 84.01 0.046 0.999
• Neptune 30.06 164.8 0.010 1.000
• Pluto 39.53 248.6 0.248 1.001
• (A.U.) (Earth years)

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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

• (15641642)

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• Siderius Nuntius (1610) Dialogo
  (1632)

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Galileo Galilei (15641642)

• 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

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Phases of Venus

• Heliocentric
• (observed)

Geocentric (not observed)
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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 Keplers
  Laws (and more!)

Isaac Newton (16421727)
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Isaac Newton (16421727)

• Major Works
• Principia (1687)
• Full title Philosophiae naturalis principia
  mathematica
• Opticks sic!(1704)
• Major findings
• Three axioms of motion
• Universal gravity

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Law of Universal Gravitation

• Force G Mearth Mman / R2

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Orbital Motion
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Cannon Thought Experiment

• http//www.phys.virginia.edu/classes/109N/more_stu
  ff/Applets/newt/newtmtn.html