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

Pyramids,

Egypt

Stonehenge,England

Newgrange,Ireland

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 long-

term 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 disentangleThe 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 LawAn imaginary line connecting the Sun to any planet sweeps out equal areas of the ellipse in equal times

Kepler’s Third LawThe 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 Eccentricity P2/a3

Mercury 0.387 0.241 0.206 1.002Venus 0.723 0.615 0.007 1.001Earth 1.000 1.000 0.017 1.000Mars 1.524 1.881 0.093 1.000Jupiter 5.203 11.86 0.048 0.999Saturn 9.539 29.46 0.056 1.000Uranus 19.19 84.01 0.046 0.999Neptune 30.06 164.8 0.010 1.000Pluto 39.53 248.6 0.248 1.001

(A.U.) (Earth years)

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

Force = G Mearth Mman / R2

MEarth

Mman

R

Orbital Motion

Cannon “Thought Experiment”

• http://www.phys.virginia.edu/classes/109N/more_stuff/Applets/newt/newtmtn.html