Ancient AstronomersAncient Astronomers

EgyptiansEgyptians BabyloniansBabylonians Native AmericansNative Americans ChineseChinese CeltsCelts Many OthersMany Others

Early ObserversEarly Observers

Stonehenge

Big Horn Medicine Wheel

Loanhead of Daviot Pyramids of Giza

Greek AstronomersGreek Astronomers

Tried to explain the motion of Tried to explain the motion of celestial objectscelestial objects

Built Cosmological Models to Built Cosmological Models to explain observationsexplain observations

Stars, Moon, Sun easily explainedStars, Moon, Sun easily explained Planets (wanderers) not so easily Planets (wanderers) not so easily

explainedexplained

Scientific MethodScientific Method

Use models to explain observationsUse models to explain observations

Test modelsTest models

Theories are never “true” just “not Theories are never “true” just “not false”false”

Chain of Scientific Chain of Scientific MethodMethod

Observation

Explanation

Prediction

Early Greek Early Greek AstronomersAstronomers Eratosthenes calculated Eratosthenes calculated

circumference of the earth within circumference of the earth within 5% accuracy5% accuracy

Aristarchus proposed a Aristarchus proposed a heliocentric cosmos and easily heliocentric cosmos and easily explained the motion of the Sun explained the motion of the Sun through the heavensthrough the heavens

His writings were lost at His writings were lost at Alexandria along with HypatiaAlexandria along with Hypatia

AristotleAristotle

384-322 B.C.384-322 B.C. Member of Plato’s AcademyMember of Plato’s Academy Philosopher and ScientistPhilosopher and Scientist One of the 1One of the 1stst Physical Models of Physical Models of

the Cosmosthe Cosmos GeocentricGeocentric

PtolemyPtolemy GeocentricGeocentric Attempted to explain retrograde Attempted to explain retrograde

motionmotion Used Hipparchus idea of epicycles Used Hipparchus idea of epicycles

and deferentsand deferents Added the equantAdded the equant Very complicatedVery complicated Inconsistent rulesInconsistent rules

Ptolemy’s SolutionPtolemy’s Solution

Earth Equant

Deferent

Epicycle

Planet

Triumph and TragedyTriumph and Tragedy

Geocentric Model persisted for Geocentric Model persisted for many, many, many, many yearsmany, many, many, many years

Lack of observable Stellar Lack of observable Stellar Parallax Parallax

Painter’s Thumb-ParallaxPainter’s Thumb-Parallax Stars are too far away!Stars are too far away!

Copernican RevolutionCopernican Revolution

1473-1543 A.D.1473-1543 A.D. Revived early heliocentric modelsRevived early heliocentric models Retrograde motion Retrograde motion simplysimply

explainedexplained Circular OrbitsCircular Orbits No public supportNo public support

Copernican RetrogradeCopernican Retrograde

http://csep10.phys.utk.edu/astr161/lect/index.html

Copernicus says…Copernicus says…

The celestial spheres do not have The celestial spheres do not have just one common centerjust one common center

The Center of Earth is not the The Center of Earth is not the center of the universe but instead center of the universe but instead only the center of gravity and of only the center of gravity and of the lunar orbitthe lunar orbit

All spheres revolve around the All spheres revolve around the SunSun

He continues….He continues….

The firmament is far awayThe firmament is far away Apparent motions of the Apparent motions of the

firmament and Sun are actually firmament and Sun are actually due to Earth’s motionsdue to Earth’s motions

The apparent retrograde and The apparent retrograde and forward motion of the planets is forward motion of the planets is actually due to the Motion of the actually due to the Motion of the EarthEarth

Finally Somebody Gets Finally Somebody Gets It!It! Johannes Kepler 1571-1630Johannes Kepler 1571-1630 Worked for Tycho Brahe, inherited Worked for Tycho Brahe, inherited

his datahis data Heliocentric model (29 years!)Heliocentric model (29 years!) Found orbit shapes by Found orbit shapes by

observation from different points observation from different points in Earth’s orbitin Earth’s orbit

""By the study of the orbit of By the study of the orbit of Mars,Mars,we must either arrive at the we must either arrive at the secrets of astronomysecrets of astronomyor forever remain in or forever remain in ignorance of them."ignorance of them."Johannes KeplerJohannes Kepler

Kepler’s 1Kepler’s 1stst Law Law

Elliptical Orbits not Circular!Elliptical Orbits not Circular!

Ellipse GeometryEllipse Geometry

Minor Axis

Major Axis

Foci

Eccentricity=How oval is it?

Kepler’s 2Kepler’s 2ndnd Law Law

Equal Areas in Equal TimesEqual Areas in Equal Times Describes Describes Single PlanetSingle Planet is orbit is orbit

around the Sunaround the Sun Perihelion-closest approach to Perihelion-closest approach to

Sun Sun Aphelion-farthest distance from Aphelion-farthest distance from

SunSun

Kepler’s 3Kepler’s 3rdrd Law Law

The more distant a planet’s orbit The more distant a planet’s orbit is from the Sun, the slower it is from the Sun, the slower it movesmoves

Compares planets to one anotherCompares planets to one another Period- time for one complete Period- time for one complete

orbit around sunorbit around sun Let’s see all three in actionLet’s see all three in action

Kepler’s TriumphKepler’s Triumph

Published first two laws in 1609Published first two laws in 1609 Calculations only for MarsCalculations only for Mars Decade later he checks all known Decade later he checks all known

planetsplanets They work!They work! Also introduces 3Also introduces 3rdrd law at this law at this

timetime

SummarySummary

Models are constantly being evaluatedModels are constantly being evaluated Theory is never “true” just not “false”Theory is never “true” just not “false” Example geocentric cosmosExample geocentric cosmos Kepler’s 3 laws accurately and simply Kepler’s 3 laws accurately and simply

describe the motion of the planetsdescribe the motion of the planets Laid the foundation for work of future Laid the foundation for work of future

scientistsscientists