A Brief History Of Astronomy
A Brief History of AstronomyReading: Chapter 3.1-3.4Development of AstronomyThe development of astronomy came about throughcareful observations and gradual application of thescientific method across the world and over thousandsof years .1
Development of AstronomyFrance: Cave paintings from 15,000 B.C. may suggestknowledge of lunar phases (29 dots) and some starpatterns (Pliades cluster)Development of AstronomyAncient people of central Africa (6500 BC) couldpredict seasons from the orientation of the crescentmoon. A carved bone shows pictographs of thecrescent moonThis allowed them to determine when to plant seedsand grow their crops2
Development of Astronomy4000BC: Egyptians institute the 365 day calendar based on theperiodic rising of Sirius.Sirius, the brightest star in the sky, which they identified withthe fertility goddess Isis, first appeared in the predawn sky eachyear just as the Nile began its life-giving floods. While therising of the Nile varied year to year, Sirius appeared withperfect regularity.Once again this helped them predictplanting seasonDevelopment of Astronomy The Egyptians were also thefirst to divide the night andday into twelve hours, usingthe rising of bright stars-latercalled decans-to mark thehours till dawn.Water clock: waterruns out of a small holelike sand in anhourglassEgyptian obelisk acts as asundial: Shadows tell timeof day.3
Development of AstronomyScotland: 4,000-year-old stone circle; Moon rises as shownhere every 18.6 years.Development of AstronomyEngland: Stonehenge (completed around 1550 B.C.)Alignments of the stones at Stonehenge mark therising and setting points of the Sun at the solstices4
Development of Astronomy2354 BC, Babylon: First female astronomer recorded inhistory. Her name was En Hedu’ AnnaHer position: Astronomer priestess of the Moon GoddessDevelopment of Astronomy2000 BC: Babylonian priests in Mesopotamia (Iraq)recorded the motions of planets on thousands of tablets.763 BC: Solar eclipse observed and recorded byBabyloniansConstellations (asterisms) of the zodiac are based onancient Babylonian names5
Development of Astronomy"On the Jisiday, the 7th dayof the month, abig new starappeared in thecompany of theHo star.""On the Xinwei day the new star dwindled."Bone or tortoise shell inscription from the 14th century BC.China, 1400 BC : Earliest known records of supernovaexplosion of a dying star. Also recorded solar, lunar eclipsescontinuously from 4 BC !Development of AstronomyAstronomy in the AmericasSW United States: “Sun Dagger” marks summer solsticeMany ancient structures were built to mark and aid inastronomical observations6
Development of AstronomyAstronomy in the AmericasMayan observatory at ChichenItza.Many ancient structures were built to mark and aid inastronomical observationsDevelopment of AstronomyAstronomy in the AmericasInuit people used the stars to mark seasons and to find theirway across the frozen land where there were no easygeographical landmarks7
Development of Astronomy1054 AD : Supernova explosion recorded by Anasazi tribes aswell as in China.It was brighter than Venus and visible during the day for 23days!Development of AstronomyThe crab nebula : This is what the supernova of 1054 ADlooks like through powerful telescopes today!8
Development of AstronomyAstronomy and ReligionThe observed connection between the planting seasons and theposition of celestial objects led to the rise of religions toattempt to understand these events.Various ceremonies were designed to please celestial beings:priests became an important classEgyptian sun god RaSun temple: IndiaDevelopment of AstronomyAstronomy vs Astrology The association of celestialobjects with one or moregods lead to the idea thatthese gods could affecthuman lives - the birth ofAstrology Astrology is the search forinfluences on human livesbased on the positions ofplanets and stars in the sky.9
Development of AstronomyDoes astrology have any scientific validity? Scientific tests have shownthat astrologicalpredictions are no moreaccurate than we shouldexpect from pure chance.Development of AstronomySummary: Astronomical observations benefitedancient societies in many ways: Keeping track of time and seasons– for practical purposes, including agriculture– for religious and ceremonial purposes Aid to navigation (polaris, southern cross)10
Development of AstronomySummary of achievements of ancient civilizations: Daily timekeeping Tracking the seasons and calendar Monitoring lunar cycles Monitoring planets and stars Predicting eclipses And more Development of AstronomyBUT, so far astronomy only involved makingobservations, recognizing basic patterns and makingrudimentary predictions.A crucial part of the scientific method - buildingmodels/hypotheses that did not resort to supernaturalor godly influence - was missing.This was about to change in Greece .11
Greek Astronomy Greeks were the firstpeople known to makemodels of nature. They tried to explainpatterns in naturewithout resorting tomyth, gods or thesupernatural.Greek geocentric model (c. 400 B.C.)Thales of Miletus first assumed that the world was understandableand proposed a model of the earth.He successfully predicted a solar eclipseGreek Astronomy500BC: Pythagoras suggests thatthe Earth is a sphere and not flat,as had been previously assumedWhat observations of the ancientGreek astronomers could you useto prove the spherical model ofthe earth to a member of the FlatEarth Society ?12
Greek AstronomyEarth as a sphere:Ships disappear sailing away from shore by sinkingbelow horizon with mast last visible; Earth's curvaturevisible over 13 mile distanceGreek AstronomyEarth as a sphere:Circular shadow projected by Earth when it eclipses theMoonWhen traveling north, new stars appeared abovenorthern horizon, while stars previously seen alongsouthern horizon no longer visible; reverse true travelingsouth13
Greek Astronomy240 BC: Eratosthenes measures theEarth’s circumferenceMeasurements:At noon on first day of summer,Syene: sun directly overheadAlexandria: sun at 7 from verticalSyene to Alexandriadistance ! 5000 stadiaCalculate circumference of Earth:7/360 ! (circum. Earth) 5000 stadia" circum. Earth 5000 ! 360/7 stadia ! 250,000 stadiaCompare to modern value (! 40,100 km):Greek stadium ! 1/6 km " 250,000 stadia ! 42,000 kmGreek AstronomyUnderpinnings of the Greek geocentric model: Earth at the center of the universe Heavens must be “perfect”: Objects moving on perfect spheresor in perfect circles. Aristotle supported this world view Hence this model lasted for 1500 ce/14
Greek AstronomySimple geocentric models make it difficult to explainapparent retrograde motion of planets Over a period of 10 weeks, Mars appears to stop, back up, thengo forward again (retrograde motion).Greek AstronomyThe modern heliocentric model easily explainsapparent retrograde motion of planets Over a period of 10 weeks, Mars appears to stop, back up, thengo forward again (retrograde motion).15
Greek AstronomyThe most sophisticatedgeocentric model was that ofPtolemy (A.D. 100-170) —the Ptolemaic model: Sufficiently accurate toremain in use for 1,500 years. Ptolemy’s model couldhandle retrograde motionPtolemyGreek AstronomyPtolemy’s wheels within wheels:Planets move on small circlescalled epicycles, attached to a bigcirclePlanets really do go backward inthis model.16
Greek AstronomyAncient Greeks couldnot detect parallaxwhich would haveproved the earth’smotion around the sun.Hence the geocentricmodel persisted for1500 years.Greek AstronomyGreek knowledge was preserved through the MiddleAges: Muslim world preserved and enhanced the knowledge theyreceived from the Greeks Al-Mamun’s House of Wisdom in Baghdad was a greatcenter of learning around A.D. 800 With the fall of Constantinople (Istanbul) in 1453, Easternscholars headed west to Europe, carrying knowledge thathelped ignite the European Renaissance.17
The Copernican RevolutionCopernicus (1473-1543): Proposed Sun-centered model(published 1543) Used model to determine layout ofsolar system (planetary distancesin AU) Driven by aesthetics rather thanobservationsBut . . . Model was no more accurate thanPtolemaic model in predictingplanetary positions, because it stillused perfect circles. Not widely acceptedThe Copernican RevolutionPossible influences on CopernicusAristarchus of Samos (Greece) in the 3rd centuryBC had developed the first serious model of aheliocentric solar system.However his work did not catch on.Aryabhatta of India anticipated Copernicus'discoveries by over 1,000 years and formulated aheliocentric model.The 8th century Arabic edition of his workwas translated into Latin.It is likely that some of Aryabhata's resultshad an influence on European astronomyand on Copernicus’ ideas.Aryabhatta’s description of eclipses18
The Copernican RevolutionTycho Brahe (1546-1601) Compiled the most accurate (onearcminute) naked eye measurementsever made of planetary positions. Still thought Earth must be at center ofsolar system (but recognized that otherplanets go around Sun) Hired Kepler, who used Tycho’sobservations to discover the truth aboutplanetary motion.The Copernican Revolution Kepler first tried to match Tycho’sobservations with circular orbits But an 8-arcminute discrepancy ledhim eventually to ellipses Johannes Kepler(1571-1630)“If I had believed that we couldignore these eight minutes [of arc], Iwould have patched up myhypothesis accordingly. But, since itwas not permissible to ignore, thoseeight minutes pointed the road to acomplete reformation in astronomy.”19
The Copernican RevolutionWhat is an ellipse?An ellipse looks like an elongated circleEccentricity quantifies the deviation from a perfect circleThe Copernican RevolutionWhat is an ellipse?Eccentricity: e distance between foci/major axisPerihelion distance (1-e) x semimajor axisAphelion distance (1 e) x semimajor axis20
The Copernican RevolutionKepler’s three laws of planetary motionKepler’s First Law: The orbit of each planet aroundthe Sun is an ellipse with the Sun at one focus.The Copernican RevolutionKepler’s Second Law: As a planet moves aroundits orbit, it sweeps out equal areas in equal times." means that a planet travels faster when it is nearer to the Sun andslower when it is farther from the Sun.21
The Copernican RevolutionKepler’s Third LawMore distant planets orbit the Sun at sloweraverage speeds, obeying the relationshipp2 a3p orbital period in yearsa avg. distance from Sun in AUThought Question:An asteroid orbits the Sun at an average distancea 4 AU. How long does it take to orbit the Sun?A.B.C.D.4 years8 years16 years64 yearsHint: Remember that p2 a322
An asteroid orbits the Sun at an average distancea 4 AU. How long does it take to orbit the Sun?A.B.C.D.4 years8 years16 years64 yearsWe need to find p so that p2 a3Since a 4, a3 43 64Therefore p 8, p2 82 64The Copernican RevolutionGalileo solidified the Copernican revolutionGalileo (1564-1642) overcame majorobjections to Copernican view. Threekey objections rooted in Aristotelianview were:1. Earth could not be moving becauseobjects in air would be left behind.2. Non-circular orbits are not “perfect”as heavens should be.3. If Earth were really orbiting Sun,we’d detect stellar parallax.23
The Copernican RevolutionOvercoming the first objection (nature of motion):Galileo’s experiments with rolling balls showed thatobjects in air would stay with a moving Earth. Aristotle thought that all objects naturally come to rest. Galileo showed that objects will stay in motion unlessa force acts to slow them down (Newton’s first law ofmotion). Hence objects moving with the earth would stay with theearthThe Copernican RevolutionOvercoming the second objection (heavenly perfection): Tycho’s observations of comet andsupernova already challenged this idea. Using his telescope, Galileo saw: Sunspots on Sun (“imperfections”) Mountains and valleys on theMoon (proving it is not a perfectsphere)24
The Copernican RevolutionOvercoming the third objection (parallax): Tycho thought he had measured stellar distances, solack of parallax seemed to rule out an orbiting Earth. Galileo showed stars must be much farther thanTycho thought — in part by using his telescope to seethe Milky Way is countless individual stars.! If stars were much farther away, then lack ofdetectable parallax was no longer so troubling.The Copernican RevolutionThe moons of JupiterGalileo also saw fourmoons orbiting Jupiter,proving that not allobjects orbit the Earth25
The Copernican RevolutionGalileo’s observations of phases of Venus proved that it orbits theSun and not Earth.Geocentric model: Only full and crescent phases of VenusobservableHeliocentric model: All phases of Venus observable as confirmedby GalileoThe Copernican RevolutionThe Catholic Church orderedGalileo to recant his claim thatEarth orbits the Sun in 1633His book on the subject wasremoved from the Church’sindex of banned books in 1824Galileo was formally vindicatedby the Church in 199226
The Beginning of Modern AstronomyCopernicus created a sun-centered model of the solar systemTycho Brahe provided the data needed to improve this modelKepler found a model that fit Tycho’s data– 1. The orbit of each planet is an ellipse with the Sun at onefocus– 2. As a planet moves around its orbit it sweeps our equal areasin equal times– 3. More distant planets orbit the Sun at slower average speeds:p2 a3Galileo’s experiments and observations overcame the remainingobjections to the Sun-centered solar systemWhat is a scientific theory? The word theory has a different meaning in science than ineveryday life. In science, a theory is NOT the same as a hypothesis oropinion, rather: A scientific theory must:—Explain a wide variety of observations with a few simpleprinciples, AND—Must be supported by a large, compelling body ofevidence.—Must NOT have failed any crucial test of its validity.27
Summary What did ancient civilizations achieve inastronomy? Daily timekeeping Tracking the seasons and calendar Monitoring lunar cycles Monitoring planets and stars Predicting eclipses Ancient astronomers observations were useful– for practical purposes, including agriculture– for religious and ceremonial purposes Aid to navigationSummary What is the Copernican Revolution ?Copernicus created a sun-centered model of the solar systemTycho Brahe provided the data needed to improve this modelKepler found a model that fit Tycho’s data– 1. The orbit of each planet is an ellipse with the Sun at one focus– 2. As a planet moves around its orbit it sweeps our equal areas inequal times– 3. More distant planets orbit the Sun at slower average speeds: p2 a3Galileo’s experiments and observations overcame the remainingobjections to the Sun-centered solar system28
Development of Astronomy Bone or tortoise shell inscription from the 14th century BC. "On the Xinwei day the new star dwindled." "On the Jisi day, the 7th day of the month, a big new star appeared in the company of the Ho star." Astronomy in the Americas Development of Astronomy Many ancient structures were built to mark and aid in astronomical .
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