Our Place in the Universe. Understanding Fundamental Astronomy from Ancient Discoveries

269 Pages·2017·14.82 MB·English

by  Sun Kwok

Sun Kwok
Our Place in
the Universe
Understanding Fundamental
Astronomy from Ancient Discoveries
Second Edition
Our Place in the Universe
Sun Kwok
Our Place in the Universe
Understanding Fundamental Astronomy
from Ancient Discoveries
Second Edition
SunKwok
FacultyofScience
TheUniversityofHongKong
HongKong,China
Thisbookisasecondeditionofthebook“OurPlaceintheUniverse”previouslypublished
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DOI10.1007/978-3-319-54172-3
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Preface
There is a common perception among the general populace that astronomy is
impracticalandirrelevant.Thiscouldnotbefurtherfromthetruth.Forthousands
ofyears,astronomywasanextremelypracticalsubject,andourancestorsreliedon
theirastronomicalknowledgetoconducttheirdailylives.Mostancientpeoplewere
far more familiar with the behavior of the Sun, the Moon, and the stars than the
average personis today. Astronomyalsomotivatedintellectual thoughtandhada
majorimpactonthesocialdevelopmentofthehumanracethroughouthistory.Our
evolvingperceptionofourplaceintheuniversehelpedbringaboutimportantsocial
changesoverthelasttwothousandyears.
This book is not just about astronomy. It uses the historical development of
astronomy to illustrate the process of rational reasoning and its effect on philoso-
phy, religion, and society. Because celestial objects followed regular patterns,
astronomical observations gave humans some of the first hints that Nature was
understandable. The complicated nature of these patterns also challenged our
intellectualpowers.
Inoureducationsystem,scienceisoftenpresentedtoourstudentsasaseriesof
facts.Infact,scienceisabouttheprocessofrationalthinkingandcreativity.What
weconsidertobethetruthisconstantlyevolvingandhascertainlychangedgreatly
over the history of humankind. The essence of science is not so much about the
current view of our world but how we changed from one set of views to another.
Thisbookisnotabouttheoutcomebuttheprocess.
I tried to achieve these goals as follows. I begin with a description of basic
observations, summarize the patterns observed and the problems they pose, and
discuss the suggested theories and their implications. The pros and cons of these
theories are evaluated alongside alternate theories. This approach differs from
typicalsciencetextbooks,whichusuallytakeanaxiomaticapproachbyfirststating
thecorrecttheoryandderivingthedeductionsbeforecomparingthemwithexper-
imentalresults.Ihopethishistoricalapproachallowsstudentstobetterunderstand
the scientific process and learn from this process when they tackle real-life prob-
lemsintheircareers.
v
vi Preface
We live in the most prosperous times in human history. It is convenient to
assume that everything important happened recently and that events from the
distant past do not matter. It is also easy for us to forget or dismiss the wisdom
andachievementsofourancestors.Asimplesurveyofmodernuniversitystudents
willrevealthatmostofthembelievewediscoveredtheEarthwasroundonlyafew
hundred years ago. But in fact the Earth’s shape was well known as long as 2500
yearsago.
Withnakedeyeobservationsandsomeverysimpleinstruments,ancientastron-
omersfoundoutagreatdealaboutourworld.Byobservingcelestialobjects,they
deduced that the Earth was round. They could explain the changing times and
locationsofsunrise.Theyhadareasonableempiricalmodeltoforecasteclipses.In
spite of the apparent erratic motions of the planets, their positions could be
predicted accurately with mathematical models hundreds of years into the future.
Althoughancientcivilizationsoccupiedonlyasmallfractionofthesurfaceofthe
Earth,theyhadaverygoodestimateofthesizeoftheentireEarth.Theycouldeven
determinethesizeofanddistancetotheMoon.
Modern humans’ disconnection from Nature also means that some common
knowledge from ancient times has been lost. Many people today believe that the
Sunrisesintheeasteveryday,butitwascommonknowledgeamongourancestors
thatthedirectionofsunrise changes everyday.Theregular yetcomplexapparent
motionoftheSunwasthemainmotivatorforthedevelopmentofrationalthought.
ThisbookisbasedonacoursedesignedfortheCommonCoreProgramofThe
UniversityofHongKong(HKU).TheHKUCommonCorecoursesarenotbased
onaspecificdisciplineandaredesignedtohelpstudentsdevelopbroaderperspec-
tivesandabilitiestocriticallyassesscomplexissues.Theclassesalsohelpstudents
appreciateourowncultureandglobalissues.
I developed this course and taught it from 2010 to 2016. Every year, the class
containedabout120studentsfromallfacultiesoftheUniversity,includingArchi-
tecture, Arts, Business and Economics, Dentistry, Education, Engineering, Law,
Medicine, Science, and Social Sciences. Because of the students’ diverse back-
ground,nomathematicalderivationsorcalculationswereused.Thestudentswere,
however, expected to understand qualitative concepts, develop geometric visuali-
zations,andperformlogicaldeductions.Inordertoconveytheconceptseffectively
without mathematics, I relied strongly on graphical illustrations and animations.
Computer simulations were used to show apparent motions of celestial objects in
thesky.Theseillustrationsgreatlyhelpedstudentsvisualizethecomplexityofsuch
motions.
Formoretechnicalreaders,Ihaveaddedsomemathematicsinthisbook,mostof
which is presented in the Appendices. Nonmathematical readers can skip these
parts. To focus on the evolution of concepts, I have deliberately omitted certain
details. For example, the apparent motions of the Sun and Moon are even more
complicatedthanIhavepresentedhere.Mygoalistoreachabroadreadership.
Jargonsaregreatobstaclestolearning.Inthisbook,Itrytominimizetheuseof
jargonsasmuchaspossibleandsometechnicaltermsarereplacedbysimplewords
Preface vii
with similar meaning. Some concepts have precise definitions, and the use of
technicaltermsisunavoidable.AlldefinitionsarepresentedintheGlossary.
Every year, students ask me whether they will be handicapped by their lack of
previousknowledgeofphysicsandastronomy.Infact,thereverseistrue.Students
in science have been told all the modern notions but have never learned how we
arrivedatthoseconclusions.Tolearnabouttheprocessofdiscovery,theyhaveto
giveuptheirpreconceptions,whichcanbehardforsomestudents.Oneexampleis
the question “How do we know that the Earth revolves around the Sun?” When I
posedthisquestiontostudents,themostcommonanswerIgotwas“ThisiswhatI
wastoldbymyteacher.”Inthisbook,wetrytoretracehistoricalstepstofindout
howwegottothisconclusion.
Inadditiontolectures,wehadweeklytutorials,quizzes,assignments,computer
laboratory exercises, a planetarium show, and exams. The planetarium show was
developed with the assistance of the Hong Kong Space Museum to illustrate the
celestialmotions observed indifferent parts ofthe world and at different timesin
history.Thelaboratoryexerciseswerebasedoncomputersoftwaresothatstudents
couldhavefirsthandexperienceviewingandrecordingdatafromsimulatedobser-
vations.Theassessmentsweredesignedtotestwhetherthestudentshadunderstood
thecoursematerials,couldconnectmaterialfromdifferentpartsofthecourse,had
achievedsomedegreeofsynthesis,andcouldapplytheacquiredknowledgetonew
situations.
IwishtothankWaiWong,whoskillfullydrewmanyofthefiguresinthisbook.
AnisiaTangandSze-LeungCheunghelpedwithbackgroundresearchandcontrib-
uted to the laboratory exercises. I thank Gray Kochhar-Lindgren, Director of the
HKUCommonCoreProgram,andY.K.Kwok,AssociateVicePresident(Teaching
and Learning), for their unyielding support for my course. Tim Wotherspoon and
Bruce Hrivnak provided helpful comments on an earlier draft. I thank Ramon
Khanna, my editor at Springer, for encouraging me to publish this book. I am
particularly grateful to my wife Emily and daughter Roberta for reading various
draftsofthisbookandgivingmecriticalcomments.
IalsowishtothanktheUniversityofBritishColumbiaforitshospitalityduring
mysabbaticalleavewhenthismanuscriptwascompleted.
Ifirstbecameinterestedinthissubjectduringmysecondyearofundergraduate
study at McMaster University, where Prof. Bertram Brockhouse (Nobel Prize in
Physics,1994)introducedmetoKepler’sworkinhisPhilosophyofSciencecourse.
His teaching made me realize that physics is more than just mechanical calcula-
tions;itisasubjectwithphilosophicalandsocialimplications.
Vancouver,Canada SunKwok
2016
Prologue
天地玄黃,宇宙洪荒。日月盈昃,辰宿列張。
寒來暑往,秋收冬藏。閏餘成歲,律呂調陽。
千字文周興嗣
“Inthebeginning,therewastheblackheavenandtheyellowearth.TheUniversewasvast
and without limit. The Sun rises and sets, the Moon goes through phases, and the stars
spreadoverdistinctconstellations inthesky.Thewarmandcoldseasonscomeandgo,
whileweharvestinthefallandstoreourgrainsforthewinter.Ayeariscomposedofan
unevennumberofmonths,andharmonyofmusicgovernsthecosmos”.
Firsteightversesfromthe“ThousandCharacterEssay”byZhouXingSi(470–521A.D.),
translatedfromChinese.
Zhou,anofficialintheCourtoftheLiangDynasty,wasaskedbytheEmperor
Wu 梁武帝(reigned 502–549 A.D.) to arrange a set of 1000 characters into an
essay for the education of the young princes. He composed a rhymed essay of
250four-characterverseswhereeachcharacterwasusedonlyonce.Fromthesixth
centurytotheearlytwentiethcentury,thisessaywascommonlyusedasaprimary
texttoteachyoungchildrentheChinesecharacters.
Theessaybeginswitheightversesthatexpresshumans’desiretounderstandthe
Universe and their appreciation for the celestial objects’ orderly movements. As
Zhou describes it, people also recognize that observations of the Sun, Moon, and
starshaveledtothedevelopmentofcalendarsandthatthestructureoftheUniverse
canbeunderstoodbytheoreticalmodels.
These verses exemplifythe yearningforknowledgeofourplace inthe Universe,
whichissharedbyallancientcultures.Throughtirelessobservations,ourancestorson
differentcontinentsobservedthebehavioroftheSun,Moon,planetsandthestars.They
wereawarethatthesepatternswereregularbutbynomeanssimple.Althoughthedata
collected were similar across cultures, the interpretations of the celestial patterns
differed.Theseinterpretationswereincorporatedintosocial,religious,andphilosoph-
icalstructures.Throughouthistory,theevolutionofourmodelsoftheUniverseledto
changesinthesestructures.Thisbookisanattempttotellthestoryoftheevolutionof
astronomicaldevelopmentovertwomillenniaanditseffectonoursociety.
ix
Contents
1 HumansandtheSky. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1
1.1 RepeatingDaysandNights. . . . . . . . . . . . . . . . . . . . . . . . . . . 3
1.2 CyclesoftheSeasons. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4
1.3 EarlySkyWatchers. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4
1.4 WorshipoftheSun. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5
1.5 TheOrderlyHeaven. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7
1.6 QuestionstoThinkAbout. . . . . . . . . . . . . . . . . . . . . . . . . . . . 9
2 EffectsofCelestialMotionsonHumanActivities. . . . . . . . . . . . . . 11
2.1 DailyMotionoftheSun. . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12
2.2 TheAnnualMotionoftheSun. . . . . . . . . . . . . . . . . . . . . . . . 12
2.3 TheSeasons. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14
2.4 RegularButNotSimple. . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15
2.5 QuestionstoThinkAbout. . . . . . . . . . . . . . . . . . . . . . . . . . . . 15
3 AncientModelsoftheUniverse. . . . . . . . . . . . . . . . . . . . . . . . . . . 17
3.1 ASphericalHeaven. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17
3.2 ChasingtheShadows. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18
3.3 NotAllDirectionsAreEqual. . . . . . . . . . . . . . . . . . . . . . . . . 18
3.4 PathoftheSun. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21
3.5 WhereDoestheSunGoatNight?. . . . . . . . . . . . . . . . . . . . . . 22
3.6 QuestionstoThinkAbout. . . . . . . . . . . . . . . . . . . . . . . . . . . . 24
4 TurningoftheHeavens. . . . . . .. . . . . . . .. . . . . . . . .. . . . . . . . .. 25
4.1 ThePoleofHeaven. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 26
4.2 TheHeavenIsTilted. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 29
4.3 AFreeFloatingEarth. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 31
4.4 QuestionstoThinkAbout. . . . . . . . . . . . . . . . . . . . . . . . . . . . 31
5 ASphericalEarth. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 33
5.1 TheSunMovesinCompleteCircles. . . . . . . . . . . . . . . . . . . . 33
5.2 ADifferentShowforEveryone. . . . . . . . . . . . . . . . . . . . . . . . 35
xi
xii Contents
5.3 EvidenceforaNon-flatEarth. . . . . . . . . . . . . . . . . . . . . . . . . 35
5.4 TheChangingHorizon. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 37
5.5 HowHighCantheSunGo?. . . . . . . . . . . . . . . . . . . . . . . . . . 41
5.6 DifferentLengthsofDaylight. . . . . . . . . . . . . . . . . . . . . . . . . 42
5.7 PoleStarandLatitude. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 42
5.8 CelestialNavigation. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 44
5.9 AreThereStarsWeCan’tSee?. . . . . . . . . . . . . . . . . . . . . . . . 46
5.10 SuccessoftheRound-EarthHypothesis. . . . . . . . . . . . . . . . . . 47
5.11 QuestionstoThinkAbout. . . . . . . . . . . . . . . . . . . . . . . . . . . . 48
6 JourneyoftheSunAmongtheStars. . . . . . . . . . . . . . . . . . . . . . . . 49
6.1 TheSunMovingThroughtheStars. . . . . . . . . . . . . . . . . . . . . 49
6.2 TwoKindsofMotionoftheSun. . . . . . . . . . . . . . . . . . . . . . . 53
6.3 InclinationoftheEcliptic. . . . . . . . . . . . . . . . . . . . . . . . . . . . 54
6.4 PlacingStarsontheCelestialSphere. . . . . . . . . . . . . . . . . . . . 56
6.5 AnAsymmetricUniverse. . . . . . . . . . . . . . . . . . . . . . . . . . . . 58
6.6 QuestionstoThinkAbout. . . . . . . . . . . . . . . . . . . . . . . . . . . . 59
7 ATwo-SphereUniverse. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 61
7.1 AnInnerSphereforHumans,anOuterSphereforCelestial
Objects. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 61
7.2 TheArmillarySphere. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 63
7.3 ArmillarySpheresasObservingInstruments. . . . . . . . . . . . . . 67
7.4 TheTwo-SphereCosmology. . . . . . . . . . . . . . . . . . . . . . . . . . 67
7.5 QuestionstoThinkAbout. . . . . . . . . . . . . . . . . . . . . . . . . . . . 69
8 DanceoftheMoon. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 71
8.1 ShiftingLocationsofMoonrise. . . . . . . . . . . . . . . . . . . . . . . . 72
8.2 TwoDifferentLengthsofaMonth. . . . . . . . . . . . . . . . . . . . . 75
8.3 EclipsesandPhasesoftheMoon. . . . . . . . . . . . . . . . . . . . . . . 77
8.4 SizeandDistancetotheMoon. . . . . . . . . . . . . . . . . . . . . . . . 78
8.5 TheSelf-spinningMoon. . . . . . . . . . . . . . . . . . . . . . . . . . . . . 80
8.6 QuestionstoThinkAbout. . . . . . . . . . . . . . . . . . . . . . . . . . . . 81
9 TheCalendars. . . .. . . . . .. . . . . .. . . . . .. . . . .. . . . . .. . . . . .. . 83
9.1 HowLongIsaYear?. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 84
9.2 StarCalendar. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 84
9.3 WhatDefinesaYear?. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 86
9.4 DifferentCalendarsAroundtheWorld. . . . . . . . . . . . . . . . . . . 87
9.5 ReformoftheJulianCalendar. . . . . . . . . . . . . . . . . . . . . . . . . 89
9.6 WhatIssoSpecialAbouta24-hourDay?. . . . . . . . . . . . . . . . 90
9.7 QuestionstoThinkAbout. . . . . . . . . . . . . . . . . . . . . . . . . . . . 91
10 TheWanderers. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 93
10.1 TheTenPatternsofVenus. . . . . . . . . . . . . . . . . . . . . . . . . . . 96
10.2 MarsatOpposition. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 100