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Chapter TheearthandtheskyCelestialsphere天球Objectsveryfarawayfromusappeartobeofequaldistancefromus(Fig.3-1)—infact,anillusion.Fig.3-1Fig.3-1(Top)Supposewearestandingunderthe“Ricecooker”(飯煲)ofScienceCentre.Mr.AandMr.BarenowstandingattwocornersofSignalFireTerrace烽火臺).(BottomAlthoughMrAisnearertousthanMr.B,sincetheyarefarawayfromustheyseemtobeofthesamedistancefromSimilarly,sinceallstarsareextremelyfarawayfromus,theyappeartobeofthesamedistancefromus.Weareatthecentreofahugehollowimaginaryspheresurroundingtheearth.Allcelestialbodiesareimaginedtoattachtotheinsideofthesphere—Celestialsphere(天球)(Fig.3-2).Aconvenientmodeloftheskyformappingtheapparentpositions(視位置)motionsofcelestialbodiesasseenfromtheBecauseweareverytinycomparedwiththeearth,thesurfaceoftheearthlooksflatandseemstoextendtoveryfaraway.Asaresult,wecanonlyseehalfofthecelestialsphere(Fig.3-3).Whichhemispherewecanseedependsonourposition.Fig.3-2Fig.3-2Thestarsareveryfaraway.Theyseemtobeofthesamedistancefromus.Asaresult,theyseemtoattachonanimaginarysphericalsurface,theCelestialsphere.Weareatthesphericalcentre.Fig.Fig.3-3WhichhalfofthecelestialspherewecanseedependsonourpositionontheDailymotion周日運動Iftheearthdoesnotspin(rotatesaboutitsownaxis),weshallalwaysseethesamehalfofthecelestialsphere(andneverseetheother,unlesswemovetoanotherThespinning/self-rotationoftheearthintroducescomplications.Beforewediscusstheself-rotationoftheearth,letustalkaboutseveralphenomenafirst.AsseenatHongKong,everystar(includingthesun)movesfromEast(E)toWest(W)alongacircle,whoseplanemakesanangleof22.5owiththevertical(Fig.3-4).1Fig.3-4Fig.3-4AsseenatHongKong,everystarmovesfromEtoWalongacircle,whoseplanemakesanangleof22.5owiththevertical.Fig.3-5(Left)AsseenattheNorthPole,allstarsmoveanticlockwisealonghorizontalcircularpathsasseenfrombelow.(Right)AttheSouthPole,wecannotseePolaris.Allstarsalsomovealonghorizontalcircularpathsbutintheclockwisesense.Fig.3-6Asseenattheequator,allstarsmovealongverticalcircles.1Youmayknowthatthis22.5oisactuallythelatitude緯度ofHongKong.Pleasedonotmixupthiswithanotherangle23.5o,whichwillbediscussedinthenextchapter.Fig.3-9Duetotheself-rotationoftheearth,peopleattheequatorFig.3-9Duetotheself-rotationoftheearth,peopleattheequatorcanseethestarsmovealongverticalcircles.Asseenattheequator,allstarsmovealongverticalcircles(Fig.3-Forallobserversinthenorthernhemisphere,Polarisseemsstationary.Starsnearitcirculatearounditanticlockwise.Nomatterwherethecircularpathis,thestaralwaysspendsonedaytocompleteonecycle.ThisdailycircularmotioniscalledtheDailymotion(周日運動)ofthestars.Alltheseeminglydifferentphenomenacanbeexplainedsimplybytherotationofthehorizon,whichisaresultoftheSelf-rotation(自轉(zhuǎn))oftheearthfromWest(W)toEast(E),thatis,anticlockwiseasseenfromabovetheNorthPole(Figs.3-throughFig.3-7Fig.3-7Themotionofstarscanbeexplainedbytheself-rotationoftheearth.Moreover,peopleatthesamelatitudecanseePolarisatthesameangleofelevation.Fig.3-8ThehorizontalpathsofthestarsasobservedattheNorthPolecanbeexplainedbytheself-rotationoftheearth.SincetheearthrotatesfromWtoE,celestialbodiesappeartomoveintheoppositedirection(EtoW)inthesky,e.g.,thesunrisesintheEandsetsintheTerminologiesofthecelestialNorth/Southcelestialpoles:TheNorth(N)andSouth(S)polesofthecelestialspherearelocateddirectlyabovetheNandSpolesoftheearth,respectively.Thepolesdefinetheaxisofdailyrotation.(Fig.3-10)Celestialequator:Animaginarycircle,whichdividesthecelestialsphereintotheNandShemispheres.(Fig.3-10)Fig.3-10Fig.3-10Thepositionsofthenorthandsouthcelestialpolesandthecelestialequator.Fig.3-11ThedefinitionsofR.A.andDEConthecelestialsphereCoordinatesonthecelestialsphere:Declination(赤緯)andRightascension(赤經(jīng))aresimilartothelatitude緯度andlongitude經(jīng)度ontheearth,respectively.(Fig.3-11)Declination(DEC):Theanglevariesfrom-90o(SCP)to90oRightascension(R.A.):MeasuredfromtheVernalEquinox(春分點)2towardstheeastinunitsofhour.1hour=15o.Weshallexplainthevernalequinoxlater.Apparentsizesandseparationsofcelestialbodiesaremeasuredin1circle=360(degrees),1=60(minutes),1=60(seconds).e.g.,apparentdiametersofthesunandmoon0.5(Fig.3-12),widthofafingeratarmslength1,widthofthefistatarmslength2Fornow,rememberthatthevernalequinoxisafixedpointonthecelestialsphere.ItsR.A.isalways0hr,anditsDECisalways0.Fig.Fig.3-12Yourhandheldatarm’slengthmakesaconvenientmeasuringtool.Yourfistisabout10,yourfingerisabout1wide,andyourspreadfingersspanabout18.Fig.3-13Fig.3-13Thecelestialspheremodelstheappearanceofthesky.Thepolesmarkthepivots,andtheequatordividestheskyintohalves.Thoseobjectsbelowourhorizonareinvisible.Theearthrotateseastward,makingobjectsintheskyappeartorisefromtheeasternhorizonandsettowardsthewesternhorizon.Fig.Fig.3-14ForanobserveratlatitudeLonEarth,theNcelestialpoleisatanangle=abovethenorthernPolaris(thePolestar)isabrightstarclosetotheNcelestialitappearsalmostPositionofPolarisdependsonthelocationoftheobserveronEarth,forianobserveratNPoleseesPolarisatthezenith天頂(directlyii.anobserveratEquatorseesPolarisatthenorthernMoregenerally,angleofPolarisabovethehorizon=latitudeofthe(Figs.3-13and3-Constellations:Visualgroupingsofstars(Fig.3-Moderndefinition:Regionsonthecelestialspherewithwell-definedTotally88today,someaddedinmoderndays(e.g.Telescopium望遠鏡座Usuallynorealcorrelationamongthestarsinthesameconstellations;theycouldbeveryfarawayfromoneanother(Fig.3-16).TheyriseandfallwiththerotationofthecelestialWhichoneswecanseedependontheobserverslocationone.g.Anobserverlivinginthenorthernhemispherecanneverseesomesouthernconstellations(theyneverriseabovethehorizon,seealsoFig.3-13),buthe/shecanalwaysseesomeconstellationsaroundtheNcelestialpole(theyneverset).Fig.Fig.3-15(aTheconstellationofOrion獵戶座Thebrighteststarsareconnectedtoformtheoutlineofahunt