(光度光的强度)课件

TheSkyChapter20Thepreviouschaptertookyouonacosmiczoomtoexploretheuniverseinspaceandtime.Thatquickpreviewonlysetsthestageforthedramatocome.NowitistimetoreturntoEarthandlookcloselyatthesky.Tounderstandwhatyouareintheuniverse,youmustknowwhereyouare.Asyoulookatthesky,youcananswerthreeessentialquestions:

Howdoastronomersrefertostarsbynameandbrightness?HowdoestheskymoveasEarthmoves?HowdoestheskyaffectEarth?Guidepost0AnsweringthosequestionswilltellyouagreatdealaboutyourselfandyourhomeonplanetEarth.Threeadditionalquestionswilltellyoumoreabouthowscienceworks.

Howdoweknow?

Whatisascientificmodel?

Howdoweknow?

Whatisthedifferencebetweenascienceandapseudoscience?

Howdoweknow?

Whyisevidencecriticalinscience?Inthenextchapter,youwillstudythemotionsofthemoonanddiscoveryetanotherwaythatmotionsintheskyaffectyourlifeonEarth.Guidepost(continued)0I.TheStars A.Constellations

(星座) B.TheNamesoftheStars C.TheBrightness(亮度)

ofStars D.Magnitude(星等)

andIntensity

(光度=光的強度)II.TheSkyandItsMotion A.TheCelestialSphere天球 B.Precession

進動III.TheCyclesoftheSun A.TheAnnualMotionoftheSun B.TheSeasonsC.TheMovingPlanetsOutline0V.AstronomicalInfluencesonEarth'sClimate A.TheHypothesis B.TheEvidenceOutline(continued)0ConstellationsInancienttimes,constellationsonlyreferredtothebrighteststarsthatappearedtoformgroups.0Constellations(2)Theywerebelievedtorepresentgreatheroesandmythologicalfigures.Theirpositionintheskyseemedtotellstoriesthatwerehandeddownfromgenerationtogenerationoverthousandsofyears.0Constellations(3)Today,constellationsarewell-definedregionsonthesky,irrespectiveofthepresenceorabsenceofbrightstarsinthoseregions.0Constellations(4)Thestarsofaconstellationonlyappeartobeclosetooneanother.Usually,thisisonlyaprojectioneffect.Thestarsofaconstellationmaybelocatedatverydifferentdistancesfromus.0Constellations(5)StarsarenamedbyaGreekletter(a,b,g)accordingtotheirrelativebrightnesswithinagivenconstellation+thepossessiveformofthenameoftheconstellation:OrionBetelgeuseRigelRigel=bOrionis0Betelgeuse=aOrionisConstellations(6)0SomeexamplesofeasilyrecognizableconstellationsandtheirbrighteststarsTheMagnitudeScaleFirstintroducedbyHipparchus(160-127B.C.):Brighteststars:~1stmagnitudeFainteststars(unaidedeye):6thmagnitude0Morequantitative:1stmag.starsappear100timesbrighterthan6thmag.stars1mag.differencegivesafactorof2.512inapparentbrightness(largermagnitude=>fainterobject!)BetelgeuseRigelMagnitude=0.41magMagnitude=0.14magTheMagnitudeScale(Example)Magn.Diff.IntensityRatio12.51222.512*2.512=(2.512)2=6.31……5(2.512)5=100Foramagnitudedifferenceof0.41–0.14=0.27,wefindanintensityratioof(2.512)0.27=1.28.0Inotherwords:Rigelis1.28timesbrighterthanBetelgeuse.TheMagnitudeScale(2)Sirius(brighteststarinthenightsky):mv=-1.42

Fullmoon:mv=-12.5

Sun:mv=-26.5Themagnitudescalesystemcanbeextendedtowardsnegativenumbers(verybright)andnumbersgreaterthan6(faintobjects):0TheCelestialSphereCelestial

equator=

projectionof

Earth’s

equatoronto

thec.s.Zenith=Pointonthecelestialspheredirectlyoverhead0Nadir=Pointonthec.s.directlyunderneath(notvisible!)North

celestialpole

=projectionof

Earth’s

northpole

ontothec.s.DistancesontheCelestialSphereThedistancebetweentwostarsonthecelestialspherecanonlybegivenasthedifferencebetweenthedirectionsinwhichweseethestars.0Therefore,distancesonthecelestialspherearemeasuredasangles,i.e.,indegrees(o):Fullcircle=360oarcminutes(‘):1o=60’arcseconds(“):1’=60”TheCelestialSphere(2)Fromgeographiclatitudel(northernhemisphere),youseethecelestialnorthpoleldegreesabovethenorthernhorizon;l90o-lCelestialequatorculminates90º–labovethehorizon.Fromgeographiclatitude–l(southernhemisphere),youseethecelestial

southpolel

degreesabove

thesouthern

horizon.0TheCelestialSphere(Example)TheCelestialSouthPoleisnotvisiblefromthenorthernhemisphere.HorizonNorthCelestialNorthPole40.70South49.30CelestialEquatorHorizonNewYorkCity:l

≈40.7º0TheCelestialSphere(3)0ApparentMotionofTheCelestialSphere0Lookingnorth,youwillseestarsapparentlycirclingcounterclockwisearoundtheCelestialNorthPole.ApparentMotionofTheCelestialSphere(2)0SomeconstellationsaroundtheCelestialNorthPoleneverset.Thesearecalled“circumpolar”.Thecircleonthecelestialspherecontainingthecircumpolarconstellationsiscalledthe“circumpolarcircle”.

ApparentMotionofTheCelestialSphere(3)0Lookingeast,youseestarsrisingandmovingtotheupperright(south)Lookingsouth,youseestarsmovingtotheright(west)Precession(1)TheSun’sgravityisdoingthesametoEarth.Theresulting“wobbling”ofEarth’saxisofrotationaroundtheverticalw.r.t.theEcliptictakesabout26,000yearsandiscalledprecession.Atleft,gravityispullingonaslantedtop.=>Wobblingaroundthevertical.0Precession(2)Asaresultofprecession,thecelestialnorthpolefollowsacircularpatternonthesky,onceevery26,000years.ItwillbeclosesttoPolaris~A.D.2100.ThereisnothingpeculiaraboutPolarisatall(neitherparticularlybrightnornearbyetc.)~12,000yearsfromnow,thecelestialnorthpolewillbeclosetoVegaintheconstellationLyra.0TheSunandItsMotionsEarth’srotationiscausingtheday/nightcycle.0TheSunandItsMotions(2)TheSun’sapparentpathontheskyiscalledtheEcliptic

(黃道).Equivalent:TheEclipticistheprojectionofEarth’sorbitontothecelestialsphere.DuetoEarth’srevolutionaroundthesun,thesunappearstomovethroughthezodiacalconstellations.0TheSunandItsMotions(3)0The13thzodiacalconstellations→Ophiuchus

(蛇夫座)TheSeasonsEarth’saxisofrotationisinclinedvs.thenormaltoitsorbitalplaneby23.5°,whichcausestheseasons.0TheSeasons(2)TheSeasonsareonlycausedbyavaryingangleofincidenceofthesun’srays.0WereceivemoreenergyfromthesunwhenitisshiningontotheEarth’ssurfaceunderasteeperangleofincidence.TheSeasons(3)TheseasonsarenotrelatedtoEarth’sdistancefromthesun.Infact,Earthisslightlyclosertothesunin(northern-hemisphere)winterthaninsummer.LightfromthesunSteepincidence→SummerShallowincidence→Winter0TheSeasons(4)Northernsummer=southernwinterNorthernwinter=

southernsummer0TheSeasons(5)Earth’sdistancefromthesunhasonlyaveryminorinfluenceonseasonaltemperaturevariations.SunEarthinJulyEarthinJanuaryEarth’sorbit

(eccentricitygreatlyexaggerated)0TheMotionofthePlanetsTheplanetsareorbitingthesunalmostexactlyintheplaneoftheEcliptic.JupiterMarsEarthVenusMercurySaturnTheMoonisorbitingEarthinalmostthesameplane(Ecliptic).0TheMotionofthePlanets(2)Allouterplanets(Mars,Jupiter,Saturn,Uranus,NeptuneandPluto)generallyappeartomoveeastwardalongtheEcliptic.TheinnerplanetsMercuryandVenuscanneverbeseenatlargeangulardistancefromthesunandappearonlyasmorningoreveningstars.0TheMotionofthePlanets(3)Mercuryappe