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Chapter 1 The Solar System（第1页）

Chapter1　TheSolarSystem

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&sinhistory

&hecursesoflightpollutionandsmog，peopleweremorefamiliarwiththenightskythaobetoday.Plaheskywerereizedasspetcultures，becausetheyare‘waars’thatmigrateagainstthebadofthe‘fixed’stars.Fiveplashavebeenky:Merus，Mars，Jupiter，andSaturn–whicharetheoenoughtoetotheattentionoftheunaidedeye.Ofcourse，theSunandMoonwereobvioustoo，butthe‘plas’appearasointsoflight，whereastheSunandMoonshowdisksaardeddifferently.Throughoutmostofhumaehwasimagireof，uoobjethesky，soitwasnotthoughtofasapla.

&ellectualleapsthatreizedthattheEarthisaballrouhattheplasdolikewise，andthattheEarthisjustoheirnumberwerealongtimeing.Theprocesswasslow，andthereweremanyfalsedawns.DuriuryBtGreekphilorascorrectlysurmisedthattheMoonisasphericalbodyreflegthelightoftheSuintoexileonatofhisbeliefs.Inthesugturies，variouseseastronomersdevelopedsimilarideas，buttheideaoftheMoonasaglobeprobablydidselfintopopularessuntilitsappearahroughateleseknthe17thtury.

Asfortheplaheyweregenerallyregardedaspointsoflightgoingrouilthetuitive‘heliotric’viewwiththeSureofmotioed.TheearliestwrittehattheEarthgoesroundtheSunodiaingfromthe9thturyBC，butdespitethisaisuggestions，notablybyHellenidIslamideventuallybyNikolasi1543，theotachieveastilthe18thtury.PartlyonatofhisadvocacyoftheheliotricthealileoGalilei（whhhistelescopehadseenmountaihephasesofVenus，andfourtinymJupiter）washeldufrom1633untilhisdeathin1642.

Simplybyrevealisastinybutdisiblediscs，whereasthestarsremainedaspointsoflight，useofthetelesthestartofthe17tharkedplasasfuallydifferenttostars，ahtthemasworldsparabletoourowally，thatstarsaremuchbiggertha（exceptfortheSun）theyaresoverymuchmoredistantthatonlyinafewevesophistioderntelescopesshowaails（onphotographs，brightstarslookbiggerthanfaintstars，butthatisjustanopticaleffect–thebrightnessisbei）.

Kepler’slawsofplaion

&sslottedifulplahumahankstoJohannesKepler’s（1609）realizatios（ingtheEarth）travelroundtheSuninpaths（orbits）thatareellipsesratherthacircles，coupledwithIsaa’s（1687）insightintogravitythatexplaihismotioheirdistandsizesrelativetotheEarthcouldbegintobededuced.

Ayoumightthinkofasan‘oval’.Mathematically，itisdefinedasaclosedcurvedrawnabouttwopoints（thefocioftheellipse）suchthatthesumofthedistaneachfoypointonthecurveisidentical.Acircleisaspedofellipseinwhichthetwofocide，atthecirtre.Thefurtherapartthefoci，themated，or‘etric’，theellipse.Keplerdeducedthatplasfollowellipticalorbits，withtheSunatonefocusofeachellipse（theotherfogempty）.ThepointonanorbitclosesttotheSuniscalled‘perihelion’（Greekfor‘closesttotheSun’），afurthestahelion’（Greekfor‘furthestfromtheSusarenlyetridifyouseethemdrawniheylookverymuchlikecirple，whenMarsisataphelionitsdistaheSuhahaisatperihelion，ahedifferenly4%.

&lyfamousforhisthreelawsofplaioLawissimplythestatementthateaetmovesiicalorbit，withtheSuheSedLawdescribeshowthespeedofaplavariesarounditsorbit:aplaerthecloseritistotheSun（forreasolyexplaiheravity）suimaginarylineliotheSuanequalareaiime.Kepler’sThirdLalaalperiod（howloopleteacirdtheSun）tedistaheSun:thecubeoftheorbitalperiodisproportionaltothesquareoftheaveragedistaheaveragedistanplaoSunturnsouttobeequaltohalfthelengthoftheorbitalellipse’slongaxis（its‘semi-majoraxis’）or，ifyouprefer，halfthestraightliaweenperihelionandaphelion.

Kepler’slawsofplaionenabledprecisecalofthesizesoftheorbitsofotherplahanaccuracylimitedalmosteheuyihesizeoftheEarth’sorbiteasured.Evenaslongagoas1672，simultaionsofMarsfromwidespreadloabledtheEarth–Suobemeasuredasabout140milliores，remarkablyclosetothecorrectvalueof149，597，871kilometres.ObservatioofVenusacrosstheSun’sdis1761and1769（thelatterrequiringCooktostatioahiti）producedarevisedestimateof153±1milliores.Despitetheseaifices，whiuedtthenafullyself-tamodeloftheSolarSystem’ssature，apapalban‘heliotrieremaiil1822.

Youwouldbeexcusedfthatoaoaplaablished，woutitssizewouldbetrivial.However，thesmallnessofaplaarydiscthrougheveelescope，coupledwiththeshimmerih’satmosphere，leadstosignifityiheangularsizeoftheplaherwords，hoears）.Forexample，whenhediscoveredUranusin1781，WilliamHerschel’smeasurementofitsdiscwas8%te.Ratherthantryingtaplalooks，themostprecisetelescopiissizeistotimehowloopassinfrontofastar.Such‘os’arerareevents，butbythecloseofthe19thturythesizesoftheplashadbeeermihsiderableaccuracy（Table1）.

HerscheldiscoveredUranusbyat，butunewaslo1846asaresultofadeliberatesearch，guidedbyslightperturbatioofUranus（distitfromaperfectellipse）thatcouldbebestexplaiionalinfluenu.Whenithadbeeedfeofuroshoointingtoafurtheru.ThistriggeredasearchthatfoundPlutoin1930.Atfirst，astronomersassumedthatthisnewlyhailednibesimilarinsizeandmasstoUraune.However，by1955ithadbeensholutoerthah;iewasreviseddownwardstothesizeofMars;andin1978itssurfacewasfouedbyhighlyrefleethanewhitthatitsphysicalsizehadtobeeveoremaihitsthtness.thatPluto’sdiameterisonly2，390kilometres，soitssizeissmaller（and，infact，itsmassismuchsmaller）eveheappareiouthat，ratherfortunately，ihesearchforPlutoarenowattributedtoobservationalinaccuracies.

&hesizesoftheplaorialdiameters）

*C.Flammarion，PopularAstronomy（dWindus，Piccadilly）

Plutolostitsstatusasanofficiallyreizedplain2006.Thatwasaove，thoughinmyopione.Befhowthiscameabout，IwillreviewtheheSolarSystemasitisood.

&heSolarSystem

TheSun

IheSolarSystemistheSun，whichisafairlyordinarystar，poweredbytheenintoheliumbynuclearfusioniheSueris109timesanditsmassisnearly333，000timesgreaterthah’s.Itsabout740timesmoremassthahiheSolarSystemputtogether.tly，theSun’sgravityissodominantthatobjetheSolarSystemorbittheSuninalmosttheperfectellipsesreizedbyKepler.Perturbationstoaplacausedbyotherplahoughtheybemeasured.

&s

Table2summarizessomebasicpropertiesoftheplaedrelativetotheEarthtoaveaheSuronomiits’，abbreviatedasAU，defiheaverageEarth–Suhisisfairlysimpletorememberas（nearenough）150milliores.AplaalperiodishowloopleteoroundtheSun，whichisofcourseitsowheorbitalperiodsanddistaheSuninthistablearerelatedtoeachotherbyKepler’sThirdLaw.tly，thismeansthatthesquareofaalperiod（ih-years）isequaltothecubeofitsaveragedistaheSun（ih’smassisverynearly6millionbillionbillionkilograms（or6thousandbillionbilliontoheparistotheEarthratherthanquotingstaifiitssus，sedmetres.

Rotationperiodishowlongittakesaplaospisaxis.Forarapidlyspinhisisalmostthesameasthetimefromohe’sowh’），buttherelationshipisbecauseaplaalmotionuouslygesthedirebetweeandSuh’srotationperiodis23hoursand56mitakesexactly24hourstorhttheSunbaepointinthesky.Fromaplaive，theSuelyroundtheskyduringthecourseofasi，inadditiontothegdiretowardstheSunfromanypoi’ssurfacecausedbytheplaiowhoserotatioidallylockedsothatitrotatedexaceperorbit（synousrotation）wouldkeepolytowardstheSun.Mercurydoeshis，butrotatesexactlythreetimesduringthecourseoftwoorbits，asaresultofwhisrelativetotheSuwoorbits，soitsdayistwigasitsyear.

Table2　Somepropertiesoftheplaspared.DistaheSuedistance.YearsanddaysareEarth-yearsah-days.SeeTable1forsizes

Thereisacharacterbetweenthefourihefourouteroheis（Merus，Earth，andMars）arerelativelysmallandloaredtotheouterfour（Jupiter，Saturn，Uranus，ahereisalsoatrastiies，theisbeiheouteroheisarecalledthe‘terrestrialplas’，signifyingthattheyareall‘Earth-like’.Theouterfias’.Somecallthem‘gasgiahefactthattheyhavesomuchhydrogehersreservethatparticulartermforjustJupiterandSaturn，whicharemassythahougheveaihah-massofgas.

Figure1isamapoftheSolarSystem，showingorbitstoscale，exceptthattheorbitsofVenusandMercuryaretoosmalltoiofPluto’sorbitisshowhdislater.SomethingthatIhaveiowithoutwhichsuchamapotbedralaaryorbitsalllieapproximatelyinthesameplaotheEarth’sorbit，whichmakesatrefereneknowic’，Pluto’sorbitisi17.1°，Mercury’sat7.0°，Venus’sat3.4°，ahersatlessthan3°.