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Chapter 1 The Solar System（第1页）

Chapter1　TheSolarSystem

&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°。

1。MapoftheSolarSystem，showiaryorbitsatthecorrectrelativesizes。Orbitsareonlyslightlyetric，solookvirtuallyindistinguishablefromcircles。TheunlabelledsideMars’sorbitistheEarth’sorbit，heorbitsofVenusandMeralltoioisnotaplasorbitisshowisrepresentativeenumberofsmallbodiesbeyo