THEEARTHANDBEYONDThesolarsystemTheuniversecontainsover100billiongalaxies.Agalaxyisagroupofbillionsofstars.OurowngalaxyiscalledtheMilkyWay,anditcontainsabout100billionstars,includingourSun.TheSunisatthecentreofthesolarsystem.Thesolarsystemcontainsdifferenttypesofobjectsincluding:*astar-theSun*planets,whichgoaroundtheSun*satellites,whichgoaroundplanets*smallerobjectssuchasasteroidsandcometsHeliocentricmodelPeopleusedtothinkthattheEarthwasatthecentreoftheuniverse,witheverythinggoingaroundit.Wenowknowthatthisisnotcorrect.Theideathatfitsscientificobservationsandallowsustopredictthemovementoftheplanetsiscalledtheheliocentricmodel.ThisjustmeansthattheSunisatthecentreofthesolarsystem,andtheEarthandotherplanetsgoaroundit.WesaythattheplanetsareinorbitaroundtheSun.TheplanetsinorderfromtheSunMercurytoPlutoAlthoughscientistshavediscoveredsomeextraobjectsorbitingtheSunthatmaybeplanets,normallyweassumethattherearenineplanetsinthesolarsystem.Inorder,workingawayfromtheSun,theseare:Mercury,Venus,Earth,Mars,Jupiter,Saturn,Uranus,NeptuneandPluto.Ifyoucantrememberthecorrectorder,trythissentence,ormakeoneupofyourown:MyVeryEasyMethodJustSpeedsUpNamingPlanetsGravitationalforcesAllobjectshaveaforcethatattractsthemtowardseachother.Thisiscalledgravity.Gravityonlybecomesnoticeablewhenthereisareallymassiveobjectlikeamoon,planetorstar.YoucanfindoutmoreaboutgravityintheRevisionBiteForcesandmotion/Massandgravity.GravityandthesolarsystemGravitationalforcesbetweentheSunandplanetskeeptheplanetsmovinginorbitaroundtheSun.Withouttheseforces,theplanetswouldflyoffintodeepspace.Theirorbitsareslightlysquashedcirclescalledellipses.Diagramsoftenshowtheorbitsasverysquashed,butthisisjusttogetasenseofperspectiveandtofitthediagramsontothepage.AstheplanetsgetfurtherfromtheSun:*thespeedthattheplanetmovesdecreases*thetimetocompleteanorbitincreasesOrbitgraphForexample,Mercurytakesjust88Earthdaystocompleteanorbit,whilePlutotakes248Earthyearstocompleteanorbit.TheplanetsfurthestfromtheSunarealsothecoldestbecausetheyreceivetheleastheatenergyfromtheSun.DaysandnightsTheplanetsspinastheyorbittheSun.Adayisthetimetakenforaplanettomakeonecompleteturnonitsaxis.Differentplanetstakedifferentamountsoftimetodothis.AnEarthdayis24hours-ittakestheEarth24hourstomakeonecompleteturnonitsaxis.ItisdaytimeforuswhenourpartoftheplanetisfacingtheSun,anditisnight-timeforuswhenourpartoftheplanetisfacingawayfromtheSun.OnewaytorememberwhichwaytheEarthturnsistorememberWEspin,whichmeanstheEarthspinsfromwesttoeast.DaysDuringtheday,theSunappearstomovethroughthesky.RememberthatthishappensbecausetheEarthisspinningonitsaxis.IfwelooksouthandplottheSunintheskyduringtheday,wefindthatitappearstomovelikethis:SunintheskyPTheSunappearstomovefromeasttowest.Itrisesintheeastandsetsinthewest.Atmiddaywefacesouthtoseeit.NightsDuringthenight,wecannotseetheSun.ButtheEarthisstillspinningonitsaxis.Thismeansthatthestarsappeartomovefromeasttowestinthesky,justastheSundoesintheday.StartrailsmadebyleavingthecamerashutteropenduringthenightYearsandseasonsYearsAplanetsyearisthetimetakenforittomakeonecompleteorbitaroundtheSun.TheEarthtakes365EarthdaystoorbittheSun.Rememberthatdifferentplanetstakedifferentlengthsoftimetodothis-thefurtherfromtheSun,thesloweraplanettravelsandthelongeritsorbittakes.SeasonsTheEarthsaxisistiltedslightlycomparedtothewayitorbitstheSun.BecauseofthistiltingoftheEarthsaxis,wegetthedifferentseasons(winter,spring,summerandautumn).*WhenitissummerintheUK,theNorthernHemisphereistiltedtowardstheSun.*WhenitiswinterintheUK,theNorthernHemisphereistiltedawayfromtheSun.SeasonsintheNorthernHemisphereThingstonoteWhenitissummerintheNorthernHemisphere,itiswinterintheSouthernHemisphere.TheSunappearstobehigherintheskyinthesummerthanitdoesinthewinter.SeasonsintheNorthernHemisphereTheMoonTheMoonisthenaturalsatelliteoftheEarth.ItorbitstheEarthbutno-onebuiltitorlauncheditintospace.GravitationalforcesbetweentheMoonandtheEarthkeeptheMooninorbit.TheMoonisnotaluminousobject,soitdoesnotmakeitsownlight.Instead,itreflectslightfromtheSunwhich,liketheotherstars,isaluminousobject.Asatellitehastotravelatthecorrectspeedtostayinorbitataparticulardistancefromaplanet.IttakestheMoon28daystomakeacompleteorbitoftheEarth.Asitorbits,weseedifferentamountsoftheMoon,soweseedifferentphasesoftheMoon.RememberthatwecanonlyseethepartoftheMoonthatreflectstheSunslight.HowmuchofthatwecanseedependsuponthepositionoftheMooninitsorbit.ThephasesoftheMoon,seenfromtheEarthAcommonmistakeistothinkthatthephasesoftheMoonhappenbecausetheEarthgetsintheway,andcastsashadow.However,ifthisdoeshappen,wegetaneclipseoftheMoon:ThephasesoftheMoon,seenfromtheEarth.ArtificialsatellitesandspaceprobesArtificialsatellitesArtificialsatellitesarebuiltbypeopleandlaunchedintoorbitaroundtheEarth.Theyareveryusefultous.Thesearesomeofthethingsthatartificialsatellitesareusedfor:*Communications,includingbroadcastingtelevisionprogrammesandrelayingtelephonecalls*GlobalPositioningSystem(GPS)andGalileosatellitesfornavigation*Collectinginformationtohelpwithweatherforecasts*ScientificsurveysoftheEarthssurface*Mapmaking*SpyingSomesatellitesareusedtocollectinformationabouttheplanetsandstars.TheHubbleSpaceTelescopecanseefurtherintospacethantelescopesbasedontheground.Itsviewisnotblockedbycloudsanditdoesnthavetowaitfornight-time.However,itisdifficultandexpensivetolaunchandmaintain.Ifanythinggoeswrong,onlyastronautscanfixit.Spaceprob