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The Celestial Sphere

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The Celestial Sphere KnowingtheHeavens SurveyofAstrophysics110 KnowingtheHeavens SurveyofAstrophysics110 KnowingtheHeavens SurveyofAstrophysics110 KnowingtheHeavens • Goals: – Toseehowtheskychangesduringanightandfrom nighttonight. – Tomeasurethepositionsofstarsincelestial coordinates. – Tounderstandthecauseoftheseasons. • ConstellationsintheSky – 6000starsvisiblebyeyeinthenightsky.Ancient civilizationstracedthepatternsofthesestarsinto figures- constellations(groupsofstars). – 88constellationsinthesky(e.g.Ursa Major- big dipper,Orion). – Starsinconstellationsaregenerallynot physically closetoeachother(e.g.Orion). KnowingtheHeavens SurveyofAstrophysics110 KnowingtheHeavens SurveyofAstrophysics110 KnowingtheHeavens SurveyofAstrophysics110 Thechangingnightsky • Positionschangeduringanight. – Duringthenightthepositionsofstarsandmoonchange “DiurnalMotion”. – EarthrotatesfromWesttoEastsostarsappeartoriseinthe eastandsetinthewest(e.g.arotatingchair). seeFigure2-4 – Earthrotates360o in24hours:15o/hr – Thisisarapidrotation. • Positionschangenighttonight. – TheearthorbitstheSunin365.25days(0.986o/day). seeFigure2-5 – Thepositionsoftheconstellationschangeslowlyduringthe year. – Skyrotatesatabout30o or2hoursamonth. – Thisisaslowrotation. KnowingtheHeavens SurveyofAstrophysics110 KnowingtheHeavens SurveyofAstrophysics110 KnowingtheHeavens SurveyofAstrophysics110 StarnamesandCatalogs • EarlyCatalogs – Mappingandcatalogingthestarshasbeeninbusinessfor1000s ofyears. – Positionsofstarshavebeenusedtotelltheseasonand navigation. • “Modern”Catalogs – JohannBayer(1603) • usedconstellationsandaddedGreekletterse.g.α-Lyrae isthe brighteststar(Vega). • 24x88(constellations)possiblenames. – BonnerDurchmusterung (1850s) • compiledbyFWArgelander (manyyears) • 324,188stars,e.g.BD+5o 1668 – HenryDraper(1911) • 225,300stars,e.g.HD87901 KnowingtheHeavens SurveyofAstrophysics110 KnowingtheHeavens SurveyofAstrophysics110 KnowingtheHeavens SurveyofAstrophysics110 Clip KnowingtheHeavens SurveyofAstrophysics110 Starnamesandcatalogs • CurrentCatalogs – Newtechnologies • PhotographicplatesorCCDimagingsurveysenablemillions ofstarstobecataloged. – HSTGuidestarCatalog(Version1) • 15,169,873stars(accuratepositions) • namesarecelestialcoordinates. – HSTGuidestarCatalog(Version2) • >100,000,000starscataloged • positionsaccurateto<1arcsec (in1500spositionswere measuredto1arcmin) • Cataloguenames – Starscanhavemanynamesinmanydifferent catalogs. KnowingtheHeavens SurveyofAstrophysics110 KnowingtheHeavens SurveyofAstrophysics110 TheCelestialSphere • Noabsolutereferenceframe – Apersononearthcanbedescribedby2angles (longitudeandlatitude). – Starsandgalaxiesappear tobeonthesurfaceofa sphere“celestialsphere”withtheearthatthecenter. – Anystarcanthenbedescribedby2 angles/coordinates(RAandDec). • Orientationofthecelestialsphere – PolesaretheaxesoftheEarth’srotation. – Directlyoverheadiscallthe“zenith”. seeFigure2-10 • Apersoncanonlysee90o inanydirection(onehemisphere). • Apersonat35o northcanseethepolestarallyearandnever seethesouthpole. KnowingtheHeavens SurveyofAstrophysics110 KnowingtheHeavens SurveyofAstrophysics110 KnowingtheHeavens SurveyofAstrophysics110 CoordinateSystems • Longitudeandlatitude – OntheskywecalltheseanglesRightAscensionandDeclination. seeBox2-2 – Declination(Latitude) • Measuredindegrees • (North)90o <Dec<-90o (South) – RightAscension(Longitude) • Measuredinhours(15o perhour). • Startingpointisdefinedasthevernalequinox(March). • RAistheangulardistanceeastwardalongthecelestialequator. – Example: • Ifastaroverhead(zenith)hasanRA8hr30m0sthen2.5hourslater astarat10hr0m0swillbeoverhead. • Usingtime(notangles)makesiteasytoknowwhenastarisvisible. KnowingtheHeavens SurveyofAstrophysics110 KnowingtheHeavens SurveyofAstrophysics110 TheSeasons • RotationoftheEartharoundthesuncausesthe seasons – ThedistancetotheSundoesNOTcauseseasons • TheseasonsareoppositeintheNorthernandSouthern Hemispheres. • TheEarthsdistancefromtheSunchangesbyonly3%. • TheEarthisclosesttotheSuninJanuary. – SeasonsarecausedbythetiltoftheEarthsaxisof rotation. seeFigure2-11 • Theearthistilted23.5o fromtheperpendicular. • WhentheNorthernhemisphereistitledtowardstheSunitis summer. • WhentheNorthernhemisphereistilterawayfromthesunit iswinter. KnowingtheHeavens SurveyofAstrophysics110 KnowingtheHeavens SurveyofAstrophysics110 TheSeasons • Summer – DuringthesummerapointontheEarthhasmore than12hoursofdaylight. – Itsishotterduetothelengthofthedayand theangle thatthesunmakes seeFigure2-12 – 1/2ayearlatertheNorthernhemispheretiltsaway fromthesun(winter). • ApparentmotionoftheSun – TheSunspositionontheskychangesovertheyear. – Itappearstotraceacircularpath- the“Ecliptic”. • TheSunmovesat1o perdayaroundtheecliptic(westto east). • Theeclipticisinclinedtothecelestialequatorby23.5o (due totheearthstilt). KnowingtheHeavens SurveyofAstrophysics110 KnowingtheHeavens SurveyofAstrophysics110 KnowingtheHeavens SurveyofAstrophysics110 KnowingtheHeavens SurveyofAstrophysics110 KnowingtheHeavens SurveyofAstrophysics110 TheEquinoxesandSolstices • Theeclipticisinclinedat23.5o – TheSunrevolvesaroundtheEarthatanangletotheCelestial Sphere seeFigure2-14 – Thepointsweretheorbitcrossesthecelestialequatorarethe equinoxes. • Thesunappearstobedirectlyovertheequator. • DayandNightareofequallength • Springequinoxoccursaround21stofMarch. – ThepointswhentheSunreachesthefarthestNorth/Southare “solstices”. • solstice=“standstill” • representsthelongestday(landofthemidnightsun). • Usuallyaround21stofJune. – ThemovementoftheSunmeansitfollowsdifferenttracks acrosstheskyandthelengthofthedayvaries. KnowingtheHeavens SurveyofAstrophysics110 PrecessionoftheEarthsTilt • Rotationofmoonaroundtheearth – MoonrotatesaroundtheEarthevery4weeks. • movesat0.5o perhour. • complicatedorbit. – ThemoonandtheSun’srotationarealmostonthesameplane. • Themoon’splaneofrotationistiltedbyabout5o totheSun. • themoonsweepsoutaband+/- 8degreeseithersideofEcliptic calledtheZodiac. • Themoonisnorthofthecelestialequatorfor2weeksandthen southoftheequator. – MoonandSun“pull”attheEarth seeFigure2-15 • Theearthisnotcompletelyspherical(43kmlargerattheequator). • TheSunandMooncauseagravitationalpull(tryingthe “straighten”therotationaxes). • Thespinoftheearthopposesthispull. KnowingtheHeavens SurveyofAstrophysics110 KnowingtheHeavens SurveyofAstrophysics110 PrecessionoftheEarth’sTilt – Thispullcausesagyroscopeeffect • The“spinningbicycle”wheelopposesachangeinthemotion. • Thisrotationiscalled“precession”. • AswiththebicyclewheeltheaxesofthetheEarth’srotation precesses. – Aslowprecessionofthepole • Thepolerotatesat23.5o totheperpendicularoftheecliptic. • Thisrotationistheslowestchange(taking26,000yearsto complete). • Theresultisachangeinwherethecelestialpolepointsrelativeto thestars. • 5000yearsagoThubon inDraco wouldhavebeenthe“PoleStar”. – PrecessioncausesRA,Dectochange • Precessionofthepolemeanstheequatoralsoprecesses. • Theequinoxesprecess sothecoordinatesofthestarschangeslowly withtime. • Coordinatesareonlygoodforaparticulartime(epoch).Most catalogsrefertoJ2000. KnowingtheHeavens SurveyofAstrophysics110 KnowingtheHeavens SurveyofAstrophysics110 KeepingTime • Asolarday – Localnoonisdefinedaswhenthesuncrossesa meridian. • meridianisacirclepassingthroughthecelestialpolesand throughthezenith. • asolardayisthetimebetween2crossingsofthemeridian. • theearthrevolvesaroundthesun(sotheearthmustrotate morethan360o betweencrossings). – Thelengthofasolardayvaries • Theearthsorbitisnotperfectlycircular(Chapter4).The earthmovesmorerapidlynearthesunsosolardaysare longerinJanuary. • The23.5o angletotheeclipticmeansthatduringthe equinoxes(spring/summer)thesunappearstomovenorth- south(dayislongerinMarch/September) • notgoodfortimekeeping. KnowingtheHeavens SurveyofAstrophysics110 KnowingtheHeavens SurveyofAstrophysics110 KeepingTime – MeanSolarDay • PaththeSunwouldtakeifitsorbitwascirculararoundthe celestialequator(averageSun). • Successivetransitsofthe“meansun”are24hourslong. • goodfortimekeeping(thoughnolongerused). – TimeZones • Wewanttimerelativetoourdaytime- theEarthhas approximately24timezones. • Timediffersbyabout1hourfromonezonetothenext. • Eachzoneisdefinesothattheapparentnoonoccursat about12pmineachzone. • TimeineachzonecalledStandardTime. – UniversalTime • SowehaveacommonreferencepointUniversaltimeisthe timezonecoveringLongitude0(GreenwichEngland) • 24hoursystem. KnowingtheHeavens SurveyofAstrophysics110 KnowingtheHeavens SurveyofAstrophysics110 KnowingtheHeavens SurveyofAstrophysics110 SiderealTime • Atimerelatedtothestars – Astronomerswanttoknowwhenastar(notthesun) isvisible. • Thevisibilityofastardependsonitsposition(RA). – Siderealtimeisbasedonthepositionofthevernal equinox seeBox2-4 • ThisisthesamestartingpointforRA. • ASiderealDayisabout4minutesshorterthanaMeanSolar Day(duetotherotationoftheEarthroundtheSun). • siderealclockstickatdifferentratesfromnormalclocks. – WhyastronomersuseSiderealTime • Astaralwaysrisesatthesamesiderealtime. • WhentheSiderealtimeequalsanobjectsRAtheobjectison themeridian. KnowingtheHeavens SurveyofAstrophysics110 Calendars • Definingacalendaryear – EarthrotatesaroundtheSunevery365.25days • Assumingayearof365dayswillcausetheseasonstoslowlymove outofsinc withthemonths. • Leapyearsaccountforthesechanges. – Siderealyear • FortheSuntoreturntothesamepositionasdefinedbythestars (365.3564meansolardays). – Tropicalyear • TimefortheSuntoreturntovernalequinox(365.2422days) • Tropicalyearis20m24sshorterthanasiderealyear(precession of theEquinoxes) – Leapyears • Tropicalyearisnot365.25daystheyearwilllose3daysevery 4 centuries • Toaccountforthisonlycenturiesdivisibleby400areleapyears (GregoryXIII).Theerrorisnow1dayevery3300years..
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