TheThe TransitTransit ofof VenusVenus JohnJohn S.S. ReidReid CruickshankCruickshank LecturerLecturer inin AstronomyAstronomy DepartmentDepartment ofof PhysicsPhysics UniversityUniversity ofof AberdeenAberdeen PublicPublic interestinterest VenusVenus crossingcrossing thethe Telescopic view by US faceface ofof thethe SunSun Naval Observatory of 1882 transit AA rarerare phenomenonphenomenon ¾ Last seen in 1882 ¾ Visible without a telescope ¾ An annular eclipse of the Sun by Venus Courtesy: www.williams.edu/astronomy/ eclipse/transits/ AstronomicalAstronomical interestinterest RefineRefine elementselements ofof Venus’sVenus’s orbitorbit usingusing observedobserved timingtiming ¾ 1717th centurycentury resultresult DetermineDetermine thethe absoluteabsolute scalescale ofof thethe solarsolar systemsystem ¾ 1818th centurycentury resultresult FindFind anan accurateaccurate valuevalue forfor 11 AUAU ¾ 1919th centurycentury interestinterest ¾ 11 AUAU isis thethe metremetre--stickstick forfor thethe UniverseUniverse SolarSolar systemsystem andand Kepler’sKepler’s lawslaws TheThe relativerelative sizessizes ofof thethe orbitsorbits inin thethe solarsolar systemsystem Average areare givengiven byby Kepler’sKepler’s 33rd distance a Sun lawlaw ¾ a3 ∝ (planetary year)2 WhatWhat isis thethe absoluteabsolute sizesize?? Planet a ¾ 1 AU is average distance Mercury 0.387 between Earth and Sun between Earth and Sun Venus 0.723 Earth 1.00 Mars 1.524 Jupiter 5.203 ParallaxParallax ofof aa planetplanet YouYou cancan findfind thethe distancedistance ofof aa planet,planet, P,P, ifif youyou cancan measuremeasure itsits parallaxparallax angleangle fromfrom separatedseparated points,points, AA andand B,B, onon thethe EarthEarth Parallax angle B P Distance A Earth Definition: Parallax angle = AB/Distance Hence: Distance = AB/Parallax angle MethodMethod waswas triedtried forfor MarsMars ObserveObserve positionposition ofof MarsMars againstagainst thethe fixedfixed backgroundbackground ofof starsstars Mars Earth ¾ DifficultDifficult ¾ ParallaxParallax fromfrom 11 EarthEarth radiusradius isis 2020"" atat bestbest ¾ CanCan useuse EarthEarth’’ss rotationrotation andand observeobserve changechange inin MarsMars’’ positionposition fromfrom eveningevening toto prepre--dawndawn EnterEnter VenusVenus VenusVenus isis closestclosest planetplanet toto EarthEarth andand inin principleprinciple bestbest forfor parallaxparallax ¾ ClosestClosest isis typicallytypically 0.280.28 AUAU MeasureMeasure parallaxparallax againstagainst backgroundbackground ofof Sun’sSun’s diskdisk ¾ WhenWhen atat greatestgreatest elongationelongation Venus orbit fromfrom SunSun (46(46ºº)),, VenusVenus 46º V ~2.5~2.5 timestimes furtherfurther fromfrom EarthEarth E Sun 0.28 AU TheThe mastermaster planplan UseUse thethe Sun’sSun’s diskdisk asas aa calibratedcalibrated screenscreen ObserveObserve thethe transittransit fromfrom differentdifferent locationslocations onon EarthEarth MeasureMeasure thethe parallaxparallax ofof VenusVenus Observed ScaleScale thethe solarsolar systemsystem andand hencehence from determinedetermine 11 AUAU inin termsterms ofof metresmetres St Helena Observed from Aberdeen Venus Earth Side view Sun’s disk TheThe refinementrefinement UseUse thethe curvedcurved edgeedge ofof thethe SunSun andand timetime thethe ingressingress andand egressegress ofof VenusVenus toto deducededuce whichwhich chordchord VenusVenus travelledtravelled onon ¾ ClocksClocks areare moremore accurateaccurate thanthan telescopetelescope anglesangles Sun AlignmentsAlignments ofof Earth,Earth, VenusVenus && SunSun TheThe Earth,Earth, VenusVenus && SunSun areare inin aa lineline lookinglooking downdown onon thethe solarsolar systemsystem everyevery 583.9583.9 daysdays (1.6(1.6 yearsyears == 8/58/5 years)years) ¾ VenusVenus orbitsorbits 2+2+ timestimes ¾ EarthEarth orbitsorbits 1+1+ timetime 2 4 E V S 5 3 Earth year: 365.25630 days Venus year: 224.70078 days TheThe problemproblem Venus’sVenus’s orbitorbit isis tiltedtilted atat 3.393.39ºº withwith respectrespect toto thethe EarthEarth’’ss orbitorbit InIn reality,reality, onlyonly 22 pointspoints inin VenusVenus’’ss orbitorbit areare inin thethe planeplane ofof thethe EarthEarth’’ss orbitorbit ¾ DescendingDescending nodenode NN View from ¾ AscendingAscending nodenode N'N' Earth to N' Venus E V S N 3.39º Plane of Earth’s orbit Line of nodes TheThe opticsoptics EarthEarth mustmust bebe inin thethe centralcentral shadowshadow conecone ofof VenusVenus toto seesee VenusVenus inin frontfront ofof thethe SunSun Sun V E Shadow cone Line of sight to Venus and Sun SomeSome ofof shadowshadow conecone mustmust bebe inin thethe planeplane ofof Earth’sEarth’s orbitorbit forfor transittransit toto bebe seenseen TheThe transittransit seasonseason 2004 TheThe transittransit seasonseason lastslasts ~3.5~3.5 daysdays Plane of Venus’s Shadow orbit cone What would be N seen E Earth’s orbit ~1 day for shadow cone 1.8 days for Earth TheThe alignmentsalignments SupposeSuppose aa transittransit occursoccurs inin oneone year,year, howhow longlong beforebefore thethe nextnext one?one? EarthEarth--VenusVenus--SunSun alignmentalignment shiftsshifts 2.42.4 daysdays (earlier)(earlier) afterafter 88 yearsyears 2012 ¾ HenceHence afterafter 88 yearsyears aa secondsecond transittransit willwill occuroccur onon thethe otherother sideside ofof thethe nodenode What would be N seen Earth’s orbit E ~2.4 days ThenThen what?what? ThereThere areare nono transitstransits forfor aa longlong timetime ¾ WhenWhen thethe EarthEarth passespasses throughthrough thethe lineline ofof nodes,nodes, VenusVenus hashas alreadyalready beenbeen therethere TheThe ~8~8 yearyear alignmentsalignments occuroccur progressivelyprogressively earlier,earlier, beforebefore thethe EarthEarth reachesreaches thethe nodenode TheThe alignmentalignment ‘spokes’‘spokes’ retreatretreat clockwiseclockwise 2.4° per 8 2 E years V 44 2 E S NN' 3 5 3 5 243243 yearsyears betweenbetween repeatrepeat pairspairs AlignmentAlignment directionsdirections everyevery 88 yearsyears lessless 2.42.4 daysdays 2, alignment after 1.6 years Alignment 153 ≡ 243 years 32 62 153 Sun N N' 72 123 alignment 72 ≡ 113.5 years 93 V 63 E 33 3, alignment after 3.2 years TheThe ascendingascending nodenode transitstransits 7171 transitstransits (113.5(113.5 years)years) afterafter thethe firstfirst one,one, thethe shadowshadow conecone passespasses closeclose toto thethe 1874 ascendingascending nodenode NN'' & 2117 ¾ AnotherAnother pairpair ofof transitstransits willwill bebe seenseen What would be N' seen Earth’s orbit E Venus moving shadow cone SeriesSeries ofof transitstransits everyevery 243243 yearsyears AscendingAscending nodenode seriesseries ReturnReturn toto 20042004 transittransit (2012 transit occurs 2.4 days earlier, mainly during our night) ExpectedExpected sightsight CloudsClouds allowing!allowing! Courtesy: http://www.eso.org/outreach/press- rel/pr-2004/images/vt-anim.gif Venus’Venus’ tracktrack acrossacross thethe SunSun WhyWhy isis Venus’Venus’ tracktrack 8.78.7ºº toto thethe ecliptic?ecliptic? → 29.78 km s-1 E N → 48.55 km s-1 ↓ 2.87 km s-1 0.52×106 km VenusVenus tracktrack acrossacross thethe SunSun isis thethe EarthEarth’’ss tracktrack acrossacross thethe shadowshadow conecone ¾ VelocityVelocity ofof EarthEarth relativerelative toto shadowshadow conecone ¾ MaxMax durationduration ~7.6~7.6 hrhr 19 km s-1 8.7º ¾ 20042004 transittransit ~~ ¾¾ maxmax ~~ 55½½ hrhr Illustrating the Earth & central parallax in transits shadow cone to experienced at Parallax effects scale different latitudes on Earth Aberdeen (57ºN) St Helena (15ºS) Each point in the shadow Equatorial cone corresponds rotational speed to Venus 0.46 km s-1 appearing at a different place on the Sun’s disk DetailsDetails ofof JuneJune 88thth transittransit Sun Approximate BST timings shown for Aberdeen Ecliptic 0640 BST 7 8 9 10 11 12 ObservingObserving thethe SpectacleSpectacle DirectlyDirectly –– useuse eclipseeclipse glassesglasses ¾ VenusVenus willwill bebe aa veryvery smallsmall dotdot ProjectProject usingusing binoculars/telescopebinoculars/telescope PhotographPhotograph withwith ~200~200 mmmm telephototelephoto lens,lens, oror longerlonger ¾ rememberremember thethe solarsolar filter!filter! JeremiahJeremiah HorrocksHorrocks (1619(1619 –– 1641)1641) Horrocks predicted the 1639 transit and observed it, as did his friend Wm Crabtree Horrocks deduced improved orbital parameters for Venus William Crabtree, Horrock’s friend 24th Nov 1639 Old-style calendar JamesJames GregoryGregory (1638(1638 –– 1675)1675) BrilliantBrilliant mathematicianmathematician && astronomerastronomer fromfrom DrumoakDrumoak FirstFirst suggestedsuggested thatthat observationobservation ofof thethe transittransit ofof VenusVenus couldcould determinedetermine thethe scalescale ofof thethe solarsolar systemsystem Courtesy: University of Aberdeen EdmondEdmond HalleyHalley (1656(1656 –– 1742)1742) St Helena Halley observed the transit of Mercury and worked out the details of finding the solar distance from the Transit of Venus observations Plantation house 1812 66thth JuneJune 17611761 transittransit AA bigbig internationalinternational CharlesCharles MasonMason (1730(1730 -- efforteffort 1786)1786) JeremiahJeremiah DixonDixon (1733(1733 -- 1779)1779) Transit of Venus from ceiling of the Paris Observatory Nevil Maskelyne (1732 – 1811) unsuccessful at St Helena http://www.bdl.fr/Granpub/Promenade/ pages6/608.html 33rdrd JuneJune 17691769 TransitTransit CharlesCharles Green’sGreen’s