Cambridge University Press 978-1-107-19728-2 — Time: From Earth Rotation to Atomic Physics Dennis D. McCarthy , P. Kenneth Seidelmann Frontmatter More Information TIME: FROM EARTH ROTATION TO ATOMIC PHYSICS In the 21st century, we take the means to measure time for granted, without contemplatingthesophisticatedconceptsonwhichourtimescalesarebased.This volumepresentstheevolutionofconceptsoftimeandmethodsoftimekeepingupto thepresentday.Itoutlinestheprogressionoftimebasedonsundials,waterclocks, andtheEarth’srotation,totimemeasurementusingpendulumclocks,quartzcrystal clocks, and atomic frequency standards. Timescales created as a result of these improvementsintechnologyandthedevelopmentofgeneralandspecialrelativity are explained. This second edition has been updated throughout to describe 20th- and21st-centuryadvancesanddiscussestheredefinitionofSIunitsandthefutureof Coordinated Universal Time (UTC). A new chapter on time and cosmology has beenadded.Thisbroad-rangingreferencebenefitsadiversereadership,including historians, scientists, engineers, and educators, and it is accessible to general readers. dennis d. mccarthy is former Director of Time at the US Naval Observatory andtheleadingauthorityintheUnitedStatesforastronomicalandtimingdata.He hasledandbeenamemberofvariouscommissionsandworkinggroupswithinthe InternationalAstronomicalUnionandhasauthoredandeditednumerouspublica- tionsdealingwithfundamentalastronomy,time,andEarthorientation. p.kenneth seidelmann isaresearchprofessorofastronomyattheUniversity of Virginia and was Director of Astrometry at the US Naval Observatory. He has ledandbeenamemberofadivision,variouscommissions,andworkinggroupsof the International Astronomical Union. He has coauthored two other books – Fundamentals of Astrometry and Celestial Mechanics and Astrodynamics – and isacoeditoroftheExplanatorySupplementtotheAstronomicalAlmanac. © in this web service Cambridge University Press www.cambridge.org Cambridge University Press 978-1-107-19728-2 — Time: From Earth Rotation to Atomic Physics Dennis D. McCarthy , P. Kenneth Seidelmann Frontmatter More Information © in this web service Cambridge University Press www.cambridge.org Cambridge University Press 978-1-107-19728-2 — Time: From Earth Rotation to Atomic Physics Dennis D. McCarthy , P. Kenneth Seidelmann Frontmatter More Information TIME: FROM EARTH ROTATION TO ATOMIC PHYSICS second edition DENNIS D. M CARTHY C USNavalObservatory(Retired) P. KENNETH SEIDELMANN UniversityofVirginia © in this web service Cambridge University Press www.cambridge.org Cambridge University Press 978-1-107-19728-2 — Time: From Earth Rotation to Atomic Physics Dennis D. McCarthy , P. Kenneth Seidelmann Frontmatter More Information UniversityPrintingHouse,CambridgeCB28BS,UnitedKingdom OneLibertyPlaza,20thFloor,NewYork,NY10006,USA 477WilliamstownRoad,PortMelbourne,VIC3207,Australia 314–321,3rdFloor,Plot3,SplendorForum,JasolaDistrictCentre,NewDelhi–110025,India 79AnsonRoad,#06–04/06,Singapore079906 CambridgeUniversityPressispartoftheUniversityofCambridge. ItfurtherstheUniversity’smissionbydisseminatingknowledgeinthepursuitof education,learning,andresearchatthehighestinternationallevelsofexcellence. www.cambridge.org Informationonthistitle:www.cambridge.org/9781107197282 DOI:10.1017/9781108178365 FirstEdition©2009WILEY-VCHVerlagGmbH&Co.KGaA,Weinheim SecondEdition©DennisD.McCarthyandP.KennethSeidelmann2018 Thispublicationisincopyright.Subjecttostatutoryexception andtotheprovisionsofrelevantcollectivelicensingagreements, noreproductionofanypartmaytakeplacewithoutthewritten permissionofCambridgeUniversityPress. Firstpublished2009 SecondEdition2018 PrintedintheUnitedKingdombyTJInternationalLtd.PadstowCornwall AcataloguerecordforthispublicationisavailablefromtheBritishLibrary. LibraryofCongressCataloging-in-PublicationData Names:McCarthy,DennisD.,author.|Seidelmann,P.Kenneth,author. Title:Time:fromEarthrotationtoatomicphysics/DennisD.McCarthy(UnitedStatesNavalObservatory (retired),P.KennethSeidelmann(UniversityofVirginia). Description:Secondedition.|Cambridge;NewYork,NY:CambridgeUniversityPress,[2018]|Includes bibliographicalreferencesandindex. Identifiers:LCCN2018030821|ISBN9781107197282 Subjects:LCSH:Timemeasurements.|Time.|Earth(Planet)–Rotation. Classification:LCCQB213.M3852018|DDC529/.7–dc23 LCrecordavailableathttps://lccn.loc.gov/2018030821 ISBN978-1-107-19728-2Hardback CambridgeUniversityPresshasnoresponsibilityforthepersistenceoraccuracyof URLsforexternalorthird-partyinternetwebsitesreferredtointhispublication anddoesnotguaranteethatanycontentonsuchwebsitesis,orwillremain, accurateorappropriate. © in this web service Cambridge University Press www.cambridge.org Cambridge University Press 978-1-107-19728-2 — Time: From Earth Rotation to Atomic Physics Dennis D. McCarthy , P. Kenneth Seidelmann Frontmatter More Information Inhonorofourwives,DianeMcCarthyandBobbieSeidelmann,andourfamilies. Dedicatedtothescientistswhoprecededusandtaught,mentored, andinspiredus. © in this web service Cambridge University Press www.cambridge.org Cambridge University Press 978-1-107-19728-2 — Time: From Earth Rotation to Atomic Physics Dennis D. McCarthy , P. Kenneth Seidelmann Frontmatter More Information © in this web service Cambridge University Press www.cambridge.org Cambridge University Press 978-1-107-19728-2 — Time: From Earth Rotation to Atomic Physics Dennis D. McCarthy , P. Kenneth Seidelmann Frontmatter More Information Contents Preface pagexv 1 TimeBeforethe20thCentury 1 1.1 IntheBeginning 1 1.2 CharacterizingTime 1 1.3 Calendars 2 1.4 AstronomicalObservations 3 1.5 Timekeeping 4 1.6 TimeEpochs 5 1.7 TimeTransfer 6 1.8 RotationoftheEarth 7 1.9 Beginningthe20thCentury 8 2 TimefromtheEarth’sRotation 10 2.1 ApparentSolarTime 10 2.2 MeanSolarTime 10 2.3 SiderealTime 13 2.4 WashingtonConferenceof1884 15 2.5 UniversalTime 15 2.6 UT1asMeanSolarTime 18 2.7 CoordinatedUniversalTime 20 2.8 GreenwichMeanTime(GMT) 21 2.9 TimeZones 22 2.10 DaylightSavingsTime 22 3 Ephemerides 25 3.1 EphemeridesandTime 25 3.2 BeforeKeplerandNewton 26 3.3 KeplerandNewton 28 3.4 Tables,GeneralTheories,andEphemerides 29 vii © in this web service Cambridge University Press www.cambridge.org Cambridge University Press 978-1-107-19728-2 — Time: From Earth Rotation to Atomic Physics Dennis D. McCarthy , P. Kenneth Seidelmann Frontmatter More Information viii Contents 3.5 LunarTheories 32 3.6 TheAdventofComputers 35 3.7 NumericalIntegrations 35 3.8 ObservationalData 36 3.8.1 RadarObservations 36 3.8.2 LunarLaserRanging 37 3.8.3 SpacecraftObservations 37 3.9 ModernEphemerides 37 3.10 ReferenceSystem 38 3.11 BesselianYear 39 3.12 TimeArguments 40 3.13 AstronomicalConstants 40 3.14 RedefinitionoftheAstronomicalUnit(au) 41 3.15 ArtificialSatelliteTheories 42 3.16 TheoryofRelativity 42 4 VariableEarthRotation 47 4.1 Pre19thCentury 47 4.2 SecularVariation 48 4.3 IrregularVariationsintheEarth’sRotation 50 4.4 EarlyExplanationsfortheVariableRotation 58 4.5 CurrentUnderstandingoftheEarth’sVariableRotation 59 4.6 Consequences 62 5 EarthOrientation 68 5.1 ReferenceSystems 68 5.1.1 CelestialReferenceFrame 69 5.1.2 TerrestrialReferenceFrame 70 5.1.3 IntermediateReferenceSystem 72 5.2 VariationsinEarthOrientation 73 5.2.1 Precession/Nutation 74 5.2.2 PolarMotion 76 5.2.3 UT1 79 5.3 VariationsinEarthOrientation 80 5.4 TransformingbetweenReferenceFrames 81 5.5 DeterminationofEarthOrientation 84 5.6 EarthOrientationData 85 6 EphemerisTime 88 6.1 NeedforaUniformTimescale 88 6.2 DanjonProposal 89 © in this web service Cambridge University Press www.cambridge.org Cambridge University Press 978-1-107-19728-2 — Time: From Earth Rotation to Atomic Physics Dennis D. McCarthy , P. Kenneth Seidelmann Frontmatter More Information Contents ix 6.3 ClemenceProposal 90 6.4 AdoptionandDefinition 91 6.5 ObservationalDetermination 92 6.6 TheEphemerisSecondandAtomicTime 95 6.7 HistoricalΔT 97 6.8 ProblemswithEphemerisTime 97 6.9 Relativity 100 6.10 DynamicalTimescales 101 7 RelativityandTime 104 7.1 NewtonianReferenceSystems 104 7.2 SpecialRelativity 104 7.3 LorentzTransformations 106 7.4 CoordinateandProperTime 107 7.5 MinkowskiDiagrams 109 7.6 TimeinSpecialRelativity 111 7.7 GeneralRelativity 112 7.7.1 MetricsinGeneralRelativity 112 7.7.2 TheEquivalencePrinciple 113 7.8 IAUResolutions 114 7.9 Timescales 121 7.9.1 InternationalAtomicTime 121 7.9.2 DynamicalTimescales 121 7.10 RelativisticEffectsinTimeTransfer 122 8 TimeandCosmology 123 8.1 Introduction 123 8.2 Space-TimeMetric 123 8.3 TheExpandingUniverse 124 8.4 AgeoftheUniverse 125 8.5 EvolutionoftheUniverse 126 8.6 CosmicTime 126 8.7 Time’sArrow 128 8.8 FutureoftheUniverse 128 9 DynamicalandCoordinateTimescales 131 9.1 ReplacingEphemerisTime 131 9.2 TerrestrialDynamicalTime(TDT)andBarycentric DynamicalTime(TDB) 132 9.3 ProblemswithTDTandTDB 135 9.4 NewReferenceSystem 136 © in this web service Cambridge University Press www.cambridge.org Cambridge University Press 978-1-107-19728-2 — Time: From Earth Rotation to Atomic Physics Dennis D. McCarthy , P. Kenneth Seidelmann Frontmatter More Information x Contents 9.5 NewTimescales 137 9.5.1 TerrestrialTime(TT) 138 9.5.2 GeocentricCoordinateTime(TCG) 139 9.5.3 BarycentricCoordinateTime(TCB) 139 9.5.4 TDBRedefined 141 9.5.5 BarycentricEphemerisTime(T ) 142 eph 9.6 ΔTandEphemerisTimeRevised 142 9.7 RelationshipsamongCoordinateTimescales 143 10 ClockDevelopments 148 10.1 Introduction 148 10.2 KeepingTimeinAntiquity 148 10.2.1 ClepsydraeandWater“Clocks” 149 10.2.2 OtherTimekeepingDevices 150 10.3 TheFirstMechanicalClocks 150 10.4 PendulumClocks 151 10.4.1 Galileo 152 10.4.2 Huygens 152 10.4.3 PendulumClockDevelopments 155 10.4.4 Chronometers 156 10.5 QuartzCrystalClocks 157 10.6 ClockPerformance 160 10.6.1 Quality(Q)Factor 161 10.6.2 Precision 162 10.6.3 Accuracy 163 10.6.4 Stability 163 11 MicrowaveAtomicClocks 171 11.1 BeyondQuartz-CrystalOscillators 171 11.2 PhysicsofAtomicClocks 172 11.3 GeneralStructureofAtomicClocks 174 11.4 DevelopmentofAtomicClocks 177 11.4.1 Caesium 177 11.4.2 Hydrogen 189 11.4.3 Rubidium 191 11.5 TrappedIonClocks 194 11.5.1 Mercury 196 11.5.2 OtherIons 197 11.6 PHARAOLaser-CooledMicrogravityAtomicClock 197 11.7 CharacterizingAtomicClocks 198 © in this web service Cambridge University Press www.cambridge.org