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Introduction

**Skills and Expertise**

## Publications

Publications (203)

IAU Commission 19 began in 1919 with the birth of the IAU at the Brussels Conference, where Standing Committee 19 on Latitude Variations was established as one of 32 standing committees. At the first IAU General Assembly in 1922, Standing Committee 19 became Commission 19 “Variation of Latitude”. In the beginning, the main topic of the Commission w...

Cambridge Core - Time: From Earth Rotation to Atomic Physics - by Dennis D. McCarthy

This report continues the practice where the IAU Working Group on Cartographic Coordinates and Rotational Elements revises recommendations regarding those topics for the planets, satellites, minor planets, and comets approximately every 3 years. The Working Group has now become a “functional working group” of the IAU, and its membership is open to...

We present our ongoing investigation of the use of U.S-produced atmospheric and oceanic angular momentum (AAM and OAM) estimates to improve the determination of near real-time Earth rotation and polar motion parameters and their short-term predictions. Previous investigations provided evidence that the use of AAM and OAM data sets could improve sho...

Very Long Baseline Interferometry (VLBI) is a primary space-geodetic technique for determining precise coordinates on the Earth, for monitoring the variable Earth rotation and orientation with highest precision, and for deriving many other parameters of the Earth system. The International VLBI Service for Geodesy and Astrometry (IVS, http://ivscc.g...

The astronomical unit (au) is a unit of length approximating the Sun -
Earth distance that is used mainly to express the scale of the solar
system. Its current definition is based on the value of the Gaussian
gravitational constant, k. This conveniently provided accurate relative
distances (expressed in astronomical units) when absolute distances
c...

To keep the calendar year synchronized with the astronomical year,
intercalary days are inserted following a commonly adopted convention
that defines the Gregorian Calendar. Similarly, to keep the day composed
of 86, 400 Système Inter national (SI) seconds synchronized with
the day defined by the Earth’s rotation, intercalary seconds are
occasional...

Astronomy has provided a means to mark the passage of time throughout history. One of the repeating phenomena that makes this possible is the Earth's rotation. The basic variability in its rotational speed, however, makes astronomical techniques unsuitable for timekeeping with the precision required for modern applications. Physical metrology from...

There were four 1.5-hour sessions of Division I business meetings during
the XXVIIth IAU General Assembly. The first three were devoted to the
reports of Commissions, Working Groups and services associated with the
Division, discussion about plans for the next triennium and future
structure of the Division. Scientific presentations on the future sp...

Newtonian Reference SystemsSpecial RelativityLorentz TransformationsCoordinate and Proper TimeMinkowski DiagramsTime in Special RelativityGeneral RelativityIAU ResolutionsTime ScalesRelativistic Effects in Time TransferReferences

Historical Transfer TechniquesTime and Frequency Dissemination ModelingTime and Frequency Dissemination SystemsReferences

Time Enables the InfrastructurePositioning and Navigation ServicesCommunicationsPower GridBanking and FinanceEmergency ServicesWater FlowSummaryReference

Time and International ActivitiesTreaty of the MeterScientific UnionsService OrganizationsReferences

Replacing Ephemeris TimeTerrestrial Dynamical Time (TDT) and Barycentric Dynamical Time (TDB)Problems with TDT and TDBNew Reference SystemNew Time ScalesΔT and Ephemeris Time RevisedRelationships Among Coordinate Time ScalesReferences

Future Needs for TimeModeling the Earth's RotationClocks of the FutureFuture Time ScalesFuture Time Distribution

Universal Time Before 1972Coordinated Universal Time After 1972Leap SecondsUT1UTC WorldwideTime DistributionThe Future of UTC – Leap Seconds or Not?References

Apparent Solar TimeMean Solar TimeEquation of TimeSidereal TimeWashington Conference of 1884Newcomb's Theory of the SunUniversal TimeCoordinated Universal Time (UTC)Greenwich Mean Time (GMT)Tropical YearBesselian YearReference SystemTime ZonesDaylight Saving TimeReferences

Ephemerides and TimeBefore Kepler and NewtonKepler and NewtonTables, General Theories, and EphemeridesLunar TheoriesThe Advent of ComputersNumerical IntegrationsObservational DataDynamical Reference FrameTime ArgumentsAstronomical ConstantsArtificial Satellite TheoriesTheory of RelativityReferences

Pre 19th CenturySecular VariationIrregular Variations in the Earth's RotationEarly Explanations for the Variable RotationCurrent Understanding of the Earth's Variable RotationConsequencesReferences

Earth OrientationVariations in the Earth's OrientationTransforming Between Reference FramesDetermination of Earth OrientationEarth Orientation DataReferences

The Solar SystemPursuit of UniformityPursuit of AccuracyTime and PhenomenaTime and DistanceSpace Mission TimesProper Times at PlanetsPulsars – An Independent Source of Time?References

The Historical SecondThe Ephemeris SecondThe SI SecondAdopting the SI SecondReferences

Beyond Quartz-Crystal OscillatorsPhysics of Atomic ClocksGeneral Structure of Atomic ClocksDevelopment of Atomic ClocksStored Ion ClocksCharacterizing Atomic ClocksReferences

Terrestrial to Celestial Reference SystemsDetermination of Earth Orientation ParametersEarth Orientation DataReferences

IntroductionKeeping Time in AntiquityThe First Mechanical ClocksPendulum ClocksQuartz Crystal ClocksClock PerformanceReferences

Constructing an Atomic Time ScaleHistory of TAIFormation of TAIStability of TAIDistribution of TAIRelationship of TAI to Terrestrial TimeReferences

In the BeginningCharacterizing TimeCalendarsAstronomical ObservationsTimekeepingTime EpochsTime TransferRotation of the EarthBeginning the Twentieth CenturyReferences

Division I provides a focus for astronomers studying a wide range of
problems related to fundamental physical phenomena such as time, the
inertial reference frame, positions and proper motions of celestial
objects and precise dynamical computation of the motions of bodies in
stellar or planetary systems in the Universe.

Recent developments in clock technology have made it possible to provide time and frequency with unprecedented precision in the laboratory. The challenge of comparing these devices at a distance remains, however. A critical component in transferring time and frequency is the consideration of the relativistic aspects of time comparisons. For example...

Filling the need for a book that conveys the current technology as well as the underlying history and physical background, this book tells physicists and engineers how to measure time to the precision required for modern-day use. The authors draw on their longstanding research experience with timekeeping and high-precision measurement to cover the...

Aleksander Brzezinski, P-C19 (Poland), Joseph A. Burns, P-C7 (USA), Pascale Defraigne, P-C31 (Belgium), Dafydd Wyn Evans, VP-C8 (UK) Toshio Fukushima, P-C4, PP (Japan), George H. Kaplan, VP-C4 (USA), Sergei A. Klioner, P-C52 (Germany), Zoran Knezevic, VP-C7 (Serbia) Irina I. Kumkova, P-C8 (Russia), Chopo Ma, VP-C19 (USA), Richard N. Manchester, VP-...

International Atomic Time (TAI) is the internationally recognized timescale based on the second of the Système International d'Unités produced by the Bureau International des Poids et Mesures using data from timing laboratories around the world. TAI is an atomic timescale without steps. Coordinated Universal Time, the basis of civil time, is derive...

A Division 1 Working Group on “Nomenclature for Fundamental Astronomy” (NFA) was formed at the 25th IAU GA in 2003 in order to provide proposals for the new nomenclature associated with the implementation of the IAU 2000 resolutions on reference systems. This WG is also intended to make related educational efforts for addressing the issue to the la...

Observations of the length of day, corrected for the effects of variations in the angular momentum due to changes in wind velocity and atmospheric pressure, ocean-tide heights and currents, and solid-Earth zonal tides, were analysed. the (1992) IERS Standards model for the effects of zonal tides on the Earth's rotation, which includes ocean-tidal e...

The definition of UTC was implemented in 1972, principally to
accommodate celestial navigation and follows recommendation 460 of the
International Radio Consultative Committee (CCIR) in 1970. Since 1972
the use of electronic means to navigate has overtaken celestial
navigation. This fact along with increasing public dissatisfaction with
the possibl...

The typical GPS receiver solves the position, velocity and timing (PVT) problem, using pseudo-range measurements, relative to an Earth centered fixed (ECF) coordinate reference frame. By design, GPS satellites broadcast their ephemeris using continuous functions of quasi-Keplerian orbital element in ECF coordinates. However, for spacecraft applicat...

The International Earth rotation and Reference system Service (IERS) provides observational determinations of the celestial pole offsets that describe quantitatively the differ-ence between the observed direction of the Celestial Intermediate Pole in the celestial reference frame and the direction predicted by the conventional precession-nutation m...

Stochastic prediction techniques including autocovariance, autoregres-sive, autoregressive moving average, and neural networks were applied to the UT1-UTC and Length of Day (LOD) International Earth Rotation and Reference Systems Service (IERS) EOPC04 time series to evaluate the capabilities of each method. All known ef-fects such as leap seconds a...

The consistency of the reference frames provided by the IERS and its different centers relies on the set of conventional models and procedures that are used to realize them. These conventional models and procedures are mostly the product of the IERS Conventions Center, provided jointly by the Bureau International des Poids et Mesures (BIPM) and the...

The IAU Joint Discussion 16 (JD16) was held in conjunction with the XXVth General Assembly in July, 2003. Papers related to the maintenance of the International Celestial Reference System were presented in the one-day session, and these were followed by discussion that pointed out the need for standard nomenclature. This issue was addressed by the...

The International Celestial Reference Frame (ICRF) is currently a radio
reference frame accessed through VLBI that is refined with
technique-dependent improvements described in this Joint Discussion. An
important component of the International Celestial Reference System
(ICRS) that is the basis for this frame is the set of conventional
models...

Practical measurement of the passage of time requires the notion of a repeating phenomenon. The Earth's rotation has traditionally fulfilled this requirement. To cope with the impracticality of making precise measures of the Sun's hour angle or altitude, particularly in uncooperative weather conditions, various devices have been employed, but all h...

GPS Time, the time scale internal to the Global Positioning System, is a uniform scale of time without leap seconds having an initial epoch of midnight, January 5/6, 1980 UTC. Galileo proposes to use a realization of TAI, another scale without leap seconds, as its internal time scale. However, TAI is ahead of GPS Time by 19 seconds, a constant valu...

Resolutions adopted by the International Astronomical Union (IAU) in
2000 recommend significant improvements in the definition of the
International Celestial Reference System and the procedures to be used
in its realization. These recommendations correspond with the
unprecedented observational accuracy that is now achievable in accessing
the refere...

This document is intended to define the standard reference systems realized by the International Earth Rotation Service (IERS) and the models and procedures used for this purpose. It is a continuation of the series of documents begun with the Project MERIT Standards (Melbourne et al., 1983) and continued with the IERS Standards (McCarthy, 1989; McC...

This paper provides expressions to be used to implement the new definition of UT1 corresponding to the IAU 2000 resolutions either in the new (CEO-based) or classical (equinox-based) transformations between the International Terrestrial Reference System (ITRS) and the Geocentric Celestial Reference System (GCRS). The new expression for Greenwich Si...

The expressions for the X and Y coordinates of the CIP in the GCRS and the quantity s(t)+XY/2 are based on the IAU2000A precession-nutation model. They are in the form : polynomial part + non-polynomial part In the non-polynomial part, ARG being for various combination of the fundamental arguments of the nutation theory) The expressions for the fun...

This paper reviews the theoretical motivation for the leap second in the context of the historical evolution of time measurement. The periodic insertion of a leap second step into the scale of Coordinated Universal Time (UTC) necessitates frequent changes in complex timekeeping systems and is currently the subject of discussion in working groups of...

Precise astrometric observations show that significant systematic differences of the order of 10 milliarcseconds (mas) exist between the observed position of the celestial pole in the International Celestial Reference Frame (ICRF) and the position determined using the International Astronomical Union (IAU) 1976 Precession (Lieske et al., 1977) and...

In this paper, four different methods for the prediction of x, y pole coordinates are investigated. We examined the accuracies of autocovariance (AC), least-squares extrap-olation (LS), a least-squares extrapolation and autoregressive combination (LS+AR), and a least-squares extrapolation and neural networks combination (LS+NN) in predicting pole p...

In the future improvement of the IERS combination solutions it will be necessary to address the role of the adopted conventions on the precision and accuracy of the product. It is suggested that the first step be the establishment of clear conventions within each technique followed by a joint effort of the Conventions Center and the Analysis Coordi...

Analyses of residuals between VLBI observations and combinations of
nutation series show that the MHB 2000 nonrigid-Earth nutation model
applied to the REN 2000 rigid-Earth model results in the best fit, and
that amplitudes of any possible periodic terms remaining in the observed
corrections to the MHB2000 theory could be expected to be less than 0...

Past endeavors in the field of Earth orientation have provided both
service to astronomical users of the data and insight into the physical
processes of the planet Earth. Since the study of Earth orientation has
required a wide geographic distribution of astronomical observing sites,
it has been an area of research that has depended heavily on
inte...

This topic addresses the issue of the compatibility of observations made
in the past with observations made using the new system. It is not
likely that there would any requirement to adjust past observations to
put them into a system that complies with the newly adopted IAU
Recommendations. However, it may be important to look at the specifics
of e...

The polar motion prediction is computed as a least-squares extrapolation of the polar motion data. The least-squares model consists of a Chandler circle with constant or variable amplitude, annual and semiannual ellipses, and a bias. The model with constant amplitude of the Chandler oscillation is fit to the last three years of polar motion data an...

Scitation is the online home of leading journals and conference proceedings from AIP Publishing and AIP Member Societies

With the beginning of the 21st Century, the timing community finds itself again facing a decades-old problem of how to synchronize a uniform time scale with time derived from the Earth's rotation. Atomic time is the basis for most everyday timing applications, However, time astronomically determined from the Earth's rotation is essential for other...

The tables contain the parameters of the development as functions of
time t (expressed in centuries since J2000.0) of the Celestial Pole
Coordinates X and Y, expressed in arcseconds, has the following form:
X=-0.017130 + 2004.193319t - 0.4271605t2 -
0.1986210t3 -0.0000461t4 + 0.0000058t5
+{Sum on
i}[(as,0)isin(ARGUMENT)+(ac,0)icos(ARGUMENT)]
+{Sum...

The current definition of Coordinated Universal Time (UTC) is related to the unpredictable, variable rotation rate of the Earth. This is accomplished by irregular insertions of leap seconds, creating unpredictable discontinuities in UTC. With the increasing importance of a continuous, uniform time scale for users, it is appropriate to re-examine th...

The adoption of the International Celestial Reference System ICRS, and
of the corresponding Frame, ICRF, by the 23rd General Assembly of the
International Astronomical Union, calls for a redefinition of the
departure point on the true equator. Several possibilities have been
suggested. This paper considers the use of the non-rotating origin
(Guinot...

After one hundred years of intensive study, some aspects of polar motion remain poorly understood. This motion of the Earth’s axis of rotation with respect to its mantle and crust has been observed and studied since the creation of the International Latitude Service. Research has shown that the motion of the pole is quite complicated and distinguis...

The last three years have been marked by changes, highlights and progress. Organizationally, commission 7 has joined Division I and plans proceed for commissions 8 and 24 to merge in 2000. They have had a common vice president during this triennium. Sadly, the Royal Greenwich Observatory was closed after over 200 years, but Her Majesty’s Nautical A...

The triennial report of Commission 19 was composed from the contributions of its members. Space does not permit a listing of their names, but their contributions are sincerely appreciated. Unfortunately because of limited space it is also not possible to provide in this report the extensive list of publication of the Commission members. The list of...

The authors present a prelminary description of the changes in the current procedures that will have to be performed in order to take into account the adoption of the ICRS as the new reference system and new nutation-precession series. A number of choices will have to be made: they are presented in this paper.

This paper presents the reflections of the Working Group of which the tasks were to examine the non-rigid Earth nutation theory. To this aim, six different levels have been identified: Level 1 concerns the input model (giving profiles of the Earth's density and theological properties) for the calculation of the Earth's transfer function of Level 2;...

Autocovariance prediction has been applied to attempt to improve polar motion and UT1-UTC predictions. The predicted polar
motion is the sum of the least-squares extrapolation model based on the Chandler circle, annual and semiannual ellipses, and
a bias fit to the past 3 years of observations and the autocovariance prediction of these extrapolati...

The solar potential perturbs light propagating in the solar system,
providing the basis for tests of gravity through Very Long Baseline
Interferometry (VLBI) observations of radio waves from extragalactic
radio sources. Such observations determine the γ parameter of the
Parameterized Post Newtonian (PPN) expansion of the spacetime metric,
with the...

The work of Commission 19 in the period 1993.5–1996.5 was concentrated both on observational and theoretical aspects of Earth rotation, namely on the mutual interaction of the rotating non-rigid Earth with other Solar System bodies as well as with the atmosphere and fluids (oceans, underground water). The activities were closely tied with the work...

Historical sources of polar motion are analysed together with modern data in order to compile a set of coordinates of the mean pole in a reference system consistent with that of the International Earth Rotation Service. the trend and quasi-periodic motion of the pole are investigated, and we find that the rotational pole appears to be moving at the...

Analyses of the orbits of the satellites of the Global Positioning System (GPS) by participants in the International GPS Service (IGS) (Mueller and Beutler, 1992) provide daily observations of high-accuracy polar motion. These data are used routinely by the International Earth Rotation Service (IERS) (see, for example, IERS Annual Report for 1993)....

Variations of the orientation of the Earth in an inertial reference frame have been observed since Hipparchus. In addition to precession and nutation, astronomical observations have shown the nature of the variable rotational speed of the Earth and the motion of its rotational pole with respect to its surface. The historical observations are import...

## Projects

Project (1)