accumulation of per riodic phenomenon vs. solution of equation of motion

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1 History of Timescales Mizuhiko Hosokawa NCT Japan IAU Commission 31, Time

2 Dichotomies i Time system vs. Time scale accumulation of per riodic phenomenon vs. solution of equation of motion proper time vs. coordinate time

3 Human life and Celestial lbodies Day - Earth rotation Month - Moon orbital motion Year - Earth orbital motion Not tdivisible, i ibl far from decimal sisytem

4 Time TIme Accumulatio n of periodic Dynamics, Low of motion Universal Time Ephemeris Time Atomic Time

5 Basics of fearth hr Rotation ti Precession and dnutation Polar motion Variation of Axisitselforbody UT (UT0, UT1,UT2) Mean solar time, Siderial TIme equation of time

6 Precession and Nutation ti Variation of axis itself Precession: 23.4deg years 50 / year Nutation: 9, 18.6 years (Moon) etc. 2 / year

7 Variation of Lo od Equation of time +29, -22 sec tidal effect Atmospheric a few 100 micr rosec/day q Earthquakes a few microsec/day

8 Universal ltime, UT Mean Solar Time, not observed Calculated from Siderial time UT0: at patiqular plac ce on the Earth UT1: Correction of Polar Motion UT2: Correction of seasonal variation

9 Ephemeris Tim me, ET Based on the Solar System Dynamics Earth orbital motion, not periodic perturbation of moon and planets Newton seq eq. ofmo otion (nonrelativistic) istic) Indicator: occultation of stars by moon Epoch: millisecond, Stability: IAU- 1952, CGPM , Definition 1960

10 Atomic Time Cs Atomic clock, developed in 1955(Essen, NPL) When developed, d no one knows the ratio with the astronomical time Three years comp parison with ET -> Hz

11 Relativistic i Dynamical Time Mid 1980,rela tivistic dynamical time Terrestrial, TDT -> TT (1991) Reference point: Geoid Realization: TAI, TT(BIPMxx) Many other systems: TDB, TCB, TCG.. Coordinate Time e (synchronization)

12 UT1 vs Atomic Time Basis of UT1: Earth rotation is periodic Approximately OK K, not exactly. Freq. : 10-8 (UT1) vs (TAI) Time: 1sec/a few ye ears, 1h/500years? Tidal: nearly const., Atmospheric?

13 Predicted diffe erene D. McCarthy Metrologia 48-4, 4 S141 (2011)

14 UTC, Coord dinated from 9.19 Ron Beard, ITU-R RWP7AChair

15 UT1 - UTC

16 Argument on leap second

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19 ITU-R RWP7Ad iscussioni After many yyears discussion, no comlete agreement. A draft was made. Draft Revised Recomm mendation ITU-R TF460-6 was proposed to modify UTC to a continuous atomic time scale and ddi discussed dat Radiocommunication Assembly and forwarded to World Radiocommunication Conference (WRC-12)

20 Resolution 653 inwrc WRC-12 recognized that a change in the eference time-scale may have operational and therefore economic consequences invited the ITU-R to conduct further studies into: the feasibility of achieving a contin nuous reference time-scale for dissemination by radiocommunication systems issues related to the possible impl ementation of a continuous reference time-scale (including technical and operational factors) to report to WRC-15 on Agenda item 1.14 : to consider the feasibility of achieving a continuous reference time-scale, whether by the modification of coordinated d univers sal time (UTC) or some other method, and take appropriate action, in accordance with Resolution 653

21 Sill Still many opin nions Currently operating systems Time keeping Daily life and culture Very few interest from majority tolerance and future technology

22 Summary Celestial bodies and Time scales periodical and dyn namical Atomic Timecales Relativity, coordinate time, synchronization UTC, compromise b etween AT and dut Digital age, inconvenie ence in leap sec. Difficulty in building consensus