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Understanding Time from an Astronomical Perspective

Explore the intricacies of time measurements in astrometry, from atomic time scales to universal time systems, with examples and conversions. Learn the significance of various time units and representations in astronomical calculations.

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Understanding Time from an Astronomical Perspective

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  1. Time • Newtonian Viewpoint • Absolute Time • Time Transformation: 1-to-1 • Ordering: Chronology • Precision • Accuracy??? Lecture Notes on Astrometry

  2. Integrated Time Scale • Assumption: Same Time Interval of Repeating Phenomenon • Counted Numbers to Expr. of Time • Examples • Month, Day • Mechanical Clock • Quartz, Atomic Time Lecture Notes on Astrometry

  3. Atomic Time • Definition of SI Second (CGPM, 1967) • 9162631770 Periods, Certain Radiation, Cs133 • International Atomic Time (TAI) • BIPM (Paris) • ~ 200 Cs + some H Masers • Relative Precision: 15-16 digits Lecture Notes on Astrometry

  4. Dynamical Time • Argument of Eq. of Motion • Inverse Function of some Observable • Example: • Mean Longitude of Sun • L(T) = • Ephemeris Time, ET = T(L) Lecture Notes on Astrometry

  5. Universal Time • Dynamical Time of Earth Rotation • UT (= Greenwich Mean Time) • UT0, UT1, UT2 • International Earth Rotation Service (IERS) • Coordinated Universal Time (UTC) • Leap Seconds Lecture Notes on Astrometry

  6. Barycentric Dynamical Time • IAU(1984-1991) • General Relativistic Effects • Adjustment of Unit of Time • Solar System Barycentric Dyn. Time: TDB • Terrestrial Dynamical Time: TDT • TDT = TAI+32.184s • <TDB> = <TDT> Lecture Notes on Astrometry

  7. Barycentric Coordinate Time • IAU(1991-) • No More Adjustment of Unit of Time • SS Barycentric Coordinate Time: TCB • Geocentric Coordinate Time: TCG • Terrestrial Time: TT • TT = TDT = TAI+32.184s Lecture Notes on Astrometry

  8. Broadcast Time • Standard Time, Time Zone: 15deg = 1 h • Japan Standard Time (JST) • JST = UTC + 9 h • Radio/Wired Time: JJY, TV, NTT • Time on Computer Network: NTP • Time from Satellites: GPS Lecture Notes on Astrometry

  9. Time Units • 1 day = 24 h = 1440 m = 86400 s • Julian Century: jc, Julian Year: jy • 1 jc = 100 jy = 36525 days • Besselian Year = Mean Solar Year • 1 mean solar year = 365.2421897… days • ms, ms, ns, ps, fs, … • c = 299792458 m/s Lecture Notes on Astrometry

  10. Time Representations • Hour, Minute, Second • Year, Month, Day • Day of Year • Julian Dates (Day Number) • 2000 Jan.1,12h = J2000.0 = 2451545.0 • Modified Julian Dates (MJD) • MJD = JD – 2400000.5 Lecture Notes on Astrometry

  11. Conversion to Julian Date • (Y,M,D,h,m,s) to JD • L=int((M-14)/12); • I=1461*(Y+4800+L); • J=367*(M-2-12*L); • K=int((Y+4900+L)/100); • N=int(I/4)+int(J/12)-int((3*K)/4) Lecture Notes on Astrometry

  12. Conv. To JD (contd.) • JD0=N+D-32075; • JD1=JD0-0.5; • JD2=h/24.0+m/1440.0+s/86400.0; • JD=JD1+JD2 or JD = (JD1,JD2) Lecture Notes on Astrometry

  13. Conversion from Julian Date • JD to (Y,M,D,h,m,s) • JD0=int(JD-0.5) or JD0=JD1-0.5; • L=JD0+68569; • N=int((4*L)/146097); • K=L-int((146097*N+3)/4); • I=int(4000*(K+1))/1461001); • P=K-int((1461*I)/4)+31; Lecture Notes on Astrometry

  14. Conv. From JD (contd.) • J=int((80*P)/2447); • D=P-int((2447*J)/80); Q=int(J/11); • M=J+2-12*Q; Y=100*(N-49)+I+Q; • JD2=JD-JD1; • h=int(JD2*24); m=int(JD2*1440-h*60); • s=JD2*86400-h*3600-m*60; Lecture Notes on Astrometry

  15. Day of Week from Julian Date • I=JD0-7*int((JD0+1)/7)+2; • I: 1,2,3,4,5,6,7 • I=1: Sunday Lecture Notes on Astrometry

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