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KOLOS 2011

KOLOS 2011 . Time. Ray Cummings (1921), later John Archibald Wheeler : Time is what keeps everything from happening at once . . Miloslav Zejda, KOLOS 2011. Accuracy of timings in astrophysical measurements. Unit of time

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KOLOS 2011

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  1. KOLOS 2011

  2. Time RayCummings (1921), later John ArchibaldWheeler: Timeiswhatkeepseverythingfrom happening atonce. Miloslav Zejda, KOLOS 2011

  3. Accuracyoftimings in astrophysicalmeasurements

  4. Unit of time • second (s) – base time unit in SI – since 1967 the duration of 9,192,631,770 periods of the radiation corresponding to the transition between the two hyperfine levels of the ground state of the caesium 133 atom • other unit oftime (in SI): minute- 1 min = 60 s • hour- 1 h = 60 min = 3600 s • day- 1 d = 24 h = 86 400 s • Planck time – shortesttime unit, based on our knowledge of natural physical units determined experimentally => present value tP = 5,391 24(27) × 10-44 s • X • longestunits = calendar units - week, month, year(365 or 366 days in leap year) • day, year – derivedfrom Earth rotation and Earth revolution around Sun => unsteady => thelenghtisvariable • calendar day x true solar day • calendar year x tropical (solar) year and sidereal year • mean Julian year 1 aj = 365,25 d = 3,155 76 × 107 s (established by IAU for usage in astronomy and astrophysics)

  5. Day • basic unit ofourbiologicalclock – derivedfromthelenghtofEarthrotation • today- 1 d = 24 hours= 1440 min = 86400 s, officially accepted for use with the SI • previously – daytime, night-time, twilight • daytimedevidedinto 10 parts + 2 twilights (morning, evening), night-timesometimesinto 12 parts • origin? • atthattimefavouriteduodecimalsystem, • 12 months in a year, • 12 phalanxes (withoutthumb) • countingofhours in a day • - antiquity, middleages– sincesunrise (noon = end of 6th hour) • - Italian (tillhalf 18th cent.), Czech (till 17th cent.) system – sinceevening (sunset, twilight) - 24 hours in a day => timeofnoondiffers in differentseasonsofyear (e.g. at 15 o’clock or even at 19 o’clock) • - middleagesIslamicsystem – sinceeveningtwilight • - Germansystem – sincemidnight

  6. Local time = time valid for a line of longitude, where we are located difference of local times = differences of longitudes (of these two locations) Zonetimes middle 19th century - system of many local times used to be inconvenient => travelling (especially using railways) led to zonetimes 1858 mathematician QuiricoFilopanti– first note in thebook Miranda! 1863engineer Charles F. Dowd – no response 1876Canadian Sir Sandford Fleming – first proposal of 24 timezones, the day starts at anti-meridian of Greenwich (longitude 180°) 1884 the International MeridianConference– 24 timezones, but a universal dayof 24 hoursbeginning atGreenwich midnight

  7. date line Deviationfromregularzonetimes: - theborder lines oftimezones are not strict - summertime/daylight-savingtime - wintertime – itis not thepresenttime!– in CR, Slovakia thereiscommonzonaltime, but a wintertimewouldbetimeofneighbourghzonetimefor a zone in western directionfromus (practically not used)

  8. Measurementsoftime • 2 ways: • direct measurementofsteadymotion and/oritsexpression • - using Sun – gnomon, sundials – 3 000 BC • - clepsydra (waterclock), hourglass/sandglass, candleclock – 300 BC

  9. Measurementsoftime • 2 ways: • 2. counting of regular movements - mechanical, electric • or atomic oscillations • - mechanical clocks – driving-wheel, beats, balance wheel – before 1320 • - marine chronometers – 1760 John Harrison • - electronic clock – quartz crystal • - atomic clock- the most accurate time standard

  10. Measurementsoftime - accuracy • - 2011 – reached accuracy of time measurements • – the difference 1 second in 32 billions years • (= relative accuracy 4 × 10-19) • - since the clock invention improvement in 16 orders! • - measurements of time – one of the most accurate physical measurements

  11. Times in astronomy • unsteady • connectedwithEarthrotation– solartime, siderealtime • – measurementsofcrossthemeridian; • today VLBI observationsofdistantquasars (accuracyms) - irregularities • derivedfromEarthrevolution– ephemeridestime (ET) • steady • atomictime(International AtomicTime, TAI) – timeofatomicclock • * * * • Julian time/date– launched in thebeginningof 17th century - Joseph Scaliger; • countingofdayssince1st January 4713 BC, • 5. 12. 2011, 0 h UT – 2455900,5

  12. Terminology • “reference frame” – refersto the geometric location from whichthetimeismeasure=>different reference frames differ bythe light-travel time between them • “time standard” – refersto the way a particular clock ticks and its arbitrary zero point,as defined by international standards; • “time stamp” – combination of the RF and TS, and determines the timing accuracy • oftheevent • two basic sourcesofuncertaintiesoftimings in astrophysics: • astrophysicaldata characterisingofobservedevent • timestamp, withthehelpofwhichwecitedtheevent; • - long-term stability • - itmustbeclearhowtheusedtimestampwasreached • but • accuracyoftimestamp– supposed • accuracyof data – problematic, does not correspond to accuracyoftimestamp

  13. Accuratetime in astronomy timestampaccuracyshouldbe much betterthanuncertaintyof data themselves!!! particularsituation: e.g. exoplanets – used JD, GJD, HJD, BJD, however 1. often in unclearorunspecifiedtimestandards 2. the most often case – in UTC  howeveritis not continual! Thecorrecttimestamp: BJDTDB= JDUTC+DR +DC +DS  +DE JDUTC– Juliandate in UTC(Coordinated UniversalTime), DR- Rømerdelay, DC - clockcorrection, DS - Shapiro delay, DE- Einstein delay timeofsignaldetectione.g. JDUTCshouldbealwaysgiven to BJDTBD 1 s

  14. Timestandards • JD canbespecified in many timestandards, • IAU has made no explicitstatement regarding the allowed time standards of the GJD,HJD, or BJD => meaning in any given time standard unambiguous • GJD,HJD, or BJD- specified in manystandards, • usuallyimplicitly => becareful not to directly • compare BJDs or HJDs in differenttimestandards • GMT • UT • UT0 • UT1 • UT2 • TAI • UTC • TT(TAI) • TT(BIMP) • TT, TDT • TDB • TCB

  15. Timestandards 1 • GMT (Greenwich Mean Time) – based on meansolartimeat Greenwich • 1847accepted on Britishislands by thecompanyRailway Clearing House ("railway time„); 1880 acceptedoficiallyfor Great Britain • 1884 – Grenwichmeridian – starting line oflongitude • UT (Universal Time) –successorofGMT, term UT established in1928 (changeofdefinitionofastronomicalbeginningdaysince Jan 1,.1925); • today – imprecise term, more variant available • UT0-Universal Time determined at an observatory by observing the diurnal motion of stars or extragalactic radio sources • UT1 - conceptually mean solar time at 0° longitude, the same everywhere on Earth, corrected for the displacement of Earth's geographic pole from its rotational pole; ±3 ms/day. • UT1Ra smoothed version of UT1,filteringoutchangesshorterthen 35 days (due to tides) • UT2 not used very often; UT1 filteringoutperiodicseasonalvariations in Earthrotation • UTC (Coordinated Universal Time) –base for civil timemeasurements and zonetimes; timein PCs via Network Time Protocol(NTP) server => used by majority ofobservers; • an atomic timescale that approximates UT1; max. difference0,9second • => leapseconds (34 s), every 6 months => not continual!

  16. .Difference between the uniform BJDTDB and the BJDUTC

  17. Timestandards 2 • TAI (International Atomic Time)– atomicclock, 1 s = “9,192,631,770 periods • ofthe radiation corresponding to the transition betweenthe two hyperfine levels of the ground state of the caesium133 atom,” – seeResolution1 from 13. Conférence Générale des Poids etMesures(CGPM) in 1967; • This definition is based onthe duration of the Ephemeris Time second, which was • previously defined as 1/31,556,925.9747 of the tropicalyear for 1900 January 0 at 12 hours Ephemeris Timeby Resolution 9 of the eleventh CGPM in 1960. • TAI – base forseriesofothertimestandards, alsofore.g. Sloan Digital SkySurvey. • TT(TAI) (Terrestrial Time) – simple offset from TAI of32.184 s released • in real time from atomic clocks andneveraltered32.184 s; maintaincontinuitybetweenit and its predecessor, the Ephemeris Time (ET). • TT(BIPM)– more preciseversionofTT(TAI); BIPM = The International Bureau • of WeightsandMeasures; currentdifferencebetweenTT(TAI) a TT(BIPM) - 30μs • TT (Terrestrial Time)–sometimescalledTerrestrialDynamicalTime (TDT), • can refer to either TT(TAI) or TT(BIPM)

  18. TDB (Barycentric Dynamical Time)– correct TT to barycentrum of Solar system; • correction TT -> TDB only series of approximations (Irwin & Fukushima 1999); difference up to 3.4 ms/year. IAU Resolution B3 (2006) converging on the same definition JPL Ephemeris Time, Teph= Coordinate Time (CT) in JPL ephemeris of Solar system objects • TCB (Barycentric Coordinate Time)– Physically and mathematically equivalent • to the TDB as defined in 2006 (Standish 1998), and differs only by an offset and rate of about 0.5 s yr-1; TDB and TCB roughly equal to TAI (1 January 1977), today difference approx. 16 s Correctionfrom UTC to TDB DC = N + 32.184s + (TDB- TT) N – numberofleapseconds

  19. Practice 1 • timingofownmeasurements • take care ofaccuratetime in PC • save data in UTC • alwaysgive reference frame in publication • reference frame – e.g. HJDUTC • to publishtimes in BJDTBD!!! • http://astroutils.astronomy.ohio-state.edu/time/

  20. Practice 2 • taking data fromanothersources • - surveys – becarefulloftimeformat!, eachsurvey has itsown reference frame and use differenttime standard • - deliberationespecially in case ofsatellitemeasurements – OMC …. • whyweshouldtake care abouttime? • long-term studies– e.g. period changes, chnages in O-C diagrams – todaychangessmallerthan 1 min! => weneed to paycloseattention to timestampsaccuracy!

  21. Albert Einstein There are onethousandsofwayshow to killthetime, but no onehow to resurrectit. I hope my talk was not onlylost (killed) time thankyouforyourattetion!

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