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 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 of time (in SI): minute - 1 min = 60 s hour - 1 h = 60 min = 3600 s day - 1 d = 24 h = s Planck time – shortest time unit, based on our knowledge of natural physical units determined experimentally => present value t P = 5,391 24(27) × s X longest units = calendar units - week, month, year (365 or 366 days in leap year) day, year – derived from Earth rotation and Earth revolution around Sun => unsteady => the lenght is variable calendar day x true solar day calendar year x tropical (solar) year and sidereal year mean Julian year 1 a j = 365,25 d = 3, × 10 7 s (established by IAU for usage in astronomy and astrophysics)

Day basic unit of our biological clock – derived from the lenght of Earth rotation today - 1 d = 24 hours = 1440 min = s, officially accepted for use with the SI previously – daytime, night-time, twilight daytime devided into 10 parts + 2 twilights (morning, evening), night-time sometimes into 12 parts origin? at that time favourite duodecimal system, 12 months in a year, 12 phalanxes (without thumb) counting of hours in a day - antiquity, middle ages – since sunrise (noon = end of 6th hour) - Italian (till half 18th cent.), Czech (till 17th cent.) system – since evening (sunset, twilight) - 24 hours in a day => time of noon differs in different seasons of year (e.g. at 15 o’clock or even at 19 o’clock) - middle ages Islamic system – since evening twilight - German system – since midnight

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 Quirico Filopanti – first note in the book Miranda! 1863 engineer Charles F. Dowd – no response 1876 Canadian Sir Sandford Fleming – first proposal of 24 timezones, the day starts at anti-meridian of Greenwich (longitude 180°) 1884 the International Meridian Conference – 24 timezones, but a universal day of 24 hours beginning at Greenwich midnight

date line Deviation from regular zonetimes: - the border lines of time zones are not strict - summer time/daylight-saving time - winter time – it is not the present time! – in CR, Slovakia there is common zonal time, but a winter time would be time of neighbourgh zonetime for a zone in western direction from us (practically not used)

Measurements of time 2 ways: 1.direct measurement of steady motion and/or its expression - using Sun – gnomon, sundials – BC - clepsydra (water clock), hourglass/sandglass, candle clock – 300 BC

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

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

Times in astronomy unsteady connected with Earth rotation – solar time, sidereal time – measurements of cross the meridian; today VLBI observations of distant quasars (accuracy  s) - irregularities derived from Earth revolution – ephemerides time (ET)steady atomic time (International Atomic Time, TAI) – time of atomic clock * * * Julian time/date – launched in the beginning of 17th century - Joseph Scaliger; counting of days since 1st January 4713 BC, , 0 h UT – ,5

Terminology “reference frame” – refers to the geometric location from which the time is measure=> different reference frames differ by the light-travel time between them “time standard” – refers to 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 of the event two basic sources of uncertainties of timings in astrophysics: astrophysical data characterising of observed event time stamp, with the help of which we cited the event; - long-term stability - it must be clear how the used time stamp was reached but accuracy of time stamp – supposed accuracy of data – problematic, does not correspond to accuracy of time stamp

The correct time stamp: BJD TDB = JD UTC +  R  +  C  +  S  +  E  JD UTC – Julian date in UTC (Coordinated Universal Time),  R  - Rømer delay,  C  - clock correction,  S  - Shapiro delay,  E  - Einstein delay time of signal detection e.g. JD UTC should be always given to BJD TBD 1 s Accurate time in astronomy time stamp accuracy should be much better than uncertainty of data themselves!!! particular situation: e.g. exoplanets – used JD, GJD, HJD, BJD, however 1. often in unclear or unspecified time standards 2. the most often case – in UTC  however it is not continual!

Time standards JD can be specified in many time standards, IAU has made no explicit statement regarding the allowed time standards of the GJD, HJD, or BJD => meaning in any given time standard unambiguous GJD, HJD, or BJD - specified in many standards, usually implicitly => be careful not to directly compare BJDs or HJDs in different time standards GMT UT UT0 UT1 UT2 TAI UTC TT(TAI) TT(BIMP) TT, TDT TDB TCB

GMT (Greenwich Mean Time) – based on mean solar time at Greenwich 1847 accepted on British islands by the company Railway Clearing House ("railway time„); 1880 accepted oficially for Great Britain 1884 – Grenwich meridian – starting line of longitude UT (Universal Time) – successor of GMT, term UT established in1928 (change of definition of astronomical beginning day since 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. UT1R a smoothed version of UT1, filtering out changes shorter then 35 days (due to tides) UT2 not used very often; UT1 filtering out periodic seasonal variations in Earth rotation UTC (Coordinated Universal Time) – base for civil time measurements and zonetimes; time in PCs via Network Time Protocol (NTP) server => used by majority of observers; an atomic timescale that approximates UT1; max. difference 0,9 second => leap seconds (34 s), every 6 months => not continual! Time standards 1

.Difference between the uniform BJD TDB and the BJD UTC

TAI (International Atomic Time) – atomic clock, 1 s = “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,” – see Resolution1 from 13. Conférence Générale des Poids et Mesures (CGPM) in 1967; This definition is based on the duration of the Ephemeris Time second, which was previously defined as 1/31,556, of the tropical year for 1900 January 0 at 12 hours Ephemeris Time by Resolution 9 of the eleventh CGPM in TAI – base for series of other time standards, also for e.g. Sloan Digital Sky Survey. TT(TAI) (Terrestrial Time) – simple offset from TAI of s released in real time from atomic clocks and never altered s; maintain continuity between it and its predecessor, the Ephemeris Time (ET). TT(BIPM) – more precise version of TT(TAI); BIPM = The International Bureau of Weights and Measures; current difference between TT(TAI) a TT(BIPM) - 30μs TT (Terrestrial Time) – sometimes called Terrestrial Dynamical Time (TDT), can refer to either TT(TAI) or TT(BIPM) Time standards 2

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, T eph = 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 Correction from UTC to TDB  C = N s + (TDB - TT) N – number of leap seconds

Practice 1 -take care of accurate time in PC -save data in UTC -always give reference frame in publication -reference frame – e.g. HJD UTC -to publish times in BJD TBD !!!  timing of own measurements

- surveys – be carefull of time format!, each survey has its own reference frame and use different time standard - deliberation especially in case of satellite measurements – OMC …. why we should take care about time? -long-term studies – e.g. period changes, chnages in O-C diagrams – today changes smaller than 1 min! => we need to pay close attention to time stamps accuracy! Practice 2  taking data from another sources

Albert Einstein There are one thousands of ways how to kill the time, but no one how to resurrect it. I hope my talk was not only lost (killed) time thank you for your attetion!