Time standard
A time standard is a specification for measuring time: either the rate at which time passes or points in time or both. In modern times, several time specifications have been officially recognized as standards, where formerly they were matters of custom and practice. An example of a kind of time standard can be a time scale, specifying a method for measuring divisions of time. A standard for civil time can specify both time intervals and time-of-day.
Standardized time measurements are made using a clock to count periods of some period changes, which may be either the changes of a natural phenomenon or of an artificial machine.
Historically, time standards were often based on the Earth's rotational period. From the late 18 century to the 19th century it was assumed that the Earth's daily rotational rate was constant. Astronomical observations of several kinds, including eclipse records, studied in the 19th century, raised suspicions that the rate at which Earth rotates is gradually slowing and also shows small-scale irregularities, and this was confirmed in the early twentieth century. Time standards based on Earth rotation were replaced (or initially supplemented) for astronomical use from 1952 onwards by an ephemeris time standard based on the Earth's orbital period and in practice on the motion of the Moon. The invention in 1955 of the caesium atomic clock has led to the replacement of older and purely astronomical time standards, for most practical purposes, by newer time standards based wholly or partly on atomic time.
Various types of second and day are used as the basic time interval for most time scales. Other intervals of time (minutes, hours, and years) are usually defined in terms of these two.
Terminology
The term "time" is generally used for many close but different concepts, including:
- instant[1] as an object – one point on the time axis. Being an object, it has no value;
- date[2] as a quantity characterising an instant. As a quantity, it has a value which may be expressed in a variety of ways, for example "2014-04-26T09:42:36,75" in ISO standard format, or more colloquially such as "today, 9:42 a.m.";
- time interval[3] as an object – part of the time axis limited by two instants. Being an object, it has no value;
- chronology, an ordered sequence of events in the past. Chronologies can be put into chronological groups (periodization). One of the most important systems of periodization is the geologic time scale, which is a system of periodizing the events that shaped the Earth and its life. Chronology, periodization, and interpretation of the past are together known as the study of history.
Definitions of the second
There have only ever been three definitions of the second: as a fraction of the day, as a fraction of an extrapolated year, and as the microwave frequency of a caesium atomic clock.[6]
In early history, clocks were not accurate enough to track seconds. After the invention of mechanical clocks, the
In the late 1940s, quartz crystal oscillator clocks could measure time more accurately than the rotation of the Earth.
Most recently, atomic clocks have been developed that offer improved accuracy. Since 1967, the
The SI second is the basis of all atomic timescales, e.g. coordinated universal time, GPS time, International Atomic Time, etc.Current time standards
Coordinated Universal Time (UTC) is an atomic time scale designed to approximate UT1. UTC differs from TAI by an integral number of seconds. UTC is kept within 0.9 second of UT1 by the introduction of one-second steps to UTC, the "leap second". To date these steps (and difference "TAI-UTC") have always been positive.
The
Standard time or civil time in a time zone deviates a fixed, round amount, usually a whole number of hours, from some form of Universal Time, usually UTC. The offset is chosen such that a new day starts approximately while the Sun is crossing the nadir meridian. Alternatively the difference is not really fixed, but it changes twice a year by a round amount, usually one hour, see Daylight saving time.
Julian day number is a count of days elapsed since Greenwich mean noon on 1 January 4713 B.C., Julian proleptic calendar. The Julian Date is the Julian day number followed by the fraction of the day elapsed since the preceding noon. Conveniently for astronomers, this avoids the date skip during an observation night. Modified Julian day (MJD) is defined as MJD = JD - 2400000.5. An MJD day thus begins at midnight, civil date. Julian dates can be expressed in UT1, TAI, TT, etc. and so for precise applications the timescale should be specified, e.g. MJD 49135.3824 TAI.[14]
Barycentric Coordinate Time (TCB) is a coordinate time having its spatial origin at the center of mass of the Solar System, which is called the barycenter.
Conversions
Conversions between atomic time systems (TAI, GPST, and UTC) are for the most part exact. However, GPS time is a measured value as opposed to a computed "paper" scale.[15] As such it may differ from UTC(USNO) by a few hundred nanoseconds,[16] which in turn may differ from official UTC by as much as 26 nanoseconds.[15] Conversions for UT1 and TT rely on published difference tables which as of 2022[update] are specified to 10 microseconds and 0.1 nanoseconds respectively.
System | Description | UT1 | UTC | TT | TAI | GPS |
---|---|---|---|---|---|---|
UT1 | Mean Solar Time | UT1 | UTC = UT1 – DUT1 | TT = UT1 – DUT1 + LS + 32.184 s + DTT | TAI = UT1 – DUT1 + LS | GPS = UT1 – DUT1 + LS – 19 s |
UTC | Civil Time | UT1 = UTC + DUT1 | UTC | TT = UTC + LS + 32.184 s + DTT | TAI = UTC + LS | GPS = UTC + LS – 19 s |
TT | Terrestrial Time | UT1 = TT – 32.184 s - DTT – LS + DUT1 | UTC = TT – 32.184 s - DTT – LS | TT | TAI = TT – 32.184 s - DTT | GPS = TT – 51.184 s - DTT |
TAI | Atomic Time | UT1 = TAI – LS + DUT1 | UTC = TAI – LS | TT = TAI + 32.184 s + DTT | TAI | GPS = TAI – 19 s |
GPS | GPS Time | UT1 = GPS + 19 s – LS + DUT1 | UTC = GPS + 19 s – LS | TT = GPS + 51.184 s + DTT | TAI = GPS + 19 s | GPS |
Definitions:
- LS = TAI – UTC = Leap Seconds from USNO Table of Leap Seconds
- DUT1 = UT1 – UTC published in IERS Bulletins or U.S. Naval Observatory EO
- DTT = TT - TAI - 32.184 s published in BIPM's TT(BIPM) tables.
TCG is linearly related to TT as: TCG - TT = LG * (JD -2443144.5) * 86400 seconds, with the scale difference LG defined as 6.969290134e-10 exactly.
TCB is a linear transformation of TDB and TDB differs from TT in small, mostly periodic terms. Neglecting these terms (on the order of 2 milliseconds for several millennia around the present epoch),[17] TCB is related to TT by: TCB - TT = LB * (JD -2443144.5) * 86400 seconds.[18] The scale difference LB has been defined by the IAU to be 1.550519768e-08 exactly.[17]
Time standards based on Earth rotation
Sidereal time is time by the stars. A sidereal rotation is the time it takes the Earth to make one revolution with rotation to the stars, approximately 23 hours 56 minutes 4 seconds. A mean solar day is about 3 minutes 56 seconds longer than a mean sidereal day, or 1⁄366 more than a mean sidereal day. In astronomy, sidereal time is used to predict when a star will reach its highest point in the sky. For accurate astronomical work on land, it was usual to observe sidereal time rather than solar time to measure mean solar time, because the observations of 'fixed' stars could be measured and reduced more accurately than observations of the Sun (in spite of the need to make various small compensations, for refraction, aberration, precession, nutation and proper motion). It is well known that observations of the Sun pose substantial obstacles to the achievement of accuracy in measurement.[19] In former times, before the distribution of accurate time signals, it was part of the routine work at any observatory to observe the sidereal times of meridian transit of selected 'clock stars' (of well-known position and movement), and to use these to correct observatory clocks running local mean sidereal time; but nowadays local sidereal time is usually generated by computer, based on time signals.[20]
Versions of Universal Time such as UT0 and UT2 have been defined but are no longer in use.[21][22]
Time standards for planetary motion calculations
For applications at the Earth's surface, ET's official replacement was
For the calculation of ephemerides,
See also
- Atomic clock
- Clock synchronization
- Clock signal
- Epoch (astronomy)
- Frequency standard
- Radio clock
- Time in astronomy
- Time signal
- Time metrology
- Time transfer
- Timekeeping on Mars
- Orbital period as unit of time
Notes
References
Citations
- ^ IEC 60050-113:2011, item 113-01-08
- ^ IEC 60050-113:2011, item 113-01-012: "mark attributed to an instant by means of a specified time scale
- ^ IEC 60050-113:2011, item 113-01-010; ISO 80000-3:2006, item 3–7
- ^ IEC 60050-113:2011, item 113-01-013: "range of a time interval (113-01-10)"
- ^ ISO 80000-3:2006, item 3–7
- ^ U.S. Naval Observatory. "Leap Seconds". Archived from the original on 2019-10-19. Retrieved 19 October 2019.
- ISBN 978-1-4081-7207-0.
- ^ "Leap Seconds". Time Service Department, United States Naval Observatory. Archived from the original on March 12, 2015. Retrieved November 22, 2015.
- BIPM. p. 112. Archived(PDF) from the original on May 3, 2019. Retrieved May 23, 2019.
- ^ McCarthy, Dennis D.; Seidelmann, P. Kenneth (2009). Time: From Earth Rotation to Atomic Physics. Weinheim: Wiley. pp. 231–232.
- NIST. Archivedfrom the original on 17 April 2011. Retrieved 9 April 2011.
- ^ TAI
- ^ "BIPM - clock comparisons". Archived from the original on 2019-08-10.
- ^ Matsakis, Demetrios. "Systems of time". Archived from the original on 2019-09-30. Retrieved 30 September 2019.
- ^ a b "International Time Scales and the B.I.P.M. — Naval Oceanography Portal". www.usno.navy.mil. Retrieved 23 March 2022.
- ^ "USNO GPS Time Transfer — Naval Oceanography Portal". www.usno.navy.mil. Retrieved 23 March 2022.
GPS time is automatically steered to UTC(USNO) on a daily basis to keep system time within one microsecond of UTC(USNO), but during the last several years has been within a few hundred nanoseconds.
- ^ a b "IAU 2006 Resolution B3: Re-definition of Barycentric Dynamical Time, TDB" (PDF). p. 2. Archived (PDF) from the original on 2022-10-09. Retrieved 4 April 2022.
- ^ "IAU (1991) RECOMMENDATION III". www.iers.org. Note 1.
- ^ See H A Harvey, "The Simpler Aspects of Celestial Mechanics", in Popular Astronomy 44 (1936), 533-541.
- ^ A E Roy, D Clarke, 'Astronomy: Principles and Practice' (4th edition, 2003) at p.89.
- ^ Urban & Seidelmann 2013, p. 81.
- ^ Schlyter, Paul. "Time Scales: UT1, UTC, TAI, ET, TT, GPS time". www.stjarnhimlen.se. Retrieved 21 March 2022.
UT2 is nowadays considered obsolete.
- ^ W Markowitz, R G Hall, L Essen, J V L Parry (1958), 'Frequency of caesium in terms of ephemeris time', Phys Rev Letters v1 (1958), 105-107; and Wm Markowitz (1988) 'Comparisons of ET(Solar), ET(Lunar), UT and TDT', in (eds.) A K Babcock & G A Wilkins, 'The Earth's Rotation and Reference Frames for Geodesy and Geophysics', IAU Symposia #128 (1988), at pp 413-418.
- V Brumberg, S Kopeikin(1990), 'Relativistic time scales in the solar system', Celestial Mechanics and Dynamical Astronomy (1990), Vol. 48, 23-44
- ^ P K Seidelmann & T Fukushima (1992), "Why new time scales?", Astronomy & Astrophysics vol.265 (1992), pages 833-838, including Fig. 1 at p.835, a graph giving an overview of the rate differences and offsets between various standard time scales, present and past, defined by the IAU.
Sources
- Urban, Sean; Seidelmann, P. Kenneth, eds. (2013). Explanatory Supplement to the Astronomical Almanac (3rd ed.). Mill Valley, California: University Science Books.
- Explanatory Supplement to the Astronomical Almanac, P. K. Seidelmann, ed., University Science Books, 1992, ISBN 0-935702-68-7.
External links
- Current time according to the bservatory (get the current time)
- Systems of Time by Demetrios Matsakis, Director, Time Service Dept., United States Naval Observatory
- USNO article on the definition of seconds and leap seconds Archived 2012-06-11 at the Wayback Machine
- A history of astronomical time scales by Steve Allen
- Why is a minute divided into 60 seconds, an hour into 60 minutes, yet there are only 24 hours in a day Ask the Experts – March 5, 2021. SCIENTIFIC AMERICAN