Time Scales
UTCUTC/GMT/GT: Coordinated Universal Time/Greenwich Mean Time/Greenwich Time
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ContentsUTC
Based on Earth's Rotation: Apparent Solar Time Based on Earth's Rotation: Mean Solar Time Sidereal Time Dynamical Time
Terrestrial Time |
Based on Earth's Rotation: Apparent Solar Time
Apparent Solar Time (Local Time, sometimes LT)
- Sunrise, sunset, passage through meridian (noon)
- See “Equation of Time” for more information
Based on Earth's Rotation: Mean Solar Time
- “mean” mean the average obtained by considering the rotation of Earth in the absence of the annual variations caused by obliquity and eccentricity
- decimal integer count of consecutive days beginning on January 1, 4713 MC in the proleptic Julian calendar
- Invented by French scholar Joseph Justus Scaliger in 1583 (named after Julius Caesar Scaliger, father)
- Product of 28, 19, and 15
- 28- year cycle of Julian calendar, 19-year cycle of Metonic cycle, 15-year cycle of Roman indiction (fiscal year)
- 4713 BC chosen as nearest past year in which all three cycles began together (Indiction, Golden Number, and Solar Number all equal to 1)
- number of days since JD 0.0 or Greenwich mean noon on January 1, 4713 B.C plus the fraction of the day since that instant
- astronomers preferred same date for all observations in a single night
- can be used in conjunction with time systems e.g. JD (TT)
- Mean Solar Time for any given longitude on Earth, differs at every longitude
- mean solar time of some conventionally chosen, standard (hopefully nearby) meridian (Refer to Current Time Scales for more information)
GMT: Greenwich Mean Time
- Used to be mean solar time of the Greenwich Meridian
- Currently can be synonymous to UTC
- Long history of confusion between UTC and GMT
- New name for mean solar time of Greenwich meridian with the hours and days reckoned from noon (defined by IAU in 1928)
- 1928, IAU approved GCT as synonymous with UT
- discontinued since legal civil time of Greenwich used Daylight Savings Time
- 1928, IAU approved use of WZ in German language as synonymous with UT
- Mean solar time of the Greenwich Meridian
- Sidereal time converted to UT since it is not easy to measure the position of the sun with great precision
- Same as GCT, WZ
- Formerly determined by astronomical observations, currently by GPS
- UT0 = Raw measure of UT based on transit observations at a single observatory
- UT1 = UT0 with corrections for polar wandering (movement of the earth's rotational or magnetic poles relative to the continents throughout geological time, due largely to continental drift), people normally mean UT1 when they say UT
- UT1R = regularized UT1 which filters out periodic variations due to tides
- UT2 = UT1 with empirical formula to remove effect of annual seasonal variations in rotation of Earth
Sidereal Time
General Definition
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Dynamical Time
- Time determined by comparing observations of motions of objects with physical models that describe that motion
Non-Relativistic Dynamical TimeNewtonian Time
Relativistic Dynamical Time
Rigorously Correct Relativistic TimeTCG: Geocentric Coordinate Time
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Atomic Time
TDT and TDB located under “Relativistic dynamical time”
SAT: Stepped Atomic Time
SAT: Stepped Atomic Time
- time scale used in long-wave radio broadcast time signals of US and Germany during 1960s
- most second intervals were atomic seconds
- frequent steps of 100 ms to 200 ms inserted to math UT2
- was used in WWVB and the PTB
- has not ticked uniformly throughout history
- weighted average of time by 400 atomic clocks
- clocks compared through GPS signals
- TAI-UTC approx. 0 on 1958 Jan 1
- initially ahead of UTC by 10 seconds (1972)
- TAI exactly 36 seconds ahead of of UTC (26 leap seconds added, current)
GA: Greenwich Atomic
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TA(k)
- Many time scales that contribute to TAI maintained in real time by many labs
- Ex. TA(NIST) or TA(PTB)
- satellite navigation systems
- Future time scale
- Nominal year to switch to it is 2022
- Defined under "UTC"
Terrestrial Time
- Defined as clarification of what TDT intended to be
- TT(TAI) = TAI + 32.184 s
- TT calculated assuming no defects in TAI since 1977-01-01 00:00:00
- 32.184 is best estimate of difference between time scales on that date
- effort to ascertain corrections to the values of TAI based on retrospective studies of behavior of chronometers which have contributed to TAI
- independent means of measuring defects of TAI inferred from TT(BIPMxy)