Hvad er UTC?

[BjarneModerator Super Nova]

Med borgerforslaget om, at Danmak officielt bør opdatere en lov fra 1893, er det måske på sin plads at spørge: Hvad er UTC? Det rette sted at gå til er U.S. Naval Observatory, 3450 Massachusetts Ave, NW, Washington, DC .

The United States Naval Observatory (USNO)

USNO strengthens national security and critical infrastructure by serving as DoD’s authoritative source for the positions and motion of celestial bodies, motions of the Earth, and precise time. USNO provides tailored products, performs relevant research, develops leading edge technologies and instrumentation, and operates state of the art systems in support of the U.S. Navy, DoD, Federal Agencies, international partners, and the general public.

USNO products support activities in the following areas:

Astrometry

Integrated Image and Catalog Archive Service, star catalogs, fundamental reference frames, space astrometry.

Astronomical Applications

Astronomical data calculators (e.g., rise/set times, Moon phase, etc.), information on astronomical phenomena and related topics, astronomical and navigational almanacs, software products.

Earth Orientation

Daily, weekly, and long-term EO products (e.g. IERS Bulletin A), GPS-based products (rapids, ultra-rapids, UTGPS), VLBI-based products, background information, technical and scientific publications and software.

Precise Time

USNO Master Clock, Network Time Protocol (NTP) servers, web-based time synchronization, GPS timing products and services, Two-Way Satellite Time Transfer, and Loran-C timing products.

Af særlig interesse er aktiviteterne i Earth Orientation:

Earth Orientation

The U.S. Naval Observatory is responsible for determining and predicting the time-varying alignment of the Earth’s terrestrial reference frame with respect to the celestial reference frame. USNO is the International Earth Rotation and Reference Systems Service (IERS) Rapid Service/Prediction Center (RS/PC) for Earth Orientation.

Earth Orientation Products

The latest determinations and predictions for polar motion, UT1-UTC, celestial pole offsets, and long-term Delta T are provided.

GPS-based Products

The latest 24-hour and 48-hour sets of GPS satellite orbits, satellite and receiver clock solutions, earth orientation parameters, and UTGPS (GPS-based UT1-like quantity).

VLBI-based Products

The latest Earth Orientation Parameters derived from Very Long Baseline Interferometry (VLBI) observations and correlator data products.

EO Information Center

Information regarding commonly used variables (General Information), Information for GPS Users (GPS User Information), frequently asked questions about Earth Orientation, and format descriptions for data sets (Read Me files).

Publications about Products

Publications providing background material (Explanatory Supplement), documentation of procedures and quality of results (Annual Reports), and technical details regarding the procedures (Scientific Publications).

Software

Supporting software for searching through Earth orientation results and for calculating the rotation matrices between terrestrial and celestial reference frames. Recommended support and auxiliary software for use with Earth Orientation products as input.

Det er måske her på sin plads at indskyde en forklaring af de begreber, som bliver omtalt på USNO’s hjemmeside. UTC er en kombination af atomtid (TAI) og astronomisk tid (UT1):

UTC – The World’s Time Standard

What Is International Atomic Time (TAI)?

What Is Universal Time?

Aktiviteterne under VLBI-based products er af særlig interesse:

VLBI-based Earth Orientation Parameters (EOP)

Observations from a number of different techniques can be used to determine Earth Orientation Parameters such as the Earth’s polar motion and the length of day. One such technique is Very Long Baseline Interferometry (VLBI). The Earth Orientation Parameters (EOP) obtained through periodic VLBI observations also connect the Celestial Reference Frame (CRF) to the Terrestrial Reference Frame (TRF). VLBI-based EOP products are updated daily as new VLBI data become available and can be used individually or combined with EOP results from other techniques.

Formålet med VLBI målinger af kvasarer er at definere et inertialsystem som reference for Jordens rotation. NASA har her beskrevet metoden:

What is VLBI?

Over its 40-year history of development and operation, the space geodetic technique called very long baseline interferometry (VLBI) has provided an unprecedented record of the motions of the solid Earth. VLBI is unique in its ability to define an inertial reference frame and to measure the Earth’s orientation in this frame. Changes in the Earth’s orientation in inertial space have two causes: the gravitational forces of the Sun and Moon and the redistribution of total angular momentum among the solid Earth, ocean, and atmosphere. VLBI makes a direct measurement of the Earth’s orientation in space from which geoscientists then study such phenomena as atmospheric angular momentum, ocean tides and currents, and the elastic response of the solid Earth.

VLBI is a geometric technique; it measures the time difference between the arrival at two Earth-based antennas of a radio wavefront emitted by a distant quasar. Using large numbers of time difference measurements from many quasars observed with a global network of antennas, VLBI determines the inertial reference frame defined by the quasars and simultaneously, the precise positions of the antennas. Because the time difference measurements are precise to a few picoseconds, VLBI determines the relative positions of the antennas to a few millimeters and the quasar positions to fractions of a milliarcsecond. Since the antennas are fixed to the Earth, their locations track the instantaneous orientation of the Earth in the inertial reference frame. Relative changes in the antenna locations from a series of measurements indicate tectonic plate motion, regional deformation, and local uplift or subsidence.

The heritage of VLBI is 40 years of NASA-led technology development that included the highly successful Crustal Dynamics Project, during which the first contemporary measurements of tectonic plate motion were made. Today VLBI observations, analysis and development are coordinated by the International VLBI Service for Geodesy and Astrometry (IVS), comprising some 80 components (including 45 antennas) sponsored by 40 organizations located in 20 countries. The IVS Coordinating Center is located at Goddard Space Flight Center in Greenblet, MD. VLBI determines with unequaled accuracy the terrestrial reference frame (antenna locations on the Earth), the International Celestial Reference Frame (quasar positions on the sky), and Earth’s orientation in space. In the future, VLBI development will continue in measurement systems technology, research on the neutral atmosphere, and integration with other space geodetic techniques.

VLBI is a valuable asset in NASA’s mission of science-driven technology leadership. Earth science research requires VLBI’s Earth orientation data coupled to a stable, accurate terrestrial reference frame.

Bemærk: Antennernes placeringer er bestemt med en nøjagtighed af nogle få mm! Der er styr på Jordens varierende rotation.

[BjarneModerator Super Nova]

Grundlaget for tidsmålinger er de 400 koordinere atomure, idet 1 s er defineret ved “the time it takes a Cesium-133 atom at the ground state to oscillate exactly 9,192,631,770 times”. Meteren er defineret ud fra 1s, idet lyshastigheden nu er en defineret størrelse, som ikke skal måles. Et skudsekund insættes som kompensation for Jordens varierende rotationshastighed. Kravet er, at et middelsoldøgn skal være så tæt på 24 timer som muligt. Man kender, som ovenfor fortalt, Jordens rotation i forhold til fjerne kvasarer meget mere nøjagtigt end strengt nødvendigt. Man har desuden brug for en dynamisk model for Solsystemet i det samme VLBI koordinatsystem. Jordens bane omkring Solen kombineret med Jordens rotation i samme VLBI-system giver middelsoldøgnets længde. Hvis det bliver længere end 24 timer indsætte et eller flere skudsekunder.

[Torben TaustrupAdmin Neutron star]

Tak for forklaringen.
Når du skriver “varierende rotationshastighed” kan den så variere både positivt og negativt?

mvh
Torben

TOC Observatory - "http://tocobs.org -14.5″ – f:4,2 Newt - Atik383 - ZWO2600-mono – SXV H9 - QHY8L-color - SkyWatcher 80 mm ED refraktor - 60 mm F:6 apocromat - TAL Apolar 125 f : 7,5.

[BjarneModerator Super Nova]

Jeg mener, at den er set negativ, men den er oftest positiv. Man medtager ikke alle variationer. Man er i stand til at måle årlige variationer som følge af ændringer i havstrømme, eller ændringer i luftstrømme. Det giver kun mening, hvis variationerne nærmer sig 1 sekund. Men det kan have betydning ved VLBI målinger af andre astronomiske objekter, for ikke at tale om geofysiske målinger af pladetektonik.

Men hvordan måler man middelsolens gang over himlen? Det gør man ikke. Man er kun interesseret i Jordens vinkelhastighed i forhold til Solens vinkelhastighed. Det betyder intet, hvor den fiktive “middelsol” befinder sig. I gamle dage (før VLBI af kvasarer) talte man om et siderisk (i forhold til stjerner) år og et siderisk døgn. Jeg vil i stedet indføre et kvasardøgn P_k og et kvasarår U_k. Disse størrelser kan måles med meget stor nøjagtighed. Det er strengt taget den beregnede jordbane, som anvendes til bestemmelse af kvasaråret. Vi interesserer os for Jordens vinkelhastighed i forhold til Solens vilkelhastighed. Jeg indfører derfor et middelsoldøgn P_s, som helst skal være tæt på 24 timer = 24*60*60 = 86400 s.

Jordens vinkelhastighed i forhold til middelsolen: Ω_sJ = 2π/P_s.
Jordens vinkelhastighed i forhold til kvasarerne: Ω_kJ = 2π/P_k.
Middelsolens vinkelhastighed i forhold til kvasarerne: Ω_kS = 2π/U_k.

Der gælder, at Ω_sJ = Ω_kJ – Ω_kS, hvorfor 1/P_s = 1/P_k – 1/U_k.

Døgnet baseret på middelsolen kan derfor let findes ud fra kvasardøgnet og kvasaråret i sekunder.

 

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