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Topic: Barycentric Dynamical Time

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Terrestrial Dynamical Time

Ephemeris time

Universal time

International Atomic Time

Coordinated Universal Time

Greenwich Mean Time

Year

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TT (disambiguation)

Terrestrial Time

	Systems of Time (Site not responding. Last check: 2007-11-04)
		Terrestrial Time (TT), (or Terrestrial Dynamical Time, TDT), with unit of duration 86400 SI seconds on the geoid, is the independent argument of apparent geocentric ephemerides.
		Geocentric Coordinate Time (TCG) is a coordinate time having its spatial origin at the center of mass of the Earth.
		Sidereal Time, with unit of duration the period of the Earth's rotation with respect to a point nearly fixed with respect to the stars, is the hour angle of the vernal equinox.
	tycho.usno.navy.mil /systime.html (560 words)

	Barycentric Dynamical Time - Wikipedia, the free encyclopedia
		Barycentric Dynamical Time (TDB) was defined by the International Astronomical Union (IAU) in 1976 to be used as the relativistic replacement for the non-relativistic Ephemeris Time which had been used in the ephemerides starting in 1960.
		Terrestrial Dynamical Time (TDT) and TDB were defined in a series of resolutions at the same meeting of the International Astronomical Union.
		Despite IAU recommendations that TCB be used for all further calculations of solar system ephemerides, as of 2002 TDB and Ephemeris Time continue to be used, the latter by the producer of the important DE200 ephemeris and its successors at the Jet Propulsion Laboratory.
	www.wikipedia.org /wiki/Barycentric_Dynamical_Time (644 words)

	Barycentric Coordinate Time - Wikipedia, the free encyclopedia
		Barycentric Coordinate Time (TCB) was defined in 1991 by the International Astronomical Union as one of the replacements for the ill-defined Barycentric Dynamical Time (TDB).
		TCB is the timescale which would be produced by a clock ticking SI seconds at rest in a coordinate frame co-moving with the barycenter of the solar system.
		The relationship between TCB and Terrestrial Time (TT) is defined with a fully general relativistic metric.
	en.wikipedia.org /wiki/Barycentric_Coordinate_Time (210 words)

	The Time
		More precisely, it is the time coordinate of a space-time coordinate system that is centered in space on the barycenter of the solar system, that is nonrotating with respect to distant galaxies, and that tends asymptotically to the proper time of an observer at rest with respect to the coordinate system.
		Barycentric Dynamical Time, or TDB, is the same as Barycentric Coordinate Time only differing by a constant factor which is chosen so as to make TDB differ from Terrestrial Time by only periodic and Poisson terms.
		Geocentric Coordinate Time, or TCG, is the same as Barycentric Coordinate Time, except that it uses the Earth's center, rather than the barycenter of the solar system (and an “asymptotic” observer is one free from the gravitational field of the Earth rather than that of all the solar system).
	www.madore.org /~david/misc/time.html (3957 words)

	Sect_2.1.4_Time
		Dynamical time is required to describe the motion of bodies in a particular reference frame and according to a particular gravitational theory.
		The time signals broadcast by the GPS satellites are synchronised with atomic clocks at the GPS Master Control Station, in Colorado Springs, Colorado.
		The precise definition of UT1 is complicated because of the motion both of the celestial equator and the earth's orbital plane with respect to inertial space, and the irregularity of the earth's polar motion.
	www.gmat.unsw.edu.au /snap/gps/gps_survey/chap2/214time.htm (991 words)

	AllRefer.com - ephemeris time (Astronomy, General) - Encyclopedia
		ephemeris time (ET), astronomical time defined by the orbital motions of the earth, moon, and planets.
		Ephemeris time is calculated from the positions of the sun and moon relative to the earth, assuming that Newton's laws are perfectly obeyed.
		In 1984 ephemeris time was renamed terrestrial dynamical time (TDT or TT); also created was barycentric dynamical time (TDB), which is based on the orbital motion of the sun, moon, and planets.
	reference.allrefer.com /encyclopedia/E/ephemeri-t.html (266 words)

	Time Scales
		Although the term GMT was not explicitly used, the standard time for legal purposes in the United States specified by 15USC261 (the Calder Act) was defined to be based on the mean astronomical time of meridians spaced at 15-degree intervals west of Greenwich; i.e., GMT.
		Although the term GMT was not explicitly used, the standard time for legal purposes in the United States specified by 15USC261 was clarified and reaffirmed to be based on the mean solar time of meridians spaced at 15-degree intervals west of Greenwich; i.e., GMT.
		Dynamical time means time determined by comparing observations of the motions of objects with physical models that describe that motion.
	www.ucolick.org /~sla/leapsecs/timescales.html (13126 words)

	Time scale (Site not responding. Last check: 2007-11-04)
		Terrestrial Time is time at the surface of the Earth.
		Barycentric Coordinate Time is a proper time scale at the center of gravity of the solar system, which is called the barycenter.
		Barycentric Dynamical Time[?] (TDB) is similar to TT but includes relativistic corrections that move the origin to the barycenter.
	www.eurofreehost.com /ti/Time_scale_3.html (446 words)

	Time equations and notes
		Universal time (UT) is counted from 0 hours at midnight, with unit of duration the mean solar day, defined to be as uniform as possible despite variations in the rotation of the Earth.
		Terrestrial Time (TT), (or Terrestrial Dynamical Time, TDT), with unit of duration 86400 SI seconds on the geoid, is the independent argument of apparent geocentric ephemerides.
		Sidereal Time, with unit of duration the period of the Earth's rotation with respect to a point nearly fixed with respect to the stars, is the hour angle of the vernal equinox.
	www.parrett.net /~wolf/Time_equations_and_notes.htm (672 words)

	4.2 Time (Site not responding. Last check: 2007-11-04)
		The apparent (mean) sidereal time is used for astronomical observation and the mean sidereal time is used to construct a time scale: the Mean Sidereal Day (a difference of 0.0084 seconds with the Earth revolution period due to the precesion).
		The Dynamical Time corresponds to the concept of the inertial time, i.e.
		Barycentric Dynamical Time (TDB) is derived from orbital motions referred to the barycenter of the solar system (Coordinate time in the terminology of General Relativity).
	maite152.upc.es /~manuel/tdgps/node18.html (645 words)

	Barycentric Dynamical Time (Site not responding. Last check: 2007-11-04)
		Barycentric Dynamical Time (TDB) was a time standard used to take account of time dilation when calculating orbits of planets, asteroids, comets and interplanetary spacecraft in the Solar system.
		In the late nineteenth century it was realised that this time scale was not uniform: the observed timing of planetary positions did not match up correctly with UT. After atomic clocks were invented, they were used from 1960 to realise a new uniform time scale, Ephemeris Time (ET).
		Although it was a uniform time scale within its reference frame, it was subject to time dilation when compared with the proper time experienced by other bodies in the Solar system.
	www.kiwipedia.com /tdb.html (897 words)

	Time Routines in CSPICE
		Atomic time is based upon the atomic second as defined by the ``oscillation of the undisturbed cesium atom.'' Atomic time is simply a count of atomic seconds that have occurred since the astronomically determined instant of midnight January 1, 1958 00:00:00 at the Royal Observatory in Greenwich, England.
		Ephemeris time is the uniform time scale represented by the independent variable in the differential equations that describe the motions of the planets, sun and moon.
		A comparison of the times kept by a clock at the solar system barycenter with a TDB clock on earth would reveal that the two clocks are in close agreement but that they run at different rates at different times of the year.
	sohowww.nascom.nasa.gov /solarsoft/stereo/gen/exe/icy/doc/html/req/time.html (11705 words)

	Dynamical Time: TT, TDB
		Dynamical time is the independent variable in the theories which describe the motions of bodies in the solar system.
		When you use published formulae which model the position of the Earth in its orbit, for example, or look up the Moon's position in a precomputed ephemeris, the date and time you use must be in terms of one of the dynamical timescales.
		Barycentric Dynamical Time TDB differs from TT by an amount which cycles back and forth by between 1 and 2 milliseconds due to relativistic effects.
	star-www.rl.ac.uk /star/dvi/sun67.htx/node224.html (522 words)

	Science Fair Projects - Geocentric Coordinate Time
		Geocentric Coordinate Time (TCG) was defined in 1991 by the International Astronomical Union as one of the replacements for the ill-defined Barycentric Dynamical Time (TDB).
		The relationship between TCG and Terrestrial Time (TT) is defined with a fully general relativistic metric.
		TCG is intended to be used as the independent variable of time for all calculations pertaining to precession, nutation, the moon, and artificial satellites of the earth.
	www.all-science-fair-projects.com /science_fair_projects_encyclopedia/Geocentric_Coordinate_Time (366 words)

	Glossary of astronomical terms
		Due to precession, this point moves over time, so positions of stars in catalogues and on atlases are usually referred to a "mean equator and equinox" of a specified standard epoch.
		Time scale used in orbital computations; TDT is tied to atomic clocks (International Atomic Time, TAI), whereas Universal Time is tied to observations.
		A measure of time used by astronomers; UT conforms (within a close approximation) to the mean daily (apparent) motion of the sun.
	cfa-www.harvard.edu /iau/icq/ICQGlossary.html (3501 words)

	Dynamical time scales (Site not responding. Last check: 2007-11-04)
		The astronomers stuck with ephemeris time until 1979, when they defined two new time scales that used the atomic second and that took into account relativity (velocity affects time).
		It was defined as being equal to TAI (Atomic time) plus 32.184 (atomic) seconds at the instant beginning 1 January 1977.
		Barycentric Dynamical Time (TDB, from the French) is time at the center of mass of the solar system.
	www.sizes.com /time/time_dynamical.htm (115 words)

	Columbia Encyclopedia - ephemeris time - AOL Research & Learn
		Ephemeris time is calculated from the positions of the sun and moon relative to the earth, assuming that Newton's laws are perfectly obeyed.
		By convention, the standard seasonal year is taken to be 1900 and to contain 31,556,925.9747 sec of ephemeris time.
		In 1984 ephemeris time was renamed terrestrial dynamical time (TDT or TT); also created was barycentric dynamical time (TDB), which is based on the orbital motion of the sun, moon, and planets.
	reference.aol.com /columbia/_a/ephemeris-time/20051206001509990026 (187 words)

	Barycentric Coordinate Time -- Facts, Info, and Encyclopedia article (Site not responding. Last check: 2007-11-04)
		Barycentric Coordinate Time (TCB) was defined in 1991 by the (Click link for more info and facts about International Astronomical Union) International Astronomical Union as one of the replacements for the ill-defined (Click link for more info and facts about Barycentric Dynamical Time) Barycentric Dynamical Time (TDB).
		As such, TCB is intended to be used as the modern replacement for ((astronomy) a measure of time defined by Earth's orbital motion; terrestrial time is mean solar time corrected for the irregularities of the Earth's motions) Ephemeris Time (ET).
		Adapting the large body of existing software to change from TDB to TCB is a formidable task, and as of 2002 the (Click link for more info and facts about ephemerides) ephemerides continue to use TDB.
	www.absoluteastronomy.com /encyclopedia/B/Ba/Barycentric_Coordinate_Time.htm (196 words)

	The ABC of XTE: A Time Tutorial
		The vectors xe and ve denote the barycentric position and velocity of the earth's center of mass, and x is the barycentric position of the observer.
		See the Timing Digest for a figure of fine clock corrections as a function of time.
		The maximum attainable absolute time precision, using the tdc.dat file corrections, is between 5 and 8 microseconds.
	heasarc.gsfc.nasa.gov /docs/xte/abc/time_tutorial.html (1043 words)

	Basics of Space Flight Section I. The Environment of Space
		Local time on another planet is conceived as the equivalent value of time for the Sun's distance from the meridian, as it is on Earth.
		TRM, Transmission time is the UTC time of uplink from Earth.
		For the European Union, Summer Time, the equivalent of the U.S. Daylight Saving Time, begins at 0100 UTC on the last Sunday in March, and ends at 0100 UTC on the last Sunday in October (all time zones switch at the same moment in the EU).
	www2.jpl.nasa.gov /basics/bsf2-3.htm (1014 words)

	Sky Publishing - Time and the Amateur Astronomer (Site not responding. Last check: 2007-11-04)
		To a person watching the sky 13 miles west of you, the time seems to be 11:59 when you swear it's 12:00 and someone 13 miles east insists it's 12:01.
		Logically enough, the "universal" time zone that was agreed upon is that of 0° longitude.
		In 1984 ET was renamed Terrestrial Dynamical Time (TDT or TT); also created was Barycentric Dynamical Time (TDB), which is referred to the solar system's center of mass.
	www.wwnorton.com /astro21/sandt/time.html (1759 words)

	Amateur Astronomers, Inc. - Newsletter (April 1999 - PG2) (Site not responding. Last check: 2007-11-04)
		So also was mean sidereal time based upon the same spin and true sidereal time which brings in the pull of the moon.
		Astronomers then went to ephemeris time which is based upon the time it took the earth to circumnavigate the sun in the year 1900 with all planetary effects removed mathematically.
		This, of course, is not the same as the time at the center of the earth which is terrestrial dynamic time.
	www.asterism.org /newsletter/l9904-2.htm (601 words)

	Search Results for time - Encyclopædia Britannica
		Time is of philosophical interest and is also the subject of mathematical and scientific investigation.
		the time of a region or country that is established by law or general usage as civil time.
		Apparent solar time is that measured by direct observation of the Sun or by a sundial.
	www.britannica.com /search?query=time&ct= (570 words)

	Absolute Time on ARGOS/USA
		This specifies that the time in the TIME column are measured in UTC seconds since the reference time of 50454.0 MJD(UTC), which is June 1, 1997 00:00:00 UTC.
		When GPS is on and locked this time is from the GPS receiver and accurate to 1 µs or better depending on quality of the solution, number of satellites being tracked and the current GPS constellation geometry.
		Many times may be as good as 1.0 s, but some may be farther off and the quality may be very difficult to determine.
	xweb.nrl.navy.mil /usa/calibration/abstime.html (2088 words)

	Terrestrial Dynamical Time (Site not responding. Last check: 2007-11-04)
		(astronomy) a measure of time defined by Earth's orbital motion; terrestrial time is mean solar time corrected for the irregularities of the Earth's motions.
		Terrestrial Dynamical Time (TDT) was defined in 1976 by the International Astronomical Union (IAU) to be the counterpart of Barycentric Dynamical Time (TDB) as measured by clocks ticking SI seconds on the surface of the earth.
		The distinction between TDT and TAI is that the rules for computing TAI have been changed several times; at one point even the rate of TAI was changed in a discontinuous fashion.
	www.websters-online-dictionary.org /definition/Terrestrial+Dynamical+Time (371 words)

	EPHEMERIDES DES PLANETES DE GRANDE PRECISION (Site not responding. Last check: 2007-11-04)
		The ephemeris for Sidereal Time (E8-V1) are : Greenwich Sidereal Time (mean and apparent), and equation of the equinoxes in UT (Universal Time).
		This time belongs to the family of time scales introduced in 1984 to replace "Ephemeris Time" (ET) which is commonly used in theories of Celestial Mechanics.
		Since the representations of the planetary orbits are expressed in Dynamical Time, the program PLANEPH uses an "estimate" of the difference DT-UT when ephemeris have to be computed in Universal Time or in one another civil time.
	artemmis.univ-mrs.fr /cybermeca/Formcont/mecaspa/EPHEMERI/ephemeri.htm (2958 words)

	CSPICE Routines: UNITIM_C (Site not responding. Last check: 2007-11-04)
		The function returns the time in the system specified by outsys that is equivalent to the epoch in the insys time scale.
		We use the term uniform time scale to refer to those representations of time that are numeric (each epoch is represented by a number) and additive.
		Given an epoch in one of the uniform time scales specified by insys, the function returns the equivalent representation in the scale specified by outsys.
	www.gps.caltech.edu /~marsdata/cspice/unitim_c.html (522 words)

	slaRcc.3 (Site not responding. Last check: 2007-11-04)
		Relativistic clock correction: the difference between proper time at a point on the surface of the Earth and coordinate time in the Solar System barycentric space-time frame of reference.
		The proper time is Terrestrial Time TT; the coordinate time is an implementation of the Barycentric Dynamical Time TDB.
		TDB may be considered to be the coordinate time in the Solar System barycentre frame of reference, and TT is the proper time given by clocks at mean sea level on the Earth.
	www.astro.uiuc.edu /~bima/starbase/slaRcc.3.html (453 words)

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