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# Topic: Ephemeris time

###### In the News (Sun 21 Apr 19)

 The Time Before even the ephemeris second was defined, the second (not “SI” because this is before SI) was defined as the fraction 1/86400 of the mean solar day (this is, after all, the historical definition of the second, the day being divided in 24 hours of 60 minutes of 60 seconds each). 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. 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   (0 words)

 Astronomical Time Keeping Moreover, siderial time is one of the constituents of Universal Time. The Ephemeris Time (ET) was consequently defined as the timescale that together with the laws of motion correctly predicts the positions of celestial bodies, and it is therefore used as the argument in the ephemerides. Ephemeris time is reckoned from the instant, near the beginning of the calendar year AD 1900, when the geometric mean longitude of the Sun was 279 degrees, 41 minutes 48.04 seconds, at which instant the measure of Ephemeris Time was 1900 January 0, 12 o'clock precisely.'' www.maa.mhn.de /Scholar/times.html   (0 words)

 [No title] This is the basis for "sidereal time" which is similar to mean solar time since both are based on the earth's spinning on its axis. An ephemeris is simply a table that predicts the positions of the sun, moon, and planets. First, the universal time family is dependent on the earth's spin on its axis. www.ieee-uffc.org /freqcontrol/fc_utc.html   (2038 words)

 Astronomical Times Several important time scales still follow the rotation of the earth, most notably civil and sidereal time, but of these are now derived from atomic time through a combination of earth rotation theory and actual measurements of the earth's rotation and orientation. By convention, the reference points for Greenwich Sidereal Time are the Greenwich Meridian and the vernal equinox (the intersection of the planes of the earth's equator and the earth's orbit, the ecliptic). It might seem strange that UT1, a solar time, is determined by measuring the earth's rotation with respect to distant celestial objects, and GMST, a sidereal time, is derived from it. www.cv.nrao.edu /~rfisher/Ephemerides/times.html   (0 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. This time scale is an effort to ascertain corrections to the values of TAI based on retrospective studies of the behavior of the clocks which have contributed to TAI. www.ucolick.org /~sla/leapsecs/timescales.html   (0 words)

 ephemeris time — FactMonster.com 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. time, sequential arrangement of all events: Physical Time and Its Measurement - Physical Time and Its Measurement The accurate measurement of time by establishing accurate time... www.factmonster.com /ce6/sci/A0817460.html   (0 words)

 ephemeris time In 1960 the 11th CGPM abandoned the definition of the second as a fraction of the mean solar day in favor of the ephemeris second. The other was that for some users, a time scale in which every second is just the same as every other, and hence that is not based on the Earth's rotation, is not ideal. 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). www.sizes.com /time/time_ephemeris.htm   (0 words)

 atomic time A scale of time based a frequency standard of 9,192,631,770 hertz obtained by comparing the outputs of a large number of “atomic clocks,” originally cesium-beam clocks but latterly including hydrogen masers and various types of fountain clocks (see below). The stability of atomic time is based on a behavior of atoms: they must absorb radiation of a precise frequency in order to pass from one energy state to another. On the basis of this work, in 1971 the 14th CGPM approved a time scale to be based on the atomic second and called International Atomic Time (TAI), leaving the definition of the scale to the CIPM, who had already prepared it. www.sizes.com /time/time_atomic.htm   (0 words)

 Analysis Of Replaced Ephemeris Files A new ephemeris file was obtained in which the the leap second was removed and TSDIS processing was continued using this file for orbits 6294 and 6295. The epoch time for this ephemeris was in the new year even though the file started on 12/31/98. A second new ephemeris was obtained, in which the epoch time was at the beginning of 12/31/98, and again the leap second was removed (to keep even 60 second intervals in the file). gcdc.gsfc.nasa.gov /ephems/analysis.html   (0 words)

 How to read the JPL Ephemeris and Perform Barycentering Other differences are that the DE200 ephemeris is nominally referenced to the FK5 coordinate system, which is tied to nearby stars, whereas the the DE405 ephemeris is referenced to the ICRS system, which is tied to distant radio quasars. The "Shapiro" delay is the time delay analog of the well-known bending of light near the sun. For the purposes of barycentering, the primary uses of the JPL Ephemeris table are in calculating rearth, vearth, and rsun (used in calculating th). lheawww.gsfc.nasa.gov /users/craigm/bary   (0 words)

 Pocket PC Freeware Downloads Note that each time a new result is computed, it is automatically copied to the clipboard, erasing its previous contents. Ephemeris is a program that lets you enter a date and a location and then computes the time and azimuth (compass bearing) or sunrise, sunset, moonrise and moonset. Allowed time zone entries to be in the range from -24.0 to 24.0 instead of -12.0 to 12.0. home.comcast.net /~jonsachs   (0 words)

 Humboldt State University Library - Quick Reference Sources: Tide Tables, Time, Calendars, & Ephemeris Sun and Moon Data for One Day - You can obtain the times of sunrise, sunset, moonrise, moonset, transits of the Sun and Moon, and the beginning and end of civil twilight, along with information on the Moon's phase by specifying the date and location. The Official U.S.Time - From the introduction: "This public service is cooperatively provided by the two time agencies of United States: a Department of Commerce agency, the National Institute of Standards and Technology (NIST), and its military counterpart, the U.S. Naval Observatory (USNO). Time Service Department -"The Official Source of Time for the Department of Defense (DoD) and the Global Positioning System (GPS), and a Standard of Time for the United States." library.humboldt.edu /infoservices/quickref/tide.htm   (0 words)

 Programming interface to the Python Port of Swiss Ephemeris If the user does not want an ephemeris for the time before 1500 he might prefer to work with the short files. In this case the default ephemeris is Swiss Ephemeris. Since the journey of the light from a planet to the earth takes some time, the planets are never seen where they actually are, but where they were a few minutes or hours before. home.earthlink.net /~soloj/swephprg.htm   (0 words)

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