The term ephemeris time (often abbreviated ET ) can in principle refer to time in association with any ephemeris (itinerary of the trajectory of an astronomical object). In practice it has been used more specifically to refer to:
90-466: Most of the following sections relate to the ephemeris time of the 1952 standard. An impression has sometimes arisen that ephemeris time was in use from 1900: this probably arose because ET, though proposed and adopted in the period 1948–1952, was defined in detail using formulae that made retrospective use of the epoch date of 1900 January 0 and of Newcomb 's Tables of the Sun . The ephemeris time of
180-496: A better realisation of Terrestrial Time (TT). Early atomic time scales consisted of quartz clocks with frequencies calibrated by a single atomic clock; the atomic clocks were not operated continuously. Atomic timekeeping services started experimentally in 1955, using the first caesium atomic clock at the National Physical Laboratory, UK (NPL) . It was used as a basis for calibrating the quartz clocks at
270-565: A constant offset. From its beginning in 1961 through December 1971, the adjustments were made regularly in fractional leap seconds so that UTC approximated UT2 . Afterwards, these adjustments were made only in whole seconds to approximate UT1 . This was a compromise arrangement in order to enable a publicly broadcast time scale. The less frequent whole-second adjustments meant that the time scale would be more stable and easier to synchronize internationally. The fact that it continues to approximate UT1 means that tasks such as navigation which require
360-495: A conventionally corrected form of Newcomb's formula, incorporating the corrections on the basis of mean solar time, would be the sum of the two preceding expressions: Clemence's 1948 proposal, however, did not adopt such a correction of mean solar time. Instead, the same numbers were used as in Newcomb's original uncorrected formula (1), but now applied somewhat prescriptively, to define a new time and time scale implicitly, based on
450-503: A current standard. As re-defined in 2006, TDB is a linear transformation of TCB . The same IAU resolution also stated (in note 4) that the "independent time argument of the JPL ephemeris DE405 , which is called T eph " (here the IAU source cites), "is for practical purposes the same as TDB defined in this Resolution". Thus the new TDB, like T eph , is essentially a more refined continuation of
540-510: A fixed standard date and time of reference (and not, as might be expected from current usage, to a change from one date and time of reference to a different date and time). Astronomical data are often specified not only in their relation to an epoch or date of reference but also in their relations to other conditions of reference, such as coordinate systems specified by " equinox ", or "equinox and equator ", or "equinox and ecliptic " – when these are needed for fully specifying astronomical data of
630-643: A formula for the Sun's mean longitude at a time, indicated by interval T (in units of Julian centuries of 36525 mean solar days), reckoned from Greenwich Mean Noon on 0 January 1900: Spencer Jones' work of 1939 showed that differences between the observed positions of the Sun and the predicted positions given by Newcomb's formula demonstrated the need for the following correction to the formula: where "the times of observation are in Universal time, not corrected to Newtonian time," and 0.0748B represents an irregular fluctuation calculated from lunar observations. Thus,
720-585: A given date defines which coordinate system is used. Most standard coordinates in use today refer to 2000 TT (i.e. to 12h (noon) on the Terrestrial Time scale on January 1, 2000, see below), which occurred about 64 seconds sooner than noon UT1 on the same date (see ΔT ). Before about 1984, coordinate systems dated to 1950 or 1900 were commonly used. There is a special meaning of the expression "equinox (and ecliptic/equator) of date ". When coordinates are expressed as polynomials in time relative to
810-543: A number of whole seconds. As of 1 January 2017 , immediately after the most recent leap second was put into effect, UTC has been exactly 37 seconds behind TAI. The 37 seconds result from the initial difference of 10 seconds at the start of 1972, plus 27 leap seconds in UTC since 1972. In 2022, the General Conference on Weights and Measures decided to abandon the leap second by or before 2035, at which point
900-409: A particular date, such as J2000.0) could be used forever, but a set of osculating elements for a particular epoch may only be (approximately) valid for a rather limited time, because osculating elements such as those exampled above do not show the effect of future perturbations which will change the values of the elements. Nevertheless, the period of validity is a different matter in principle and not
990-510: A particular theory of the orbit of the Earth around the Sun, that of Newcomb (1895), which is now obsolete; for that reason among others, the use of Besselian years has also become or is becoming obsolete. Lieske 1979 , p. 282 says that a "Besselian epoch" can be calculated from the Julian date according to Lieske's definition is not exactly consistent with the earlier definition in terms of
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#17327831800291080-461: A recent epoch for all of the elements: but some of the data are dependent on a chosen coordinate system, and then it is usual to specify the coordinate system of a standard epoch which often is not the same as the epoch of the data. An example is as follows: For minor planet (5145) Pholus , orbital elements have been given including the following data: where the epoch is expressed in terms of Terrestrial Time, with an equivalent Julian date. Four of
1170-460: A reference frame defined in this way, that means the values obtained for the coordinates in respect of any interval t after the stated epoch, are in terms of the coordinate system of the same date as the obtained values themselves, i.e. the date of the coordinate system is equal to (epoch + t). It can be seen that the date of the coordinate system need not be the same as the epoch of the astronomical quantities themselves. But in that case (apart from
1260-517: A revision of the faulty Circular T or by errata in a subsequent Circular T. Aside from this, once published in Circular T, the TAI scale is not revised. In hindsight, it is possible to discover errors in TAI and to make better estimates of the true proper time scale. Since the published circulars are definitive, better estimates do not create another version of TAI; it is instead considered to be creating
1350-546: A series of decisions that designated the BIPM time scale International Atomic Time (TAI). In the 1970s, it became clear that the clocks participating in TAI were ticking at different rates due to gravitational time dilation , and the combined TAI scale, therefore, corresponded to an average of the altitudes of the various clocks. Starting from the Julian Date 2443144.5 (1 January 1977 00:00:00 TAI), corrections were applied to
1440-409: A specific time and place on the Earth, the coordinates of the object are needed relative to a coordinate system of the current date. If coordinates relative to some other date are used, then that will cause errors in the results. The magnitude of those errors increases with the time difference between the date and time of observation and the date of the coordinate system used, because of the precession of
1530-470: A standard until superseded in the 1970s by further time scales (see Revision ). During the currency of ephemeris time as a standard, the details were revised a little. The unit was redefined in terms of the tropical year at 1900.0 instead of the sidereal year; and the standard second was defined first as 1/31556925.975 of the tropical year at 1900.0, and then as the slightly modified fraction 1/31556925.9747 instead, finally being redefined in 1967/8 in terms of
1620-482: A year with decimals ( 2000 + x ), where x is either positive or negative and is quoted to 1 or 2 decimal places, has come to mean a date that is an interval of x Julian years of 365.25 days away from the epoch J2000 = JD 2451545.0 (TT), still corresponding (in spite of the use of the prefix "J" or word "Julian") to the Gregorian calendar date of January 1, 2000, at 12h TT (about 64 seconds before noon UTC on
1710-418: Is a moment in time used as a reference point for some time-varying astronomical quantity. It is useful for the celestial coordinates or orbital elements of a celestial body , as they are subject to perturbations and vary with time. These time-varying astronomical quantities might include, for example, the mean longitude or mean anomaly of a body, the node of its orbit relative to a reference plane ,
1800-439: Is a weighted average of the time kept by over 450 atomic clocks in over 80 national laboratories worldwide. It is a continuous scale of time, without leap seconds , and it is the principal realisation of Terrestrial Time (with a fixed offset of epoch ). It is the basis for Coordinated Universal Time (UTC), which is used for civil timekeeping all over the Earth's surface and which has leap seconds. UTC deviates from TAI by
1890-577: Is defined as the independent variable of the equations of celestial mechanics". De Sitter offered a correction to be applied to the mean solar time given by the Earth's rotation to get uniform time. Other astronomers of the period also made suggestions for obtaining uniform time, including A Danjon (1929), who suggested in effect that observed positions of the Moon, Sun and planets, when compared with their well-established gravitational ephemerides, could better and more uniformly define and determine time. Thus
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#17327831800291980-412: Is defined by international agreement to be equivalent to: Over shorter timescales, there are a variety of practices for defining when each day begins. In ordinary usage, the civil day is reckoned by the midnight epoch, that is, the civil day begins at midnight. But in older astronomical usage, it was usual, until January 1, 1925, to reckon by a noon epoch, 12 hours after the start of the civil day of
2070-439: Is widely known, although not always the same date in the year: thus "J2000" refers to the instant of 12 noon (midday) on January 1, 2000, and J1900 refers to the instant of 12 noon on January 0 , 1900, equal to December 31, 1899. It is also usual now to specify on what time scale the time of day is expressed in that epoch-designation, e.g. often Terrestrial Time . In addition, an epoch optionally prefixed by "J" and designated as
2160-478: The IAU , so astronomers worldwide can collaborate more effectively. It is inefficient and error-prone if data or observations of one group have to be translated in non-standard ways so that other groups could compare the data with information from other sources. An example of how this works: if a star's position is measured by someone today, they then use a standard transformation to obtain the position expressed in terms of
2250-735: The Royal Greenwich Observatory and to establish a time scale, called Greenwich Atomic (GA). The United States Naval Observatory began the A.1 scale on 13 September 1956, using an Atomichron commercial atomic clock, followed by the NBS-A scale at the National Bureau of Standards , Boulder, Colorado on 9 October 1957. The International Time Bureau (BIH) began a time scale, T m or AM, in July 1955, using both local caesium clocks and comparisons to distant clocks using
2340-488: The TCB time scale adopted in 1991 as a standard by the IAU . Thus for clocks on or near the geoid , T eph (within 2 milliseconds), but not so closely TCB, can be used as approximations to Terrestrial Time, and via the standard ephemerides T eph is in widespread use. Partly in acknowledgement of the widespread use of T eph via the JPL ephemerides, IAU resolution 3 of 2006 (re-)defined Barycentric Dynamical Time (TDB) as
2430-425: The atomic time scale , and to what was first called Terrestrial Dynamical Time and is now Terrestrial Time , defined to provide continuity with ET. The availability of atomic clocks, together with the increasing accuracy of astronomical observations (which meant that relativistic corrections were at least in the foreseeable future no longer going to be small enough to be neglected), led to the eventual replacement of
2520-627: The heliacal rising of the star Sirius , a phenomenon which occurs in the morning just before dawn. In some cultures following a lunar or lunisolar calendar , in which the beginning of the month is determined by the appearance of the New Moon in the evening, the beginning of the day was reckoned from sunset to sunset, following an evening epoch, e.g. the Jewish and Islamic calendars and in Medieval Western Europe in reckoning
2610-409: The signal averaging TAI is an order of magnitude more stable than its best constituent clock. The participating institutions each broadcast, in real time , a frequency signal with timecodes , which is their estimate of TAI. Time codes are usually published in the form of UTC, which differs from TAI by a well-known integer number of seconds. These time scales are denoted in the form UTC(NPL) in
2700-406: The "equinox of date" case described above), two dates will be associated with the data: one date is the epoch for the time-dependent expressions giving the values, and the other date is that of the coordinate system in which the values are expressed. For example, orbital elements , especially osculating elements for minor planets, are routinely given with reference to two dates: first, relative to
2790-401: The 1952 standard leaves a continuing legacy, through its historical unit ephemeris second which became closely duplicated in the length of the current standard SI second (see below: Redefinition of the second ). Ephemeris time ( ET ), adopted as standard in 1952, was originally designed as an approach to a uniform time scale, to be freed from the effects of irregularity in the rotation of
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2880-565: The 1990s by time scales Terrestrial Time (TT) , Geocentric Coordinate Time GCT (TCG) and Barycentric Coordinate Time BCT (TCB) . High-precision ephemerides of sun, moon and planets were developed and calculated at the Jet Propulsion Laboratory (JPL) over a long period, and the latest available were adopted for the ephemerides in the Astronomical Almanac starting in 1984. Although not an IAU standard,
2970-431: The 24.349 seconds of time corresponding to the 1.00" in ΔLs. Clemence's formula (today superseded by more modern estimations) was included in the original conference decision on ephemeris time. In view of the fluctuation term, practical determination of the difference between ephemeris time and UT depended on observation. Inspection of the formulae above shows that the (ideally constant) units of ephemeris time have been, for
3060-530: The Earth, "for the convenience of astronomers and other scientists", for example for use in ephemerides of the Sun (as observed from the Earth), the Moon, and the planets. It was proposed in 1948 by G M Clemence . From the time of John Flamsteed (1646–1719) it had been believed that the Earth's daily rotation was uniform. But in the later nineteenth and early twentieth centuries, with increasing precision of astronomical measurements, it began to be suspected, and
3150-464: The German mathematician and astronomer Friedrich Bessel (1784–1846). Meeus 1991 , p. 125 defines the beginning of a Besselian year to be the moment at which the mean longitude of the Sun, including the effect of aberration and measured from the mean equinox of the date, is exactly 280 degrees. This moment falls near the beginning of the corresponding Gregorian year . The definition depended on
3240-489: The Moon moves against the background of stars about 13 times as fast as the Sun's corresponding rate of motion, and the accuracy of time determinations from lunar measurements is correspondingly greater. When ephemeris time was first adopted, time scales were still based on astronomical observation, as they always had been. The accuracy was limited by the accuracy of optical observation, and corrections of clocks and time signals were published in arrear. A few years later, with
3330-565: The Nautical Almanac, by then a separate publication for the use of navigators, continued to be expressed in terms of UT.) The ephemerides continued on this basis through 1983 (with some changes due to adoption of improved values of astronomical constants), after which, for 1984 onwards, they adopted the JPL ephemerides. Previous to the 1960 change, the 'Improved Lunar Ephemeris' had already been made available in terms of ephemeris time for
3420-413: The Sun, it was usually measured in practice by the orbital motion of the Moon around the Earth. These measurements can be considered as secondary realizations (in a metrological sense) of the primary definition of ET in terms of the solar motion, after a calibration of the mean motion of the Moon with respect to the mean motion of the Sun. Reasons for the use of lunar measurements were practically based:
3510-471: The UTC form, where NPL here identifies the National Physical Laboratory, UK . The TAI form may be denoted TAI(NPL) . The latter is not to be confused with TA(NPL) , which denotes an independent atomic time scale, not synchronised to TAI or to anything else. The clocks at different institutions are regularly compared against each other. The International Bureau of Weights and Measures (BIPM, France), combines these measurements to retrospectively calculate
3600-505: The age of the observations and their epoch, and the equinox and equator to which they are referred, get older. After a while, it is easier or better to switch to newer data, generally referred to as a newer epoch and equinox/equator, than to keep applying corrections to the older data. Epochs and equinoxes are moments in time, so they can be specified in the same way as moments that indicate things other than epochs and equinoxes. The following standard ways of specifying epochs and equinoxes seem
3690-415: The aim developed, to provide a new time scale for astronomical and scientific purposes, to avoid the unpredictable irregularities of the mean solar time scale, and to replace for these purposes Universal Time (UT) and any other time scale based on the rotation of the Earth around its axis, such as sidereal time . The American astronomer G M Clemence (1948) made a detailed proposal of this type based on
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3780-445: The caesium 133 atom. Although this is an independent definition that does not refer to the older basis of ephemeris time, it uses the same quantity as the value of the ephemeris second measured by the cesium clock in 1958. This SI second referred to atomic time was later verified by Markowitz (1988) to be in agreement, within 1 part in 10, with the second of ephemeris time as determined from lunar observations. For practical purposes
3870-496: The cesium atomic clock standard (see below). Although ET is no longer directly in use, it leaves a continuing legacy. Its successor time scales, such as TDT, as well as the atomic time scale IAT (TAI) , were designed with a relationship that "provides continuity with ephemeris time". ET was used for the calibration of atomic clocks in the 1950s. Close equality between the ET second with the later SI second (as defined with reference to
3960-421: The cesium atomic clock) has been verified to within 1 part in 10. In this way, decisions made by the original designers of ephemeris time influenced the length of today's standard SI second , and in turn, this has a continuing influence on the number of leap seconds which have been needed for insertion into current broadcast time scales, to keep them approximately in step with mean solar time . Ephemeris time
4050-406: The considered type. When the data are dependent for their values on a particular coordinate system, the date of that coordinate system needs to be specified directly or indirectly. Celestial coordinate systems most commonly used in astronomy are equatorial coordinates and ecliptic coordinates . These are defined relative to the (moving) vernal equinox position, which itself is determined by
4140-426: The current position of that comet must be expressed in the coordinate system of 1875 (equinox/equator of 1875). Thus that coordinate system can still be used today, even though most comet predictions made originally for 1875 (epoch = 1875) would no longer be useful today, because of the lack of information about their time-dependence and perturbations. To calculate the visibility of a celestial object for an observer at
4230-451: The dates of religious festivals, while in others a morning epoch was followed, e.g. the Hindu and Buddhist calendars . International Atomic Time International Atomic Time (abbreviated TAI , from its French name temps atomique international ) is a high-precision atomic coordinate time standard based on the notional passage of proper time on Earth's geoid . TAI
4320-415: The difference between TAI and UTC will remain fixed. TAI may be reported using traditional means of specifying days, carried over from non-uniform time standards based on the rotation of the Earth. Specifically, both Julian days and the Gregorian calendar are used. TAI in this form was synchronised with Universal Time at the beginning of 1958, and the two have drifted apart ever since, due primarily to
4410-442: The direction of the apogee or aphelion of its orbit, or the size of the major axis of its orbit. The main use of astronomical quantities specified in this way is to calculate other relevant parameters of motion, in order to predict future positions and velocities. The applied tools of the disciplines of celestial mechanics or its subfield orbital mechanics (for predicting orbital paths and positions for bodies in motion under
4500-500: The elements are independent of any particular coordinate system: M is mean anomaly (deg), n: mean daily motion (deg/d), a: size of semi-major axis (AU), e: eccentricity (dimensionless). But the argument of perihelion, longitude of the ascending node and the inclination are all coordinate-dependent, and are specified relative to the reference frame of the equinox and ecliptic of another date "2000.0", otherwise known as J2000, i.e. January 1.5, 2000 (12h on January 1) or JD 2451545.0. In
4590-532: The ephemeris second corresponded to 9 192 631 770 ± 20 cycles of the chosen cesium resonance. Following this, in 1967/68, the General Conference on Weights and Measures (CGPM) replaced the definition of the SI second by the following: The second is the duration of 9 192 631 770 periods of the radiation corresponding to the transition between the two hyperfine levels of the ground state of
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#17327831800294680-425: The ephemeris time argument T eph has been in use at that institution since the 1960s. The time scale represented by T eph has been characterized as a relativistic coordinate time that differs from Terrestrial Time only by small periodic terms with an amplitude not exceeding 2 milliseconds of time: it is linearly related to, but distinct (by an offset and constant rate which is of the order of 0.5 s/a) from
4770-608: The ephemeris time standard by more refined time scales including terrestrial time and barycentric dynamical time , to which ET can be seen as an approximation. In 1976, the IAU resolved that the theoretical basis for its then-current (since 1952) standard of Ephemeris Time was non-relativistic, and that therefore, beginning in 1984, Ephemeris Time would be replaced by two relativistic timescales intended to constitute dynamical timescales : Terrestrial Dynamical Time (TDT) and Barycentric Dynamical Time (TDB) . Difficulties were recognized, which led to these, in turn, being superseded in
4860-434: The epoch for Barycentric Coordinate Time (TCB), Geocentric Coordinate Time (TCG), and Terrestrial Time (TT), which represent three fundamental time scales in the solar system. All three of these time scales were defined to read JD 2443144.5003725 (1 January 1977 00:00:32.184) exactly at that instant. TAI was henceforth a realisation of TT, with the equation TT(TAI) = TAI + 32.184 s. The continued existence of TAI
4950-425: The equator and of the ecliptic. The epoch of the coordinate system need not be the same, and often in practice is not the same, as the epoch for the data themselves. The difference between reference to an epoch alone, and a reference to a certain equinox with equator or ecliptic, is therefore that the reference to the epoch contributes to specifying the date of the values of astronomical variables themselves; while
5040-492: The equinoxes. If the time difference is small, then fairly easy and small corrections for the precession may well suffice. If the time difference gets large, then fuller and more accurate corrections must be applied. For this reason, a star position read from a star atlas or catalog based on a sufficiently old equinox and equator cannot be used without corrections if reasonable accuracy is required. Additionally, stars move relative to each other through space. Apparent motion across
5130-456: The formula given above, A Julian year is an interval with the length of a mean year in the Julian calendar , i.e. 365.25 days. This interval measure does not itself define any epoch: the Gregorian calendar is in general use for dating. But, standard conventional epochs which are not Besselian epochs have been often designated nowadays with a prefix "J", and the calendar date to which they refer
5220-435: The gravitational effects of other bodies) can be used to generate an ephemeris , a table of values giving the positions and velocities of astronomical objects in the sky at a given time or times. Astronomical quantities can be specified in any of several ways, for example, as a polynomial function of the time interval, with an epoch as a temporal point of origin (this is a common current way of using an epoch). Alternatively,
5310-429: The invention of the cesium atomic clock , an alternative offered itself. Increasingly, after the calibration in 1958 of the cesium atomic clock by reference to ephemeris time, cesium atomic clocks running on the basis of ephemeris seconds began to be used and kept in step with ephemeris time. The atomic clocks offered a further secondary realization of ET, on a quasi-real time basis that soon proved to be more useful than
5400-416: The length of the ephemeris second can be taken as equal to the length of the second of Barycentric Dynamical Time (TDB) or Terrestrial Time (TT) or its predecessor TDT. The difference between ET and UT is called ΔT ; it changes irregularly, but the long-term trend is parabolic , decreasing from ancient times until the nineteenth century, and increasing since then at a rate corresponding to an increase in
5490-530: The mean longitude of the Sun. When using Besselian years, specify which definition is being used. To distinguish between calendar years and Besselian years, it became customary to add ".0" to the Besselian years. Since the switch to Julian years in the mid-1980s, it has become customary to prefix "B" to Besselian years. So, "1950" is the calendar year 1950, and "1950.0" = "B1950.0" is the beginning of Besselian year 1950. According to Meeus, and also according to
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#17327831800295580-572: The mean solar second is unsatisfactory as a unit of time by reason of its variability, the unit adopted should be the sidereal year at 1900.0, that the time reckoned in this unit be designated ephemeris time ", and gave Clemence's formula (see Definition of ephemeris time (1952) ) for translating mean solar time to ephemeris time. The International Astronomical Union approved this recommendation at its 1952 general assembly. Practical introduction took some time (see Use of ephemeris time in official almanacs and ephemerides ); ephemeris time (ET) remained
5670-449: The most popular: All three of these are expressed in TT = Terrestrial Time . Besselian years, used mostly for star positions, can be encountered in older catalogs but are now becoming obsolete. The Hipparcos catalog summary, for example, defines the "catalog epoch" as "J1991.25" (8.75 Julian years before January 1.5, 2000 TT, e.g., April 2.5625, 1991 TT). A Besselian year is named after
5760-543: The older ephemeris time ET and (apart from the < 2 ms periodic fluctuations) has the same mean rate as that established for ET in the 1950s. Ephemeris time based on the standard adopted in 1952 was introduced into the Astronomical Ephemeris (UK) and the American Ephemeris and Nautical Almanac , replacing UT in the main ephemerides in the issues for 1960 and after. (But the ephemerides in
5850-578: The orientations of the Earth 's rotation axis and orbit around the Sun . Their orientations vary (though slowly, e.g. due to precession ), and there is an infinity of such coordinate systems possible. Thus the coordinate systems most used in astronomy need their own date-reference because the coordinate systems of that type are themselves in motion, e.g. by the precession of the equinoxes , nowadays often resolved into precessional components, separate precessions of
5940-457: The output of all participating clocks, so that TAI would correspond to proper time at the geoid ( mean sea level ). Because the clocks were, on average, well above sea level, this meant that TAI slowed by about one part in a trillion. The former uncorrected time scale continues to be published under the name EAL ( Échelle Atomique Libre , meaning Free Atomic Scale ). The instant that the gravitational correction started to be applied serves as
6030-419: The particular set of coordinates exampled above, much of the elements has been omitted as unknown or undetermined; for example, the element n allows an approximate time-dependence of the element M to be calculated, but the other elements and n itself are treated as constant, which represents a temporary approximation (see Osculating elements ). Thus a particular coordinate system (equinox and equator/ecliptic of
6120-556: The phase of VLF radio signals. The BIH scale, A.1, and NBS-A were defined by an epoch at the beginning of 1958 The procedures used by the BIH evolved, and the name for the time scale changed: A3 in 1964 and TA(BIH) in 1969. The SI second was defined in terms of the caesium atom in 1967. From 1971 to 1975 the General Conference on Weights and Measures and the International Committee for Weights and Measures made
6210-440: The primary ET standard: not only more convenient, but also more precisely uniform than the primary standard itself. Such secondary realizations were used and described as 'ET', with an awareness that the time scales based on the atomic clocks were not identical to that defined by the primary ephemeris time standard, but rather, an improvement over it on account of their closer approximation to uniformity. The atomic clocks gave rise to
6300-495: The real position of the Sun: With this reapplication, the time variable, now given as E, represents time in ephemeris centuries of 36525 ephemeris days of 86400 ephemeris seconds each. The 1961 official reference summarized the concept as such: "The origin and rate of ephemeris time are defined to make the Sun's mean longitude agree with Newcomb's expression" From the comparison of formulae (2) and (3), both of which express
6390-464: The reference to an equinox along with equator/ecliptic, of a certain date, addresses the identification of, or changes in, the coordinate system in terms of which those astronomical variables are expressed. (Sometimes the word 'equinox' may be used alone, e.g. where it is obvious from the context to users of the data in which form the considered astronomical variables are expressed, in equatorial form or ecliptic form.) The equinox with equator/ecliptic of
6480-487: The result of the use of an epoch to express the data. In other cases, e.g. the case of a complete analytical theory of the motion of some astronomical body, all of the elements will usually be given in the form of polynomials in interval of time from the epoch, and they will also be accompanied by trigonometrical terms of periodical perturbations specified appropriately. In that case, their period of validity may stretch over several centuries or even millennia on either side of
6570-652: The results of the English Astronomer Royal H Spencer Jones (1939). Clemence (1948) made it clear that his proposal was intended "for the convenience of astronomers and other scientists only" and that it was "logical to continue the use of mean solar time for civil purposes". De Sitter and Clemence both referred to the proposal as 'Newtonian' or 'uniform' time. D Brouwer suggested the name 'ephemeris time'. Following this, an astronomical conference held in Paris in 1950 recommended "that in all cases where
6660-516: The same calendar day). (See also Julian year (astronomy) .) Like the Besselian epoch, an arbitrary Julian epoch is therefore related to the Julian date by The IAU decided at their General Assembly of 1976 that the new standard equinox of J2000.0 should be used starting in 1984. Before that, the equinox of B1950.0 seems to have been the standard. Different astronomers or groups of astronomers used to define individually, but today standard epochs are generally defined by international agreements through
6750-479: The same denomination, so that the day began when the mean sun crossed the meridian at noon. This is still reflected in the definition of J2000, which started at noon, Terrestrial Time. In traditional cultures and in antiquity other epochs were used. In ancient Egypt , days were reckoned from sunrise to sunrise, following a morning epoch. This may be related to the fact that the Egyptians regulated their year by
6840-521: The same real solar motion in the same real time but defined on separate time scales, Clemence arrived at an explicit expression, estimating the difference in seconds of time between ephemeris time and mean solar time, in the sense (ET-UT): δ t = + 24 s .349 + 72 s .3165 T + 29 s .949 T 2 + 1.821 B {\displaystyle \delta t=+24^{s}.349+72^{s}.3165T+29^{s}.949T^{2}+1.821B} . . . . . (4) with
6930-427: The sky relative to other stars is called proper motion . Most stars have very small proper motions, but a few have proper motions that accumulate to noticeable distances after a few tens of years. So, some stellar positions read from a star atlas or catalog for a sufficiently old epoch require proper motion corrections as well, for reasonable accuracy. Due to precession and proper motion, star data become less useful as
7020-481: The slowing rotation of the Earth. TAI is a weighted average of the time kept by over 450 atomic clocks in over 80 national laboratories worldwide. The majority of the clocks involved are caesium clocks ; the International System of Units (SI) definition of the second is based on caesium . The clocks are compared using GPS signals and two-way satellite time and frequency transfer . Due to
7110-508: The solar day length of 1.7 ms per century (see leap seconds ). International Atomic Time (TAI) was set equal to UT2 at 1 January 1958 0:00:00. At that time, ΔT was already about 32.18 seconds. The difference between Terrestrial Time (TT) (the successor to ephemeris time) and atomic time was later defined as follows: This difference may be assumed constant—the rates of TT and TAI are designed to be identical. Epoch (astronomy) In astronomy , an epoch or reference epoch
7200-400: The standard reference frame of J2000, and it is often then this J2000 position which is shared with others. On the other hand, there has also been an astronomical tradition of retaining observations in just the form in which they were made, so that others can later correct the reductions to standard if that proves desirable, as has sometimes occurred. The currently used standard epoch "J2000"
7290-418: The stated epoch. Some data and some epochs have a long period of use for other reasons. For example, the boundaries of the IAU constellations are specified relative to an equinox from near the beginning of the year 1875. This is a matter of convention, but the convention is defined in terms of the equator and ecliptic as they were in 1875. To find out in which constellation a particular comet stands today,
7380-477: The time as in formula (3) above. The relation with Newcomb's coefficient can be seen from: Caesium atomic clocks became operational in 1955, and quickly confirmed the evidence that the rotation of the Earth fluctuated irregularly. This confirmed the unsuitability of the mean solar second of Universal Time as a measure of time interval for the most precise purposes. After three years of comparisons with lunar observations, Markowitz et al. (1958) determined that
7470-433: The time-varying astronomical quantity can be expressed as a constant, equal to the measure that it had at the epoch, leaving its variation over time to be specified in some other way—for example, by a table, as was common during the 17th and 18th centuries. The word epoch was often used in a different way in older astronomical literature, e.g. during the 18th century, in connection with astronomical tables. At that time, it
7560-468: The weighted average that forms the most stable time scale possible. This combined time scale is published monthly in "Circular T", and is the canonical TAI. This time scale is expressed in the form of tables of differences UTC − UTC( k ) (equal to TAI − TAI( k )) for each participating institution k . The same circular also gives tables of TAI − TA( k ), for the various unsynchronised atomic time scales. Errors in publication may be corrected by issuing
7650-405: The whole of the twentieth century, very slightly shorter than the corresponding (but not precisely constant) units of mean solar time (which, besides their irregular fluctuations, tend to lengthen gradually). This finding is consistent with the modern results of Morrison and Stephenson (see article ΔT ). Although ephemeris time was defined in principle by the orbital motion of the Earth around
7740-415: The years 1952—1959 (computed by W J Eckert from Brown 's theory with modifications recommended by Clemence (1948)). Successive definitions of the unit of ephemeris time are mentioned above ( History ). The value adopted for the 1956/1960 standard second: was obtained from the linear time-coefficient in Newcomb's expression for the solar mean longitude (above), taken and applied with the same meaning for
7830-439: Was customary to denote as "epochs", not the standard date and time of origin for time-varying astronomical quantities, but rather the values at that date and time of those time-varying quantities themselves . In accordance with that alternative historical usage, an expression such as 'correcting the epochs' would refer to the adjustment, usually by a small amount, of the values of the tabulated astronomical quantities applicable to
7920-405: Was defined in principle by the orbital motion of the Earth around the Sun (but its practical implementation was usually achieved in another way, see below). Its detailed definition was based on Simon Newcomb 's Tables of the Sun (1895), implemented in a new way to accommodate certain observed discrepancies: In the introduction to Tables of the Sun, the basis of the tables (p. 9) includes
8010-439: Was eventually established, that the rotation of the Earth ( i.e. the length of the day ) showed irregularities on short time scales, and was slowing down on longer time scales. The evidence was compiled by W de Sitter (1927) who wrote "If we accept this hypothesis, then the 'astronomical time', given by the Earth's rotation, and used in all practical astronomical computations, differs from the 'uniform' or 'Newtonian' time, which
8100-548: Was questioned in a 2007 letter from the BIPM to the ITU-R which stated, "In the case of a redefinition of UTC without leap seconds, the CCTF would consider discussing the possibility of suppressing TAI, as it would remain parallel to the continuous UTC." Contrary to TAI, UTC is a discontinuous time scale. It is occasionally adjusted by leap seconds. Between these adjustments, it is composed of segments that are mapped to atomic time by
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