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50-588: It erupted in 2500 BP in the southern part of the Krafla fissure swarm. The crater is approximately 1 km in diameter. Tephra has been carried from Hverfjall all over the Mývatn area. A landslide apparently occurred in the south part of the crater during the eruption, which accounts for the disruption to the round shape of the mountain. During the Age of Settlement, lava flowed from Svörtuborgir [ˈsvœr̥tʏˌpɔrcɪr̥] , at

100-423: 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 , 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

150-401: A 1950-based reference sample of oxalic acid . According to scientist A. Currie Lloyd: The problem was tackled by the international radiocarbon community in the late 1950s, in cooperation with the U.S. National Bureau of Standards . A large quantity of contemporary oxalic acid dihydrate was prepared as NBS Standard Reference Material (SRM) 4990B. Its C concentration was about 5% above what

200-577: 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

250-409: 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

300-548: 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

350-461: 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

400-460: 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

450-409: 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

500-534: 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 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

550-477: 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

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600-415: Is a stub . You can help Misplaced Pages by expanding it . Before Present Before Present ( BP ) or " years before present ( YBP )" is a time scale used mainly in archaeology , geology, and other scientific disciplines to specify when events occurred relative to the origin of practical radiocarbon dating in the 1950s. Because the "present" time changes, standard practice is to use 1 January 1950 as

650-412: 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

700-438: 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 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

750-439: 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

800-475: 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

850-608: 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

900-598: The unit "a" (for "annum", Latin for "year") and reserve the term "BP" for radiocarbon estimations. Some archaeologists use the lowercase letters bp , bc and ad as terminology for uncalibrated dates for these eras. The Centre for Ice and Climate at the University of Copenhagen instead uses the unambiguous "b2k", for "years before 2000 AD", often in combination with the Greenland Ice Core Chronology 2005 (GICC05) time scale. Some authors who use

950-406: 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

1000-460: 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

1050-509: The YBP dating format also use "YAP" ("years after present") to denote years after 1950. SI prefix multipliers may be used to express larger periods of time, e.g. ka BP (thousand years BP), Ma BP (million years BP) and many others . Radiocarbon dating was first used in 1949. Beginning in 1954, metrologists established 1950 as the origin year for the BP scale for use with radiocarbon dating, using

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1100-505: 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

1150-422: The commencement date (epoch) of the age scale, with 1950 being labelled as the "standard year". The abbreviation "BP" has been interpreted retrospectively as "Before Physics", which refers to the time before nuclear weapons testing artificially altered the proportion of the carbon isotopes in the atmosphere, which scientists must account for. In a convention that is not always observed, many sources restrict

1200-406: 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

1250-426: 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

1300-500: 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

1350-649: The epochs' would refer to the adjustment, usually by a small amount, of the values of the tabulated astronomical quantities applicable to 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

1400-425: 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

1450-492: 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

1500-471: The exponential decay relation and the "Libby half-life" 5568 a. The ages are expressed in years before present (BP) where "present" is defined as AD 1950. The year 1950 was chosen because it was the standard astronomical epoch at that time. It also marked the publication of the first radiocarbon dates in December 1949, and 1950 also antedates large-scale atmospheric testing of nuclear weapons , which altered

1550-452: 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

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1600-480: The global ratio of carbon-14 to carbon-12 . Dates determined using radiocarbon dating come as two kinds: uncalibrated (also called Libby or raw ) and calibrated (also called Cambridge ) dates. Uncalibrated radiocarbon dates should be clearly noted as such by "uncalibrated years BP", because they are not identical to calendar dates. This has to do with the fact that the level of atmospheric radiocarbon ( carbon-14 or C) has not been strictly constant during

1650-525: 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

1700-446: 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

1750-463: The name (standard codes are used) of the laboratory concerned, and other information such as confidence levels, because of differences between the methods used by different laboratories and changes in calibrating methods. Conversion from Gregorian calendar years to Before Present years is by starting with the 1950-01-01 epoch of the Gregorian calendar and increasing the BP year count with each year into

1800-571: 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

1850-419: 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

1900-412: The past from that Gregorian date. For example, 1000 BP corresponds to 950 AD, 1949 BP corresponds to 1 AD, 1950 BP corresponds to 1 BC, 2000 BP corresponds to 51 BC. Astronomical epoch In astronomy , an epoch or reference epoch 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

1950-464: 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

2000-487: 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

2050-516: 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

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2100-474: 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

2150-471: 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, 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

2200-427: 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

2250-496: The southern end of Námafjall [ˈnauːmaˌfjatl̥] , around Hverfjall, which was nearly engulfed by the lava. At the same time an eruption occurred in the slopes above the valley of Hlíðardalur [ˈl̥iːðarˌtaːlʏr̥] . The rim of the crater is only accessible by two paths, from the northwest and south. It is strictly forbidden to use other routes in ascent or descent. [REDACTED] Media related to Hverfjall at Wikimedia Commons This Iceland location article

2300-596: The span of time that can be radiocarbon-dated. Uncalibrated radiocarbon ages can be converted to calendar dates by calibration curves based on comparison of raw radiocarbon dates of samples independently dated by other methods, such as dendrochronology (dating based on tree growth-rings) and stratigraphy (dating based on sediment layers in mud or sedimentary rock). Such calibrated dates are expressed as cal BP, where "cal" indicates "calibrated years", or "calendar years", before 1950. Many scholarly and scientific journals require that published calibrated results be accompanied by

2350-400: 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"

2400-418: 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,

2450-520: The use of BP dates to those produced with radiocarbon dating; the alternative notation "RCYBP" stands for the explicit "radio carbon years before present". The BP scale is sometimes used for dates established by means other than radiocarbon dating, such as stratigraphy . This usage differs from the recommendation by van der Plicht & Hogg, followed by the Quaternary Science Reviews , both of which requested that publications should use

2500-419: Was believed to be the natural level, so the standard for radiocarbon dating was defined as 0.95 times the C concentration of this material, adjusted to a C reference value of −19 per mil (PDB). This value is defined as "modern carbon" referenced to AD 1950. Radiocarbon measurements are compared to this modern carbon value, and expressed as "fraction of modern" (fM). "Radiocarbon ages" are calculated from fM using

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