Epoch (reference date)

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For other uses, see epoch.

epochal date, epochal event, epochal criteria, epoch date,epoch event and epoch criteria all redirect to here.

In the fields of chronology and periodization, an epoch means an instant in time chosen as the origin of a particular era. The "epoch" then serves as a reference point from which time is measured. Time measurement units are counted from the epoch so that the date and time of events can be specified unambiguously.

Contents

Events taking place before the epoch can be dated by counting negatively from the epoch, though in pragmatic periodization practice, epochs are defined for the past, and another epoch is used to start the next era, therefore serving as the ending of the older preceding era. The whole purpose and criteria of such definitions is to clarify and co-ordinate scholarship about a period, at times, across disciplines.

Epochs are generally chosen to be convenient or significant by a consensus of the time scale's initial users, or by authoritarian fiat. The epoch moment or date is usually defined by a specific clear event, condition, or criteria— the epoch event or epoch criteria —from which the period or era or age is usually characterized or described.

Examples:
by events:
The assassination of the Roman Emperor Alexander Severus triggering the Crisis of the Third Century
The defenestration of Prague triggering the horrible depopulation of Europe and the Thirty Years' War
Queen Victoria ascending to the throne giving the start of the Victorian era
by criteria:
The spurt in exploration, mercantilism, and colonization in the Age of Discovery
Particular ratios of animal fossils in a rock strata —various Geology epochs

Calendars

Each calendar era starts from an arbitrary epoch, which is often chosen to commemorate an important historical or mythological event. For example, the epoch of the anno Domini calendar era (the civil calendar era used internationally and in many countries) is the traditionally-reckoned Incarnation of Jesus.[1] Many other current and historical calendar eras exist, each with its own epoch.

Asian national eras

  • The official Japanese system numbers years from the accession of the current emperor, regarding the calendar year during which the accession occurred as the first year.
  • A similar system existed in China before 1912, being based on the accession year of the emperor (1911 was thus the fourth year of the Xuantong period). With the establishment of the Republic of China in 1912, the republican era was introduced. It is still very common in Taiwan to date events via the republican era. The People's Republic of China adopted the common era calendar in 1949 (the 38th year of the Chinese Republic).
  • North Korea uses a system that starts in 1912 (= Juche 1), the year of the birth of their founder Kim Il-Sung. The year 2007 is "Juche 96". Juche means "autarky, self-reliance".
  • In Thailand in 1888 King Chulalongkorn decreed a National Thai Era dating from the founding of Bangkok on April 6, 1782. In 1912, New Year's Day was shifted to April 1. In 1941, Prime Minister Phibunsongkhram decided to count the years since 543 BC. This is the Thai solar calendar using the Thai Buddhist Era. Except for this era, it is the Gregorian calendar.

Religious eras

  • In Israel, the traditional Hebrew calendar, using an era dating from Creation, is the official calendar. However, the Gregorian calendar is the de facto calendar and is commonly used. Government documents usually display a dual date. The beginning of year 1 of the Hebrew calendar occurred in the autumn of 3761 BC. Therefore, "Rosh Hashanah, the Jewish New Year, in September 2003 marked the transition from 5763 to 5764". [2] [3]
  • In the Islamic world, traditional Islamic dating according to the Anno Hegiræ (in the year of the hijra) or AH era remains in use to a varying extent, especially for religious purposes. The official Iranian calendar (used in Afghanistan as well as Iran) also dates from the hijra, but as it is a solar calendar its year numbering does not coincide with the religious calendar.

Other

  • In the French Republican Calendar, a calendar used by the French government for about twelve years from late 1793, the epoch was the beginning of the 'Republican Era', 22 September 1792 (the day the French First Republic was proclaimed, one day after the Convention abolished the monarchy).
  • In the scientific Before Present system of numbering years for purposes of radiocarbon dating, the reference date is January 1, 1950 (though the use of January 1 is mostly irrelevant, as radiocarbon dating is approximate to years and days can rarely be calculated accurately).
  • Different branches of Freemasonry have selected different years to date their documents according to a Masonic era.

Astronomy

Main article: Epoch (astronomy)

In astronomy, an epoch is a specific moment in time for which celestial coordinates or orbital elements are specified, and from which other orbital parametrics are thereafter calculated in order to predict future position. The applied tools of the mathematics disciplines of Celestial mechanics or its subfield Orbital mechanics (both predict orbital paths and positions) about a center of gravity are used to generate an ephemeris (plural: ephemerides; from the Greek word ephemeros = daily) which is a table of values that gives the positions of astronomical objects in the sky at a given time or times, or a formula to calculate such given the proper time offset from the epoch. Such calculations generally result in an elliptical path on a plane defined by some point on the orbit, and the two focii of the ellipse. Viewing from another orbiting body, following it's own trace and orbit, creates shifts in three dimensions in the spherical trigonometry used to calculate relative positions. Interestingly, these dynamics in three dimensions are also elliptical, which means the ephemeris need only specify one set of equations to be a useful predictive tool to predict future location of the object of interest.

Over time, inexactitudes and other errors accumulate, creating more and greater errors of prediction, so ephemeris factors need recalculated from time to time, and that requires a new epoch to be defined. Different astronomers or groups of astronomers used to define epochs to suite themselves, but these days of speedy communications, the epochs are generally defined in an international agreement, so astronomers world wide can collaborate more effectively. It was inefficient and error prone for data observed by one group to need translated (mathematically transformed) so other groups could compare information.

J2000.0

The current standard epoch is called "J2000.0" (Julian 2000, based on the Julian Calendar), is defined by international agreement, and is precisely defined to be approximately noon at the Royal Observatory, Greenwich, in London England:

  1. The Julian date 2451545.0 TT (Terrestrial Time), or January 1, 2000, noon TT.
  2. This is equivalent to January 1, 2000, 11:59:27.816 TAI (International Atomic Time) or
  3. January 1, 2000, 11:58:55.816 UTC (Coordinated Universal Time).

Computing

In computers, time is often expressed as the number of seconds or days (including a fraction) since midnight, Universal Time, from a convenient epoch defined by the operating system. Applications programs get their time keeping from the automatically updated times maintained continuously by the operating system, which typically is counting away so many times a second, usually in some tic only a few milliseconds large.

The tendency writing operating system in the early days of computing was to use as small a data element size as seemed practicable, for servicing a long word was costly in time the computer could be doing something more important than counting every few milliseconds. So small data sizes meant both greater speed and simpler operating system software, that would also then be smaller. That choice in data size, the related epoch moment defining what each count would then mean and so forth in turn depended directly on the hardware of the computer, specifically it's natural word size (the number of bits in it's registers or double or quadruple that computer register size).

The problem becomes one of expressing unambiguously times before the smallest number it's internal words can hold (always zero) and the largest number, for in counting up, when reaching maximum, a binary word cyclically returns to zero, generating a carry bit. So when the number is greater than the largest, when counting, the binary word becomes a new zero. If the software has no space to do something with that carry bit (incrementing the next word like we humans increment the tens place when adding one to nine) because all the words defined to mean clock tick counts by the operating system have been used up. No carry can take place, and the count of all ticks resets to zero.

This creates an ambiguity, and one that lead directly to the now infamous Y2K scare. Contrary to human calendars, computers usually start counting from 0 at the epoch instant, and recycling through all possible counts and so returning to zero made precise dates a choice of two. There is no precision in "two choices", only in "one choice". So the interaction between software, hardware and meaning in human terms is thereafter "broken", an anomaly and exception have been generated when clarity is part of the specified behavior. Sufficient software changes or a larger word size relative to the epoch mitigate the problem.

Famous epoch dates include the :

January 1, 1904, was chosen as the base for the Macintosh clock because it was the first leap year of the twentieth century. [...] This means that by starting with 1904, Macintosh system programmers could save a half dozen instructions in their leap-year checking code.

System time is measured in seconds or ticks of arbitrary length past the epoch. Unspecified problems may occur when this number exceeds a predefined capacity, which is not necessarily a rare event; on a machine counting 10 ticks per second, a signed 32-bit count of ticks allows for only 6.8 years of accurate timekeeping. The 1-tick-per-second clock of Unix will overflow on January 19, 2038, creating the Year 2038 problem on systems that still store time as a 32-bit signed integer. David Mills, author of NTP, acknowledges that the protocol's ultra-precise 64-bit timestamps will roll over on February 6, 2036 and advises that:

Should NTP be in use in 2036, some external means will be necessary to qualify time relative to 1900 and time relative to 2036 (and other multiples of 136 years). (quoted from RFC 1305)

The evolving definition of official time over history introduces more subtle problems for computer-based linear representations. Leap years and the Gregorian calendar are generally taken into account, but leap seconds are more challenging due to their non-linear rate of past occurrences and the impossibility to accurately predict their future occurrences. These complications are discussed at length in the Unix time article.

The fictional (or Julian) leap day February 29, 1900 in Microsoft Excel was introduced intentionally in order to maintain compatibility with then market leader Lotus 1-2-3. Designers of Lotus 1-2-3 had probably chosen this simplified behaviour in order to save some precious processing time and program space. For the rest of its time range 1900 – 9999 Excel uses the Gregorian calendar, hence e.g. there is no February 29, 2100.

Geology

Main article: Epoch (geology)

In geology, an Epoch is a time division criteria based on specific physical and chemical characteristics in rock layers (Lithography) indicative of the global environment, including tectonic activity (mountain building, continental drift, etc.) and include macroscopic (visible by inspection) and microscopic Fauna features or characteristics that are clear and distinct from other series defining criteria. For example, the formations, rock beds, and members of differing rock types that were being laid down in different environments at the same time.

Each defined set of characteristics—in a word, Epochs —directly correspond to specific rock series, the equivalent of an "era" (both are measured usually in time units) in rock layers. Since such layers correspond to time (or era) of formation, they are used to date pre-historic events in other sciences.

Notes

  1. ^ Blackburn, B. & Holford-Strevens, L. (2003). The Oxford Companion to the Year: An exploration of calendar customs and time-reckoning. Oxford University Press. Glossary entry for "Incarnation era", p. 881.
  2. ^ My Jewish Learning:Counting the Years
  3. ^ Rosetta Calendar
  4. ^ Timing on the Macintosh by Martin Minow
  5. ^ USNO GPS Time Transfer

External links

Retrieved from "http://en.wikipedia.org/wiki/Epoch_%28reference_date%29"

This article is from Wikipedia. All text is available under the terms of the GNU Free Documentation License.

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