The Sothic cycle or Canicular period is a period of 1,461 Egyptian civil years of 365 days each or 1,460 Julian years averaging 365¼ days each. During a Sothic cycle, the 365-day year loses enough time that the start of its year once again coincides with the heliacal rising of the star Sirius (Ancient Egyptian: Spdt or Sopdet, "Triangle"; Greek: Σῶθις, Sō̂this) on 19 July in the Julian calendar.[a] It is an important aspect of Egyptology, particularly with regard to reconstructions of the Egyptian calendar and its history. Astronomical records of this displacement may have been responsible for the later establishment of the more accurate Julian and Alexandrian calendars.
The ancient Egyptian civil year, its holidays, and religious records reflect its apparent establishment at a point when the return of the bright star Sirius to the night sky was considered to herald the annual flooding of the Nile. However, because the civil calendar was exactly 365 days long and did not incorporate leap years until 22 BC, its months "wandered" backwards through the solar year at the rate of about one day in every four years. This almost exactly corresponded to its displacement against the Sothic year as well. (The Sothic year is about a minute longer than a solar year.) The sidereal year of 365.25636 days is only valid for stars on the ecliptic (the apparent path of the Sun across the sky), whereas Sirius's displacement ~40˚ below the ecliptic, its proper motion, and the wobbling of the celestial equator cause the period between its heliacal risings to be almost exactly 365.25 days long instead. This steady loss of one relative day every four years over the course of the 365-day calendar meant that the "wandering" day would return to its original place relative to the solar and Sothic year after precisely 1461 civil or 1460 Julian years.
This cycle was first noticed by Eduard Meyer in 1904, who then carefully combed known Egyptian inscriptions and written materials to find any mention of the calendar dates when Sirius rose at dawn. He found six of them, on which the dates of much of the conventional Egyptian chronology are based. A heliacal rise of Sirius was recorded by Censorinus as having happened on the Egyptian New Year's Day between AD 139 and 142. The record actually refers to 21 July AD 140 but is astronomically calculated as a definite 20 July AD 139. This correlates the Egyptian calendar to the Julian calendar. Leap day occurs in AD 140, and so the new year on 1 Thoth is 20 July in AD 139 but it is 19 July for AD 140-142. Thus he was able to compare the day on which Sirius rose in the Egyptian calendar to the day on which Sirius ought to have risen in the Julian calendar, count the number of intercalary days needed, and determine how many years were between the beginning of a cycle and the observation. One also needs to know the place of observation, since the latitude of the observation changes the day when the heliacal rising of Sirius occurs, and mislocating an observation can potentially change the resulting chronology by several decades. (Official observations were made at Heliopolis or Memphis near Cairo, Thebes, and Elephantine near Aswan, with the rising of Sirius observed at Cairo about 8 days after it is seen at Aswan.) Meyer concluded from an ivory tablet from the reign of Djer that the Egyptian civil calendar was created in 4241 BC, a date that appears in a number of old books. But research and discoveries have since shown that the first dynasty of Egypt did not begin before c. 3100 BC, and the claim that 19 July 4241 BC is the "earliest fixed date" has since been discredited. Most scholars either move the observation upon which he based this forward by one cycle of Sirius to 19 July 2781 BC or reject the assumption that the document in question indicates a rise of Sirius at all.
Three specific observations of the heliacal rise of Sirius are extremely important for Egyptian chronology. The first is the aforementioned ivory tablet from the reign of Djer which supposedly indicates the beginning of a Sothic cycle, the rising of Sirius on the same day as the new year. If this does indicate the beginning of a Sothic cycle, it must date to about 17 July 2773 BC. However, this date is too late for Djer's reign, so many scholars believe that it indicates a correlation between the rising of Sirius and the Egyptian lunar calendar, instead of the solar civil calendar, which would render the tablet essentially devoid of chronological value. In 2017 it was claimed that a newly discovered Sothis date from the Old Kingdom and a subsequent astronomic study confirms the Sothic cycle model. 
The second observation is clearly a reference to a heliacal rising, and is believed to date to the seventh year of Senusret III. This observation was almost certainly made at Itj-Tawy, the Twelfth Dynasty capital, which would date the Twelfth Dynasty from 1963 to 1786 BC. The Ramses or Turin Papyrus Canon says 213 years (1991-1778 BC), Parker reduces it to 206 years (1991-1785 BC), based on 17 July 1872 BC as the Sothic date (120th year of 12th dynasty, a drift of 30 leap days). Prior to Parker's investigation of lunar dates the 12th dynasty was placed as 213 years of 2007-1794 BC perceiving the date as 21 July 1888 BC as the 120th year, and then as 2003-1790 BC perceiving the date as 20 July 1884 BC as the 120th year.
The third observation was in the reign of Amenhotep I, and, assuming it was made in Thebes, dates his reign between 1525 and 1504 BC. If made in Memphis, Heliopolis, or some other Delta site instead, as a minority of scholars still argue, the entire chronology of the 18th Dynasty needs to be expanded by some 20 years.
The Sothic cycle is a specific example of two cycles of differing length interacting to cycle together, here called a tertiary cycle. This is mathematically defined by the formula 1/a + 1/b = 1/t or half the harmonic mean. In the case of the Sothic cycle the two cycles are the Egyptian civil year and the Sothic year.
The Sothic year is the length of time for the star Sirius to visually return to the same position in relation to the sun. Star years measured in this way vary due to axial precession, the movement of the Earth's axis in relation to the sun. The length of time for a star to make a yearly path can be marked when it rises to a defined altitude above a local horizon at the time of sunrise. This altitude does not have to be the altitude of first possible visibility. Throughout the year the star will rise approximately four minutes earlier each successive sunrise. Eventually the star will return to its same relative location at sunrise. This length of time can be called an observational year. Stars that reside close to the ecliptic or the ecliptic meridian will on average exhibit observational years close to the sidereal year of 365.2564 days. The ecliptic and the meridian cut the sky into four quadrants. The axis of the earth wobbles around slowly moving the observer and changing the observation of the event. If the axis swings the observer closer to the event its observational year will be shortened. Likewise, the observational year can be lengthened when the axis swings away from the observer. This depends upon which quadrant of the sky the phenomenon is observed.
The Sothic year is remarkable because its average duration was exactly 365.25 days in the early 4th millennium BC before the unification of Egypt. The slow rate of change from this value is also of note. If observations and records could have been maintained during predynastic times the Sothic rise would optimally return to the same calendar day after 1461 calendar years. This value would drop to about 1456 calendar years by the Middle Kingdom. The 1461 value could also be maintained if the date of the Sothic rise were artificially maintained by moving the feast in celebration of this event one day every fourth year instead of rarely adjusting it according to observation.
It has been noticed, and the Sothic cycle confirms, that Sirius does not move retrograde across the sky like other stars, a phenomenon widely known as the precession of the equinox. Professor Jed Buchwald wrote "Sirius remains about the same distance from the equinoxes—and so from the solstices—throughout these many centuries, despite precession."  For the same reason, the helical rising or zenith of Sirius does not slip through the calendar at the precession rate of about one day per 71.6 years as other stars do but much slower. This remarkable stability within the solar year may be one reason that the Egyptians used it as a basis for their calendar. The coincidence of a heliacal rising of Sirius and the New Year reported by Censorinus occurred about the 20th of July, that is a month later after the summer solstice.
Determining the date of a heliacal rise of Sirius has been shown to be difficult, especially considering the need to know the exact latitude of the observation. Another problem is that because the Egyptian calendar loses one day every four years, a heliacal rise will take place on the same day for four years in a row, and any observation of that rise can date to any of those four years, making the observation imprecise.
A number of criticisms have been leveled against the reliability of dating by the Sothic cycle. Some are serious enough to be considered problematic. Firstly, none of the astronomical observations have dates that mention the specific pharaoh in whose reign they were observed, forcing Egyptologists to supply that information on the basis of a certain amount of informed speculation. Secondly, there is no information regarding the nature of the civil calendar throughout the course of Egyptian history, forcing Egyptologists to assume that it existed unchanged for thousands of years; the Egyptians would only have needed to carry out one calendar reform in a few thousand years for these calculations to be worthless. Other criticisms are not considered as problematic, e.g. there is no extant mention of the Sothic cycle in ancient Egyptian writing, which may simply be a result of it either being so obvious to Egyptians that it didn't merit mention or to relevant texts being destroyed over time or still awaiting discovery.
One recent popular history of the Ancient Near East by Marc Van de Mieroop, in his discussion of chronology and dating, does not mention the Sothic cycle at all, and believes that the bulk of historians nowadays would consider that it is not possible to put forward exact dates earlier than the 8th century BCE. 
Some have recently claimed that the Theran eruption marks the beginning of the Eighteenth dynasty due to Theran ash and pumice discoveries in the ruins of Avaris in layers that mark the end of the Hyksos era. Because the evidence of dendrochronologists indicates the eruption took place in 1626 BC, this has been taken to indicate that dating by the Sothic cycle is off by 50–80 years at the outset of the 18th dynasty. Claims that the Thera eruption is the subject of the Tempest Stele of Ahmose I have been disputed by writers such as Peter James.
The Armenian calendar is the calendar traditionally used in Armenia.
The older Armenian calendar was based on an invariant year length of 365 days. As a result, the correspondence between it and both the solar year and the Julian calendar slowly drifted over time, shifting across a year of the Julian calendar once in 1,461 calendar years (see Sothic cycle). Thus, the Armenian year 1461 (Gregorian 2010/2011) completed the first full cycle.
Armenian year 1 began on 11 July 552 of the Julian calendar, and Armenian year 1462 began on 11 July 2012 of the Julian calendar which co-incided with 24 July 2012 of the Gregorian calendar.
An analytical expression of the Armenian date includes ancient name of Day of week, Christian name of Day of week, named Day of month, Date, Month, Year number after 552 A.D. and the religious feasts.
The Armenian calendar is divided into 12 months of 30 days each, plus an additional (epagomenal) five days are called aweleacʿ ("superfluous").
Years are usually given in Armenian numerals, letters of the Armenian alphabet preceded by the abbreviation ԹՎ for t’vin "in the year" (for example, ԹՎ ՌՆԾԵ "in the year 1455").Chronological synchronism
Chronological synchronism is an event that links two chronologies. For example, it is used in Egyptology to ground Egyptian chronology. The main types of chronological synchronism are synchronisms with other historical chronologies and synchronisms with precisely datable astronomical events.
Synchronisms with other chronologies often rely on some form of recorded communication between regions. For example, in Egyptology, the earliest such synchronisms appear in the 15th century BC, during the Amarna Period by the considerable quantity of diplomatic correspondence between Amenhotep III and Akhenaten and various Near Eastern monarchs; that links Egyptian chronology with other Near Eastern chronologies.
Astronomical synchronisms rely on precise identification of astronomical events recorded in the historical record. The best known of these is the Sothic cycle whose careful study led Richard Anthony Parker to argue that the dates of the Twelfth dynasty of Egypt could be fixed with absolute precision. More recent research has eroded that confidence and questioned many of the assumptions used with the Sothic Cycle. As a result, experts have moved away from relying on it.Chronostratigraphy
Chronostratigraphy is the branch of stratigraphy that studies the age of rock strata in relation to time.
The ultimate aim of chronostratigraphy is to arrange the sequence of deposition and the time of deposition of all rocks within a geological region, and eventually, the entire geologic record of the Earth.
The standard stratigraphic nomenclature is a chronostratigraphic system based on palaeontological intervals of time defined by recognised fossil assemblages (biostratigraphy). The aim of chronostratigraphy is to give a meaningful age date to these fossil assemblage intervals and interfaces.Circa
Circa (from Latin, meaning 'around, about') – frequently abbreviated c., ca. or ca and less frequently circ. or cca. – signifies "approximately" in several European languages and as a loanword in English, usually in reference to a date. Circa is widely used in historical writing when the dates of events are not accurately known.
When used in date ranges, circa is applied before each approximate date, while dates without circa immediately preceding them are generally assumed to be known with certainty.
1732–1799: Both years are known precisely.
c. 1732 – 1799: The beginning year is approximate; the end year is known precisely.
1732 – c. 1799: The beginning year is known precisely ; the end year is approximate.
c. 1732 – c. 1799: Both years are approximate.Egyptian chronology
The majority of Egyptologists agree on the outline and many details of the chronology of Ancient Egypt. This scholarly consensus is the so-called Conventional Egyptian chronology, which places the beginning of the Old Kingdom in the 27th century BC, the beginning of the Middle Kingdom in the 21st century BC and the beginning of the New Kingdom in the mid-16th century BC.
Despite this consensus, disagreements remain within the scholarly community, resulting in variant chronologies diverging by about 300 years for the Early Dynastic Period, up to 30 years in the New Kingdom, and a few years in the Late Period.In addition, there are a number of "alternative chronologies" outside scholarly consensus, such as the "New Chronology" proposed in the 1990s, which lowers New Kingdom dates by as much as 350 years, or the "Glasgow Chronology" (proposed 1978–1982), which lowers New Kingdom dates by as much as 500 years.Era (geology)
A geologic era is a subdivision of geologic time that divides an eon into smaller units of time. The Phanerozoic Eon is divided into three such time frames: the Paleozoic, Mesozoic, and Cenozoic (meaning "old life", "middle life" and "recent life") that represent the major stages in the macroscopic fossil record. These eras are separated by catastrophic extinction boundaries, the P-T boundary between the Paleozoic and the Mesozoic and the K-Pg boundary between the Mesozoic and the Cenozoic. There is evidence that catastrophic meteorite impacts played a role in demarcating the differences between the eras.
The Hadean, Archean and Proterozoic eons were as a whole formerly called the Precambrian. This covered the four billion years of Earth history prior to the appearance of hard-shelled animals. More recently, however, the Archean and Proterozoic eons have been subdivided into eras of their own.
Geologic eras are further subdivided into geologic periods, although the Archean eras have yet to be subdivided in this way.Floruit
Floruit (UK: , US: ), abbreviated fl. (or occasionally flor.), Latin for "he/she flourished", denotes a date or period during which a person was known to have been alive or active. In English, the word may also be used as a noun indicating the time when someone flourished.Fluorine absorption dating
Fluorine absorption dating is a method used to determine the amount of time an object has been underground.
Fluorine absorption dating can be carried out based on the fact that groundwater contains fluoride ions. Items such as bone that are in the soil will absorb fluoride from the groundwater over time. From the amount of absorbed fluoride in the item, the time that the item has been in the soil can be estimated.
Many instances of this dating method compare the amount of fluorine and uranium in the bones to nitrogen dating to create more accurate estimation of date. Older bones have more fluorine and uranium and less nitrogen. But because decomposition happens at different speeds in different places, it's not possible to compare bones from different sites.
As not all objects absorb fluorine at the same rate, this also undermines the accuracy of such a dating technique. Although this can be compensated for by accommodating for the rate of absorption in calculations, such an accommodation tends to have a rather large margin of error.
In 1953 this test was used to easily identify that the 'Piltdown Man' was forged, almost 50 years after it was originally 'unearthed'.Geologic Calendar
The Geologic Calendar is a scale in which the geological lifetime of the earth is mapped onto a calendrical year; that is to say, the day one of the earth took place on a geologic January 1 at precisely midnight, and today's date and time is December 31 at midnight. On this calendar, the inferred appearance of the first living single-celled organisms, prokaryotes, occurred on a geologic February 25 around 12:30pm to 1:07pm, dinosaurs first appeared on December 13, the first flower plants on December 22 and the first primates on December 28 at about 9:43pm. The first Anatomically modern humans did not arrive until around 11:48 p.m. on New Year's Eve, and all of human history since the end of the last ice-age occurred in the last 82.2 seconds before midnight of the new year.Geological period
A geological period is one of the several subdivisions of geologic time enabling cross-referencing of rocks and geologic events from place to place.
These periods form elements of a hierarchy of divisions into which geologists have split the Earth's history.
Eons and eras are larger subdivisions than periods while periods themselves may be divided into epochs and ages.
The rocks formed during a period belong to a stratigraphic unit called a system.Law of superposition
The law of superposition is an axiom that forms one of the bases of the sciences of geology, archaeology, and other fields dealing with geological stratigraphy. It is a form of relative dating. In its plainest form, it states that in undeformed stratigraphic sequences, the oldest strata will be at the bottom of the sequence. This is important to stratigraphic dating, which assumes that the law of superposition holds true and that an object cannot be older than the materials of which it is composed.Limmu
Limmu was an Assyrian eponym. At the beginning of the reign of an Assyrian king, the limmu, an appointed royal official, would preside over the New Year festival at the capital. Each year a new limmu would be chosen. Although picked by lot, there was most likely a limited group, such as the men of the most prominent families or perhaps members of the city assembly. The Assyrians used the name of the limmu for that year to designate the year on official documents. Lists of limmus have been found accounting for every year between 892 BC and 648 BC.
During the Old Assyrian period, the king himself was never the limmum, as it was called in their language. In the Middle Assyrian and Neo-Assyrian periods, however, the king could take this office.List of cycles
This is a list of recurring cycles. See also Index of wave articles, Time, and Pattern.New Earth Time
New Earth Time (or NET) is an alternative naming system for measuring the time of day. In NET the day is split into 360 NET degrees, each NET degree is split into 60 NET minutes and each NET minute is split into 60 NET seconds. One NET degree is therefore equivalent to four standard minutes, and one standard hour is equivalent to 15 NET degrees.
NET is equivalent to the UTC read from a 24-hour analog clock as the clockwise angle past midnight of the hour hand. For example, noon is 180°0'0" NET and at that time the hour hand is pointing straight down forming a 180° angle when measured from the top, at midnight. A full circle is 360 degrees and one NET day.Nitrogen dating
Nitrogen dating is a form of relative dating which relies on the reliable breakdown and release of amino acids from bone samples to estimate the age of the object. For human bones, the assumption of about 5% nitrogen in the bone, mostly in the form of collogen, allows fairly consistent dating techniques.Compared to other dating techniques, Nitrogen dating can be unreliable because leaching from bone is dependent on temperature, soil pH, ground water, and the presence of microorganism that digest nitrogen rich elements, like collagen. Some studies compare nitrogen dating results with dating results from methods like fluorine absorption dating to create more accurate estimates. Though some situations, like thin porous bones might more rapidly change the dating created by multiple methods.Proleptic Gregorian calendar
The proleptic Gregorian calendar is produced by extending the Gregorian calendar backward to dates preceding its official introduction in 1582. In countries that adopted the Gregorian calendar later, dates occurring in the interim (between 1582 and the local adoption) are sometimes "Gregorianized" as well. For example, George Washington was born on February 11, 1731 (Old Style), as Great Britain and its possessions were using the Julian calendar with English years starting on March 25 until September 1752. After the switch, that day became February 22, 1732, which is the date commonly given as Washington's birthday.Spanish era
The Spanish era or era of Caesar (Latin: Æra Hispanica) was a dating system commonly used in the states of the Iberian Peninsula from the 3rd century until the 14th–15th centuries, when it was phased out in favour of the Anno Domini system. Year one of this calendar era coincides with what is now known as 38 BC, possibly the date of a new tax imposed by the Roman Republic on the subdued population of Iberia. Whatever the case, the date signifies the beginning of the Pax Romana in Iberia.
To convert an Anno Domini (AD) date to the corresponding year in the Spanish era, add 38 to the Anno Domini year, such that Era 941 would be equivalent to AD 903.
Official usage ceased in different parts of the peninsula at different times: Aragon in AD 1349, Valencia 1358, Castile 1383, and Portugal 1422. While the year officially began on 1 January under the Spanish era, that was changed to 25 December when the Anno Domini system was adopted (while the Church used 11 January).Stratotype
A stratotype or type section is a geological term that names the physical location or outcrop of a particular reference exposure of a stratigraphic sequence or stratigraphic boundary. If the stratigraphic unit is layered, it is called a stratotype, whereas the standard of reference for unlayered rocks is the type locality.Terminus post quem
Terminus post quem ("limit after which", often abbreviated to TPQ) and terminus ante quem ("limit before which", abbreviated to TAQ) specify the known limits of dating for events. A terminus post quem is the earliest time the event may have happened, and a terminus ante quem is the latest. An event may well have both a terminus post quem and a terminus ante quem, in which case the limits of the possible range of dates are known at both ends, but many events have just one or the other. Similarly, terminus ad quem ("limit to which") is the latest possible date of a non-punctual event (period, era, etc.), while terminus a quo ("limit from which") is the earliest. The concepts are similar to those of upper and lower bounds in mathematics.