Time is the indefinite continued progress of existence and events that occur in irreversible succession through the past, in the present, the future. Time is a component quantity of various measurements used to sequence events, to compare the duration of events or the intervals between them, to quantify rates of change of quantities in material reality or in the conscious experience. Time is referred to as a fourth dimension, along with three spatial dimensions. Time has long been an important subject of study in religion and science, but defining it in a manner applicable to all fields without circularity has eluded scholars. Diverse fields such as business, sports, the sciences, the performing arts all incorporate some notion of time into their respective measuring systems. Time in physics is unambiguously operationally defined as "what a clock reads". See Units of Time. Time is one of the seven fundamental physical quantities in both the International System of Units and International System of Quantities.
Time is used to define other quantities – such as velocity – so defining time in terms of such quantities would result in circularity of definition. An operational definition of time, wherein one says that observing a certain number of repetitions of one or another standard cyclical event constitutes one standard unit such as the second, is useful in the conduct of both advanced experiments and everyday affairs of life; the operational definition leaves aside the question whether there is something called time, apart from the counting activity just mentioned, that flows and that can be measured. Investigations of a single continuum called spacetime bring questions about space into questions about time, questions that have their roots in the works of early students of natural philosophy. Temporal measurement has occupied scientists and technologists, was a prime motivation in navigation and astronomy. Periodic events and periodic motion have long served as standards for units of time. Examples include the apparent motion of the sun across the sky, the phases of the moon, the swing of a pendulum, the beat of a heart.
The international unit of time, the second, is defined by measuring the electronic transition frequency of caesium atoms. Time is of significant social importance, having economic value as well as personal value, due to an awareness of the limited time in each day and in human life spans. Speaking, methods of temporal measurement, or chronometry, take two distinct forms: the calendar, a mathematical tool for organising intervals of time, the clock, a physical mechanism that counts the passage of time. In day-to-day life, the clock is consulted for periods less than a day whereas the calendar is consulted for periods longer than a day. Personal electronic devices display both calendars and clocks simultaneously; the number that marks the occurrence of a specified event as to hour or date is obtained by counting from a fiducial epoch – a central reference point. Artifacts from the Paleolithic suggest that the moon was used to reckon time as early as 6,000 years ago. Lunar calendars were among the first to appear, with years of either 13 lunar months.
Without intercalation to add days or months to some years, seasons drift in a calendar based on twelve lunar months. Lunisolar calendars have a thirteenth month added to some years to make up for the difference between a full year and a year of just twelve lunar months; the numbers twelve and thirteen came to feature prominently in many cultures, at least due to this relationship of months to years. Other early forms of calendars originated in Mesoamerica in ancient Mayan civilization; these calendars were religiously and astronomically based, with 18 months in a year and 20 days in a month, plus five epagomenal days at the end of the year. The reforms of Julius Caesar in 45 BC put the Roman world on a solar calendar; this Julian calendar was faulty in that its intercalation still allowed the astronomical solstices and equinoxes to advance against it by about 11 minutes per year. Pope Gregory XIII introduced a correction in 1582. During the French Revolution, a new clock and calendar were invented in attempt to de-Christianize time and create a more rational system in order to replace the Gregorian calendar.
The French Republican Calendar's days consisted of ten hours of a hundred minutes of a hundred seconds, which marked a deviation from the 12-based duodecimal system used in many other devices by many cultures. The system was abolished in 1806. A large variety of devices have been invented to measure time; the study of these devices is called horology. An Egyptian device that dates to c. 1500 BC, similar in shape to a bent T-square, measured the passage of time from the shadow cast by its crossbar on a nonlinear rule. The T was oriented eastward in the mornings. At noon, the device was turned around so. A sundial uses a gnomon to cast a shadow on a set of markings calibrated to the hour; the position of the shadow marks the hour in local time. The idea to separate the day into smaller parts is credited to Egyptians because of their sundials, which operated on a duodecimal system; the importance of the number 12 is due to the number of lunar cycles in a year and the number of stars used to count the passage of night.
The most precise timekeeping device of the ancient
History is the study of the past as it is described in written documents. Events occurring before written record are considered prehistory, it is an umbrella term that relates to past events as well as the memory, collection, organization and interpretation of information about these events. Scholars who write about history are called historians. History can refer to the academic discipline which uses a narrative to examine and analyse a sequence of past events, objectively determine the patterns of cause and effect that determine them. Historians sometimes debate the nature of history and its usefulness by discussing the study of the discipline as an end in itself and as a way of providing "perspective" on the problems of the present. Stories common to a particular culture, but not supported by external sources, are classified as cultural heritage or legends, because they do not show the "disinterested investigation" required of the discipline of history. Herodotus, a 5th-century BC Greek historian is considered within the Western tradition to be the "father of history", along with his contemporary Thucydides, helped form the foundations for the modern study of human history.
Their works continue to be read today, the gap between the culture-focused Herodotus and the military-focused Thucydides remains a point of contention or approach in modern historical writing. In East Asia, a state chronicle, the Spring and Autumn Annals was known to be compiled from as early as 722 BC although only 2nd-century BC texts have survived. Ancient influences have helped spawn variant interpretations of the nature of history which have evolved over the centuries and continue to change today; the modern study of history is wide-ranging, includes the study of specific regions and the study of certain topical or thematical elements of historical investigation. History is taught as part of primary and secondary education, the academic study of history is a major discipline in university studies; the word history comes from the Ancient Greek ἱστορία, meaning'inquiry','knowledge from inquiry', or'judge'. It was in that sense; the ancestor word ἵστωρ is attested early on in Homeric Hymns, the Athenian ephebes' oath, in Boiotic inscriptions.
The Greek word was borrowed into Classical Latin as historia, meaning "investigation, research, description, written account of past events, writing of history, historical narrative, recorded knowledge of past events, narrative". History was borrowed from Latin into Old English as stær, but this word fell out of use in the late Old English period. Meanwhile, as Latin became Old French, historia developed into forms such as istorie and historie, with new developments in the meaning: "account of the events of a person's life, account of events as relevant to a group of people or people in general, dramatic or pictorial representation of historical events, body of knowledge relative to human evolution, narrative of real or imaginary events, story", it was from Anglo-Norman that history was borrowed into Middle English, this time the loan stuck. It appears in the 13th-century Ancrene Wisse, but seems to have become a common word in the late 14th century, with an early attestation appearing in John Gower's Confessio Amantis of the 1390s: "I finde in a bok compiled | To this matiere an old histoire, | The which comth nou to mi memoire".
In Middle English, the meaning of history was "story" in general. The restriction to the meaning "the branch of knowledge that deals with past events. With the Renaissance, older senses of the word were revived, it was in the Greek sense that Francis Bacon used the term in the late 16th century, when he wrote about "Natural History". For him, historia was "the knowledge of objects determined by space and time", that sort of knowledge provided by memory. In an expression of the linguistic synthetic vs. analytic/isolating dichotomy, English like Chinese now designates separate words for human history and storytelling in general. In modern German and most Germanic and Romance languages, which are solidly synthetic and inflected, the same word is still used to mean both'history' and'story'. Historian in the sense of a "researcher of history" is attested from 1531. In all European languages, the substantive history is still used to mean both "what happened with men", "the scholarly study of the happened", the latter sense sometimes distinguished with a capital letter, or the word historiography.
The adjective historical is attested from 1661, historic from 1669. Historians write in the context of their own time, with due regard to the current dominant ideas of how to interpret the past, sometimes write to provide lessons for their own society. In the words of Benedetto Croce, "All history is contemporary history". History is facilitated by the formation of a "true discourse of past" through the production of narrative and analysis of past events relating to the human race; the modern discipline of history is dedicated to the institutional production of this discourse. All events that are remembered and preserved in some authentic form constitute the historical record; the task of histori
The terms anno Domini and before Christ are used to label or number years in the Julian and Gregorian calendars. The term anno Domini is Medieval Latin and means "in the year of the Lord", but is presented using "our Lord" instead of "the Lord", taken from the full original phrase "anno Domini nostri Jesu Christi", which translates to "in the year of our Lord Jesus Christ"; this calendar era is based on the traditionally reckoned year of the conception or birth of Jesus of Nazareth, with AD counting years from the start of this epoch, BC denoting years before the start of the era. There is no year zero in this scheme, so the year AD 1 follows the year 1 BC; this dating system was devised in 525 by Dionysius Exiguus of Scythia Minor, but was not used until after 800. The Gregorian calendar is the most used calendar in the world today. For decades, it has been the unofficial global standard, adopted in the pragmatic interests of international communication and commercial integration, recognized by international institutions such as the United Nations.
Traditionally, English followed Latin usage by placing the "AD" abbreviation before the year number. However, BC is placed after the year number, which preserves syntactic order; the abbreviation is widely used after the number of a century or millennium, as in "fourth century AD" or "second millennium AD". Because BC is the English abbreviation for Before Christ, it is sometimes incorrectly concluded that AD means After Death, i.e. after the death of Jesus. However, this would mean that the approximate 33 years associated with the life of Jesus would neither be included in the BC nor the AD time scales. Terminology, viewed by some as being more neutral and inclusive of non-Christian people is to call this the Current or Common Era, with the preceding years referred to as Before the Common or Current Era. Astronomical year numbering and ISO 8601 avoid words or abbreviations related to Christianity, but use the same numbers for AD years; the Anno Domini dating system was devised in 525 by Dionysius Exiguus to enumerate the years in his Easter table.
His system was to replace the Diocletian era, used in an old Easter table because he did not wish to continue the memory of a tyrant who persecuted Christians. The last year of the old table, Diocletian 247, was followed by the first year of his table, AD 532; when he devised his table, Julian calendar years were identified by naming the consuls who held office that year—he himself stated that the "present year" was "the consulship of Probus Junior", 525 years "since the incarnation of our Lord Jesus Christ". Thus Dionysius implied that Jesus' incarnation occurred 525 years earlier, without stating the specific year during which his birth or conception occurred. "However, nowhere in his exposition of his table does Dionysius relate his epoch to any other dating system, whether consulate, year of the world, or regnal year of Augustus. Among the sources of confusion are: In modern times, incarnation is synonymous with the conception, but some ancient writers, such as Bede, considered incarnation to be synonymous with the Nativity.
The civil or consular year began on 1 January but the Diocletian year began on 29 August. There were inaccuracies in the lists of consuls. There were confused summations of emperors' regnal years, it is not known. Two major theories are that Dionysius based his calculation on the Gospel of Luke, which states that Jesus was "about thirty years old" shortly after "the fifteenth year of the reign of Tiberius Caesar", hence subtracted thirty years from that date, or that Dionysius counted back 532 years from the first year of his new table, it has been speculated by Georges Declercq that Dionysius' desire to replace Diocletian years with a calendar based on the incarnation of Christ was intended to prevent people from believing the imminent end of the world. At the time, it was believed by some that the resurrection of the dead and end of the world would occur 500 years after the birth of Jesus; the old Anno Mundi calendar theoretically commenced with the creation of the world based on information in the Old Testament.
It was believed that, based on the Anno Mundi calendar, Jesus was born in the year 5500 with the year 6000 of the Anno Mundi calendar marking the end of the world. Anno Mundi 6000 was thus equated with the resurrection and the end of the world but this date had passed in the time of Dionysius; the Anglo-Saxon historian the Venerable Bede, familiar with the work of Dionysius Exiguus, used Anno Domini dating in his Ecclesiastical History of the English People, completed in 731. In this same history, he used another Latin term, ante vero incarnationis dominicae tempus anno sexagesimo, equivalent to the English "before Christ", to identify years before the first year of this era. Both Dionysius and Bede regarded Anno Domini as beginning at the incarnation of Jesus, but "the distinction between Incarnation and Nativity was not drawn until the late 9th century, when in some places the Incarnation epoch was identified with Christ's conception, i.e. the Annunciation on March 25". On the continent of Europe, Anno
Ab urbe condita
Ab urbe condita, or Anno urbis conditæ abbreviated as AUC in either case, is a convention, used in antiquity and by classical historians to refer to a given year in Ancient Rome. Ab urbe condita means "from the founding of the City," while anno urbis conditæ means "in the year since the City's founding." Therefore, the traditional year of the foundation of Rome, 753 BC, would be written AUC 1, while AD 1 would be AUC 754. The foundation of the Empire in 27 BC would be AUC 727. Usage of the term was more common during the Renaissance, when editors sometimes added AUC to Roman manuscripts they published, giving the false impression that the convention was used in antiquity. In reality, the dominant method of identifying years in Roman times was to name the two consuls who held office that year. In late antiquity, regnal years were in use, as was the Diocletian era in Roman Egypt after AD 293, in the Byzantine Empire after AD 537, following a decree by Justinian; the traditional date for the founding of Rome, 21 April 753 BC, is due to Marcus Terentius Varro.
Varro may have used the consular list and called the year of the first consuls "ab urbe condita 245," accepting the 244-year interval from Dionysius of Halicarnassus for the kings after the foundation of Rome. The correctness of this calculation has not been confirmed. From the time of Claudius onward, this calculation superseded other contemporary calculations. Celebrating the anniversary of the city became part of imperial propaganda. Claudius was the first to hold magnificent celebrations in honor of the anniversary of the city, in AD 48, the eight hundredth year from the founding of the city. Hadrian and Antoninus Pius held similar celebrations, in AD 121, in AD 147 and AD 148, respectively. In AD 248, Philip the Arab celebrated Rome's first millennium, together with Ludi saeculares for Rome's alleged tenth sæculum. Coins from his reign commemorate the celebrations. A coin by a contender for the imperial throne, explicitly states "ear one thousand and first", an indication that the citizens of the empire had a sense of the beginning of a new era, a Sæculum Novum.
The Anno Domini year numbering was developed by a monk named Dionysius Exiguus in Rome in AD 525, as a result of his work on calculating the date of Easter. Dionysius did not use the AUC convention, but instead based his calculations on the Diocletian era; this convention had been in use since AD 293, the year of the tetrarchy, as it became impractical to use regnal years of the current emperor. In his Easter table, the year AD 532 was equated with the 248th regnal year of Diocletian; the table counted the years starting from the presumed birth of Christ, rather than the accession of the emperor Diocletian on 20 November AD 284, or as stated by Dionysius: "sed magis elegimus ab incarnatione Domini nostri Jesu Christi annorum tempora praenotare". Blackburn and Holford-Strevens review interpretations of Dionysius which place the Incarnation in 2 BC, 1 BC, or AD 1, it has been calculated that the year AD 1 corresponds to AUC 754, based on the epoch of Varro. Thus, AUC 1 = 753 BC AUC 753 = 1 BC AUC 754 = AD 1 AUC 1000 = AD 247 AUC 1229 = AD 476 AUC 2206 = AD 1453 AUC 2753 = AD 2000 AUC 2772 = AD 2019 List of Latin phrases
The Roman calendar was the calendar used by the Roman kingdom and republic. The term includes the Julian calendar established by the reforms of the dictator Julius Caesar and emperor Augustus in the late 1st century BC and sometimes includes any system dated by inclusive counting towards months' kalends and ides in the Roman manner; the term excludes the Alexandrian calendar of Roman Egypt, which continued the unique months of that land's former calendar. Roman dates were counted inclusively forward to the next of three principal days: the first of the month, a day less than the middle of the month, eight days—nine, counting inclusively—before this; the original calendar consisted of 10 months beginning in spring with March. These months ran for 38 nundinal cycles, each forming an eight-day week ended by religious rituals and a public market; the winter period was divided into two months and February. The legendary early kings Romulus and Numa Pompilius were traditionally credited with establishing this early fixed calendar, which bears traces of its origin as an observational lunar one.
In particular, the kalends and ides seem to have derived from the first sighting of the crescent moon, the first-quarter moon, the full moon respectively. The system ran well short of the solar year, it needed constant intercalation to keep religious festivals and other activities in their proper seasons. For superstitious reasons, such intercalation occurred within the month of February after it was no longer considered the last month. After the establishment of the Roman Republic, years began to be dated by consulships and control over intercalation was granted to the pontifices, who abused their power by lengthening years controlled by their political allies and shortening the years in their rivals' terms of office. Having won his war with Pompey, Caesar used his position as Rome's chief pontiff to enact a calendar reform in 46 BC, coincidentally making the year of his third consulship last for 446 days. In order to avoid interfering with Rome's religious ceremonies, the reform added all its days towards the ends of months and did not adjust any nones or ides in months which came to have 31 days.
The Julian calendar was supposed to have a single leap day on 24 February every fourth year but following Caesar's assassination the priests figured this using inclusive counting and mistakenly added the bissextile day every three years. In order to bring the calendar back to its proper place, Augustus was obliged to suspend intercalation for one or two decades; the revised calendar remaining longer than the solar year, the date of Easter shifted far enough away from the vernal equinox that Pope Gregory XIII ordered its adjustment in the 16th century. The original Roman calendar is believed to have been an observational lunar calendar whose months began from the first signs of a new crescent moon; because a lunar cycle is about 29 1⁄2 days long, such months would have varied between 29 and 30 days. Twelve such months would have fallen 11 days short of the solar year. Given the seasonal aspects of the calendar and its associated religious festivals, this was avoided through some form of intercalation or through the suspension of the calendar during winter.
Rome's 8-day week, the nundinal cycle, was shared with the Etruscans, who used it as the schedule of royal audiences. It was a part of the early calendar and was credited in Roman legend variously to Romulus and Servius Tullius; the Romans themselves described their first organized year as one with ten fixed months, each of 30 or 31 days. Such a decimal division fitted general Roman practice; the four 31-day months were called "full" and the others "hollow". Its 304 days made up 38 nundinal cycles; the system is said to have left the remaining 50-odd days of the year as an unorganized "winter", although Licinius Macer's lost history stated the earliest Roman calendar employed intercalation instead and Macrobius claims the 10-month calendar was allowed to shift until the summer and winter months were misplaced, at which time additional days belonging to no month were inserted into the calendar until it seemed things were restored to their proper place. Roman writers credited this calendar to Romulus, their legendary first king and culture hero, although this was common with other practices and traditions whose origin had been lost to them.
Some scholars doubt the existence of this calendar at all, as it is only attested in late Republican and Imperial sources and supported only by the misplaced names of the months from September to December. Rüpke finds the coincidence of the length of the supposed "Romulan" year with the length of the first ten months of the Julian calendar to be suspicious. Other traditions existed alongside this one, however. Plutarch's Parallel Lives recounts that Romulus's calendar had been solar but adhered to the general principle that the year should last for 360 days. Months were employed secondarily and haphazardly, with some counted as 20 days and others as 35 or more; the attested calendar of the Roman Republic was quite different. It followed Gre
Big History is an academic discipline which examines history from the Big Bang to the present. Big History resists specialization, searches for universal patterns or trends, it examines long time frames using a multidisciplinary approach based on combining numerous disciplines from science and the humanities, explores human existence in the context of this bigger picture. It integrates studies of the cosmos, Earth and humanity using empirical evidence to explore cause-and-effect relations, is taught at universities and primary and secondary schools using web-based interactive presentations. Historian David Christian has been credited with coining the term "Big History" while teaching one of the first such courses at Macquarie University. An all-encompassing study of humanity's relationship to cosmology and natural history has been pursued by scholars since the Renaissance, the new field, Big History, continues such work. Big History examines the past using numerous time scales, from the Big Bang to modernity, unlike conventional history courses which begin with the introduction of farming and civilization, or with the beginning of written records.
It explores common patterns. Courses do not focus on humans until one-third to halfway through, unlike conventional history courses, there is not much focus on kingdoms or civilizations or wars or national borders. If conventional history focuses on human civilization with humankind at the center, Big History focuses on the universe and shows how humankind fits within this framework and places human history in the wider context of the universe's history. Unlike conventional history, Big History tends to go through detailed historical eras such as the Renaissance or Ancient Egypt, it draws on the latest findings from biology, geology, prehistory, anthropology, evolutionary biology, psychology, geography, ancient history, economics, natural history, population and environmental studies as well as standard history. One teacher explained: We're taking the best evidence from physics and the best evidence from chemistry and biology, we're weaving it together into a story... They're not going to learn how to balance equations, but they're going to learn how the chemical elements came out of the death of stars, that's interesting.
Big History arose from a desire to go beyond the specialized and self-contained fields that emerged in the 20th century. It tries to grasp history as a whole, looking for common themes across multiple time scales in history. Conventional history begins with the invention of writing, is limited to past events relating directly to the human race. Big Historians point out that this limits study to the past 5,000 years and neglects the much longer time when humans existed on Earth. Henry Kannberg sees Big History as being a product of the Information Age, a stage in history itself following speech and printing. Big History covers the formation of the universe and galaxies, includes the beginning of life as well as the period of several hundred thousand years when humans were hunter-gatherers, it sees the transition to civilization as a gradual one, with many causes and effects, rather than an abrupt transformation from uncivilized static cavemen to dynamic civilized farmers. An account in The Boston Globe describes what it polemically asserts to be the conventional "history" view: Early humans were slump-shouldered, slope-browed, hairy brutes.
They ate scorched meat. Sometimes they carried spears. Once in a while they scratched pictures of antelopes on the walls of their caves. That's. History didn't start with the first humans - they were cavemen! The Stone Age wasn't history. History started with agriculture, nation-states, written documents. History began in Mesopotamia's Fertile Crescent, somewhere around 4000 BC, it began when we overcame our savage legacy, culture surpassed biology. Big History, in contrast to conventional history, has more of an interdisciplinary basis. Advocates sometimes view conventional history as "microhistory" or "shallow history", note that three-quarters of historians specialize in understanding the last 250 years while ignoring the "long march of human existence." However, one historian disputed that the discipline of history has overlooked the big view, described the "grand narrative" of Big History as a "cliché that gets thrown around a lot." One account suggested that conventional history had the "sense of grinding the nuts into an finer powder."
It emphasizes long-term processes rather than history-making individuals or events. Historian Dipesh Chakrabarty of the University of Chicago suggested that Big History was less politicized than contemporary history because it enables people to "take a step back." It uses more kinds of evidence than the standard historical written records, such as fossils, household items, structures, ecological changes and genetic variations. Critics of Big History, including sociologist Frank Furedi, have deemed the discipline an "anti-humanist turn of history." The Big History narrative has been challenged for failing to engage with the methodology of the conventional history discipline. According to historian and educator Sam Wineburg of Stanford University, Big History eschews the interpretation of texts in favor of a purely scientific approach, thus becoming "less history and more of a kind of evolutionary biology o
Mesoamerican calendars are the calendrical systems devised and used by the pre-Columbian cultures of Mesoamerica. Besides keeping time, Mesoamerican calendars were used in religious observances and social rituals, such as for divination; the existence of Mesoamerican calendars is known as early as ca. 500 BCE, with the essentials appearing defined and functional. These calendars are still used today in the Guatemalan highlands, Veracruz and Chiapas, Mexico. Among the various calendar systems in use, two were central and widespread across Mesoamerica. Common to all recorded Mesoamerican cultures, the most important, was the 260-day calendar, a ritual calendar with no confirmed correlation to astronomical or agricultural cycles; the earliest Mesoamerican calendar to be developed, it was known by a variety of local terms, its named components and the glyphs used to depict them were culture-specific. However, it is clear that this calendar functioned in the same way across cultures, down through the chronological periods it was maintained.
The second of the major calendars was one representing a 365-day period approximating the tropical year, known sometimes as the "vague year". Because it was an approximation, over time the seasons and the true tropical year "wandered" with respect to this calendar, owing to the accumulation of the differences in length. There is little hard evidence to suggest that the ancient Mesoamericans used any intercalary days to bring their calendar back into alignment. However, there is evidence to show Mesoamericans were aware of this gradual shifting, which they accounted for in other ways without amending the calendar itself; these two 260- and 365-day calendars could be synchronised to generate the Calendar Round, a period of 18980 days or 52 years. The completion and observance of this Calendar Round sequence was of ritual significance to a number of Mesoamerican cultures. A third major calendar form known as the Long Count is found in the inscriptions of several Mesoamerican cultures, most famously those of the Maya civilization who developed it to its fullest extent during the Classic period.
The Long Count provided the ability to uniquely identify days over a much longer period of time, by combining a sequence of day-counts or cycles of increasing length, calculated or set from a particular date in the mythical past. Most five such higher-order cycles in a modified vigesimal count were used; the use of Mesoamerican calendrics is one of the cultural traits that Paul Kirchoff used in his original formulation to define Mesoamerica as a culture area. Therefore, the use of Mesoamerican calendars is specific to Mesoamerica and is not found outside its boundaries. In the 260-day cycle 20 day names pairs with 13 day numbers; this cycle was used for divination purposes to foretell unlucky days. The date of birth was used to give names to both humans and gods in many Mesoamerican cultures; each day sign was presided over by a god and many had associations with specific natural phenomena. The exact origin of the 260-day count is not known. One theory is that the calendar came from mathematical operations based on the numbers thirteen and twenty, which were important numbers to the Maya.
The numbers multiplied together equal 260. Another theory is; this is close to the average number of days between the first missed menstrual period and birth, unlike Naegele's rule, 40 weeks between the last menstrual period and birth. It is postulated that midwives developed the calendar to predict babies' expected birth dates. A third theory comes from understanding of astronomy and paleontology; the mesoamerican calendar originated with the Olmecs, a settlement existed at Izapa, in southeast Chiapas Mexico, before 1200 BCE. There, at a latitude of about 15° N, the Sun passes through zenith twice a year, there are 260 days between zenithal passages, gnomons, were found at this and other sites; the sacred almanac may well have been set in motion on 1359 BCE, in Izapa. In the post-classic Aztec calendar the periods of 13 days called in Spanish a trecena were important; the days of a trecena were numbered from 1 to 13. There were some exceptions, such as in the Tlapanec area where they were counted from 2 to 14.
The first day of the trecena, the god, its patron, ruled the following thirteen days. If the first day of a trecena was auspicious so were the next twelve days; this 365-day calendar corresponded was divided into 18'months' of 20 days each, plus 5'nameless' days at the end of the year. The 365 day year had no leap year; the years were given their name in much the same way as the days of the 260-day calendar, 20 names were paired with 13 numbers giving 52 different possibilities for year names In the post-classic Aztec calendar the 20 days called veintenas in Spanish and meztli, meaning moon in Nahuatl, were important. The five unlucky days were called nemontemi in Mexico. Most believe them to have come at the end of each year, but since we do not know when the year started, we cannot know for sure. We do know though, that in the Maya-area these five days were always the last days of the year; the nemontemi were seen as'the useless days' or the days that were dedicated to no