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, 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", which lowers New Kingdom dates by as much as 500 years. Scholarly consensus on the general outline of the conventional chronology current in Egyptology has not fluctuated much over the last 100 years.
For the Old Kingdom, consensus fluctuates by as much as a few centuries, but for the Middle and New Kingdoms, it has been stable to within a few decades. This is illustrated by comparing the chronology as given by two Egyptologists, the first writing in 1906, the second in 2000; the disparities between the two sets of dates result from additional discoveries and refined understanding of the still incomplete source evidence. For example, Breasted adds a ruler in the Twentieth dynasty that further research showed did not exist. Following Manetho, Breasted believed all the dynasties were sequential, whereas it is now known that several existed at the same time; these revisions have resulted in a lowering of the conventional chronology by up to 400 years at the beginning of Dynasty I. Forming the backbone of Egyptian chronology are the regnal years as recorded in Ancient Egyptian king lists. Surviving king lists are either comprehensive but have significant gaps in their text, or are textually complete but fail to provide a complete list of rulers for a short period of Egyptian history.
The situation is further complicated by occasional conflicting information on the same regnal period from different versions of the same text. Regnal periods have to be pieced together from inscriptions, which will give a date in the form of the regnal year of the ruling pharaoh, yet this only provides a minimum length of that reign and may or may not include any coregencies with a predecessor or successor. In addition, some Egyptian dynasties overlapped, with different pharaohs ruling in different regions at the same time, rather than serially. Not knowing whether monarchies were simultaneous or sequential results in differing chronological interpretations. Where the total number of regnal years for a given ruler is not known, Egyptologists have identified two indicators to deduce that total number: for the Old Kingdom, the number of cattle censuses. A number of Old Kingdom inscriptions allude to a periodic census of cattle, which experts at first believed took place every second year. However, further research has shown that these censuses were sometimes taken in consecutive years, or after two or more years had passed.
The Sed festival was celebrated on the thirtieth anniversary of a pharaoh's ascension, thus rulers who recorded celebrating one could be assumed to have ruled at least 30 years. However, once again, this may not have been standard practice in all cases. In the early days of Egyptology, the compilation of regnal periods was hampered by a profound biblical bias on the part of Egyptologists; this was most pervasive before the mid 19th century, when Manetho's figures were recognized as conflicting with biblical chronology, based on Old Testament references to Egypt. In the 20th century, such biblical bias has been confined to alternative chronologies outside the scholarly mainstream. A useful way to work around these gaps in knowledge is to find chronological synchronisms, which can lead to a precise date. Over the past decades, a number of these have been found, although they are of varying degrees of usefulness and reliability. Seriation, i.e. archeological sequences. This does not fix a person or event to a specific year, but establishing a sequence of events can provide indirect evidence to provide or support a precise date.
For example, some inscribed stone vessels of the rulers of the first two dynasties were collected and deposited in storage galleries beneath and sealed off when the Step Pyramid of Djoser, a Pharaoh of the Third Dynasty, was built. Another example are blocks from the Old Kingdom bearing the names of several kings, which were reused in the construction of Middle Kingdom pyramid-temples at Lisht in the structures of Amenemhat I; the third pylon at Karnak, built by Amenhotep III contained as "fill" material from the kiosk of Sesostris I, along with various stelae of the Second Intermediate Period and the Eighteenth Dynasty of the New Kingdom. Synchronisms with other chronologies, the most important of these being with the Assyrian and Babylonian chronologies, but synchronisms with the Hittites, ancient Palestine, in the final period with ancient Greece, are used; the earliest such synchronism is in the 18th century
Obsidian is a occurring volcanic glass formed as an extrusive igneous rock. Obsidian is produced when felsic lava extruded from a volcano cools with minimal crystal growth, it is found within the margins of rhyolitic lava flows known as obsidian flows, where the chemical composition causes a high viscosity which, upon rapid cooling, forms a natural glass from the lava. The inhibition of atomic diffusion through this viscous lava explains the lack of crystal growth. Obsidian is hard and amorphous. In the past it was used to manufacture cutting and piercing tools and it has been used experimentally as surgical scalpel blades.... among the various forms of glass we may reckon Obsidian glass, a substance similar to the stone found by Obsidius in Ethiopia. The translation into English of Natural History written by Pliny the Elder of Rome shows a few sentences on the subject of a volcanic glass called obsidian, discovered in Ethiopia by Obsidius, a Roman explorer. Obsidian is the rock formed as a result of cooled lava, the parent material.
Extrusive formation of obsidian may occur when felsic lava cools at the edges of a felsic lava flow or volcanic dome or when lava cools during sudden contact with water or air. Intrusive formation of obsidian may occur. Tektites were once thought by many to be obsidian produced by lunar volcanic eruptions, though few scientists now adhere to this hypothesis. Obsidian is mineral-like, but not a true mineral, it is sometimes classified as a mineraloid. Though obsidian is dark in color, similar to mafic rocks such as basalt, obsidian's composition is felsic. Obsidian consists of SiO2 70% or more. Crystalline rocks with obsidian's composition include rhyolite; because obsidian is metastable at the Earth's surface, no obsidian has been found, older than Cretaceous age. This breakdown of obsidian is accelerated by the presence of water. Having a low water content when newly formed less than 1% water by weight, obsidian becomes progressively hydrated when exposed to groundwater, forming perlite. Pure obsidian is dark in appearance, though the color varies depending on the presence of impurities.
Iron and other transition elements may give the obsidian a dark brown to black color. Few samples are nearly colorless. In some stones, the inclusion of small, radially clustered crystals spherulites of the mineral cristobalite in the black glass produce a blotchy or snowflake pattern. Obsidian may contain patterns of gas bubbles remaining from the lava flow, aligned along layers created as the molten rock was flowing before being cooled; these bubbles can produce interesting effects such as a golden sheen. An iridescent, rainbow-like sheen is caused by inclusions of magnetite nanoparticles. Obsidian can be found in locations, it can be found in Argentina, Azerbaijan, Canada, Georgia, Greece, El Salvador, Iceland, Japan, Mexico, New Zealand, Papua New Guinea, Scotland and the United States. Obsidian flows which may be hiked on are found within the calderas of Newberry Volcano and Medicine Lake Volcano in the Cascade Range of western North America, at Inyo Craters east of the Sierra Nevada in California.
Yellowstone National Park has a mountainside containing obsidian located between Mammoth Hot Springs and the Norris Geyser Basin, deposits can be found in many other western U. S. states including Arizona, New Mexico, Utah, Washington and Idaho. Obsidian can be found in the eastern U. S. states of Virginia, as well as North Carolina. There are only four major deposit areas in the central Mediterranean: Lipari, Pantelleria and Monte Arci. Ancient sources in the Aegean were Gyali. Acıgöl town and the Göllü Dağ volcano were the most important sources in central Anatolia, one of the more important source areas in the prehistoric Near East; the first known archaeological evidence of usage was in Kariandusi and other sites of the Acheulian age dated 700,000 BC, although the number of objects found at these sites were low relative to the Neolithic. Use of obsidian in pottery of the Neolithic in the area around Lipari was found to be less at a distance representing two weeks journeying. Anatolian sources of obsidian are known to have been the material used in the Levant and modern-day Iraqi Kurdistan from a time beginning sometime about 12,500 BC.
The first attested civilized use is dated to the late fifth millennium BC, known from excavations at Tell Brak. Obsidian was valued in Stone Age cultures because, like flint, it could be fractured to produce sharp blades or arrowheads. Like all glass and some other types of occurring rocks, obsidian breaks with a characteristic conchoidal fracture, it was polished to create early mirrors. Modern archaeologists have developed a relative dating system, obsidian hydration dating, to calculate the age of obsidian artifacts. In the Ubaid in the 5th millennium BC, blades were manufactured from obsidian extracted from outcrops located in modern-day Turkey. Ancient Egyptians used obsidian imported from the eastern Mediterranean and southern Red Sea regions. Obsidian was used in ritual circumcisions because of its deftness and sharpness. In the eastern Mediterranean
Revised Julian calendar
The Revised Julian calendar known as the Milanković calendar, or, less formally, new calendar, is a calendar proposed by the Serbian scientist Milutin Milanković in 1923, which discontinued the 340 years of divergence between the naming of dates sanctioned by those Eastern Orthodox churches adopting it and the Gregorian calendar that has come to predominate worldwide. This calendar was intended to replace the ecclesiastical calendar based on the Julian calendar hitherto in use by all of the Eastern Orthodox Church; the Revised Julian calendar temporarily aligns its dates with the Gregorian calendar proclaimed in 1582 by Pope Gregory XIII for adoption by the Christian world. The calendar has been adopted by the Orthodox churches of Constantinople, Alexandria, Bulgaria, Greece and Romania; the Revised Julian calendar has the same months and month lengths as the Julian calendar, but, in the Revised Julian calendar, years evenly divisible by 100 are not leap years, except that years with remainders of 200 or 600 when divided by 900 remain leap years.
A committee composed of members of the Greek government and Greek Orthodox Church was set up to look into the question of calendar reform. It reported in January 1923, it recommended a switch to the "political calendar" devised in 1785 and advocated by Maksim Trpković. Trpković advocated this calendar in preference to the Gregorian because of its greater accuracy and because the vernal equinox would fall on 21 March, the date allocated to it by the church. In the Gregorian, it falls on 20 March; as in the Gregorian, end-century years are not leap years, but years that give remainder 0 or 400 on division by 900 are leap years. The changeover went into effect on 17 February/1 March. After the promulgation of the royal decree, the Ecumenical Patriarch, Patriarch Meletius IV of Constantinople, issued an encyclical on 3 February recommending the calendar's adoption by Orthodox churches; the matter came up for discussion at a "Pan-Orthodox" Congress of Constantinople, which deliberated in May and June.
Subsequently it was adopted by several of the autocephalous Orthodox churches. The synod was chaired by the controversial patriarch and representatives were present from the churches of Cyprus, Greece and Serbia. There were no representatives of the other members of the original Orthodox Pentarchy or from the largest Orthodox church, the Russian Orthodox Church. Discussion was lengthy because although Serbia supported the political calendar, Milanković pressed for the adoption of his own version, in which the centennial leap years would be those giving remainder 200 or 600 when divided by 900 and the equinox would fall on 20 March. Under the official proposal the equinox would sometimes fall on 22 March; this might make Easter fall outside its canonical limits due to the requirement that the Easter full moon follow the equinox. His scheme maximised the time during which the political calendar and the Gregorian would run in tandem. Milanković's arguments won the day. In its decision the conference noted that "the difference between the length of the political year of the new calendar and the Gregorian is so small that only after 877 years it is observed difference of dates."
The same decision provided that the coming 1 October should be called 14 October, thus dropping thirteen days. It adopted the leap year rule of Milanković; the political calendar was preferred over the Gregorian because its mean year was within two seconds of the current length of the mean tropical year. The present vernal equinox year, however, is about 12 seconds longer, in terms of mean solar days; the synod proposed the adoption of an astronomical rule for Easter: Easter was to be the Sunday after the midnight-to-midnight day at the meridian of the Church of the Holy Sepulchre in Jerusalem during which the first full moon after the vernal equinox occurs. Although the instant of the full moon must occur after the instant of the vernal equinox, it may occur on the same day. If the full moon occurs on a Sunday, Easter is the following Sunday. Churches that adopted this calendar did so on varying dates. However, all Eastern Orthodox churches continue to use the Julian calendar to determine the date of Easter.
The following are Gregorian minus Revised Julian date differences, calculated for the beginning of March in each century year, where differences arise or disappear, until 10000 AD. These are exact arithmetic calculations. A negative difference means that the proleptic Revised Julian calendar was behind the proleptic Gregorian calendar; the Revised Julian calendar is the same as the Gregorian calendar from 1 March 1600 to 28 February 2800. A positive difference means that the Revised Julian calendar will be ahead of the Gregorian calendar, which will first occur on 1 March 2800: In 900 Julian years there are 900⁄4 = 225 leap days; the Revised Julian leap rule omits seven of nine century leap years, leaving 225−7 = 218 leap days per 900-year cycle. Thus the calendar mean year is 365 + 218⁄900 days, but this is a double-cycle that reduces to 365 + 109⁄450 = 365.242 days, or 365 days 5 hours 48 minutes 48 seconds, 24 seconds shorter than the Gregorian mean year of 365.2425 days, so in the long term on average the Revised Julian calendar pulls ahead of the Gregorian calen
Mesoamerican Long Count calendar
The Mesoamerican Long Count calendar is a non-repeating and base-18 calendar used by several pre-Columbian Mesoamerican cultures, most notably the Maya. For this reason, it is known as the Maya Long Count calendar. Using a modified vigesimal tally, the Long Count calendar identifies a day by counting the number of days passed since a mythical creation date that corresponds to August 11, 3114 BCE in the Proleptic Gregorian calendar; the Long Count calendar was used on monuments. The two most used calendars in pre-Columbian Mesoamerica, were the 260-day Tzolkʼin and the 365 day Haabʼ; the equivalent Aztec calendars are known in Nahuatl as Xiuhpohualli. The combination of a Haabʼ and a Tzolkʼin date identifies a day in a combination which does not occur again for 18,980 days, a period known as the Calendar Round. To identify days over periods longer than this, Mesoamericans used the Long Count calendar; the Long Count calendar identifies a date by counting the number of days from a starting date, calculated to be August 11, 3114 BCE in the proleptic Gregorian calendar or September 6 in the Julian calendar.
There has been much debate over the precise correlation between the Western calendars and the Long Count calendars. The August 11 date is based on the GMT correlation; the completion of 13 bʼakʼtuns marks the Creation of the world of human beings according to the Maya. On this day, Raised-up-Sky-Lord caused three stones to be set by associated gods at Lying-Down-Sky, First-Three-Stone-Place; because the sky still lay on the primordial sea, it was black. The setting of the three stones centered the cosmos which allowed the sky to be raised, revealing the sun. Rather than using a base-10 scheme, like Western numbering, the Long Count days were tallied in a modified base-20 scheme. In a pure base-20 scheme, 0.0.0.1.5 is equal to 25 and 0.0.0.2.0 is equal to 40. The Long Count is not pure base-20, since the second digit from the right rolls over to zero when it reaches 18, thus 0.0.1.0.0 does not represent 400 days, but rather only 360 days and 0.0.0.17.19 represents 359 days. The name bʼakʼtun was invented by modern scholars.
The numbered Long Count was no longer in use by the time the Spanish arrived in the Yucatán Peninsula, although unnumbered kʼatuns and tuns were still in use. Instead the Maya were using an abbreviated Short Count. Long Count dates are written with Mesoamerican numerals. A dot represents 1 while a bar equals 5; the shell glyph was used to represent the zero concept. The Long Count calendar required the use of zero as a place-holder and presents one of the earliest uses of the zero concept in history. On Maya monuments, the Long Count syntax is more complex; the date sequence is given once, at the beginning of the inscription and opens with the so-called ISIG which reads tzik-a habʼ. Next come the 5 digits of the Long Count, followed by supplementary series; the supplementary series is optional and contains lunar data, for example, the age of the moon on the day and the calculated length of current lunation. The text continues with whatever activity occurred on that date. A drawing of a full Maya Long Count inscription is shown below.
The earliest contemporaneous Long Count inscription yet discovered is on Stela 2 at Chiapa de Corzo, Mexico, showing a date of 36 BCE, although Stela 2 from Takalik Abaj, Guatemala might be earlier. Takalik Abaj Stela 2's battered Long Count inscription shows 7 bak'tuns, followed by k'atuns with a tentative 6 coefficient, but that could be 11 or 16, giving the range of possible dates to fall between 236 and 19 BCE. Although Takalik Abaj Stela 2 remains controversial, this table includes it, as well as six other artifacts with the eight oldest Long Count inscriptions according to Dartmouth professor Vincent H. Malmström. Interpretations of inscriptions on some artifacts differ. Of the six sites, three are on the western edge of the Maya homeland and three are several hundred kilometers further west, leading some researchers to believe that the Long Count calendar predates the Maya. La Mojarra Stela 1, the Tuxtla Statuette, Tres Zapotes Stela C and Chiapa Stela 2 are all inscribed in an Epi-Olmec, not Maya, style.
El Baúl Stela 2, on the other hand, was created in the Izapan style. The first unequivocally Maya artifact is Stela 29 from Tikal, with the Long Count date of 292 CE, more than 300 years after Stela 2 from Chiapa de Corzo. More with the discovery in Guatemala of the San Bartolo stone block text, it has been argued that this text celebrates an upcoming time period ending celebration; this time period may have been projected to end sometime between 22.214.171.124.0 and 126.96.36.199.0 — 295 and 256 BCE, respectively. Besides this being the earliest Maya hieroglyphic text so far uncovered, it would arguably be the earliest glyphic evidence to date of Long Count notation in Mesoamerica; the Maya and Western calendars are correlated by using a Julian day number of the starting date of the current creation — 188.8.131.52.0, 4 Ajaw, 8 Kumkʼu. This is referred to; the accepted correlation constant is the Modified Thompson 2, "Goodman–Martinez–Thomps
Astronomy is a natural science that studies celestial objects and phenomena. It applies mathematics and chemistry in an effort to explain the origin of those objects and phenomena and their evolution. Objects of interest include planets, stars, nebulae and comets. More all phenomena that originate outside Earth's atmosphere are within the purview of astronomy. A related but distinct subject is physical cosmology, the study of the Universe as a whole. Astronomy is one of the oldest of the natural sciences; the early civilizations in recorded history, such as the Babylonians, Indians, Nubians, Chinese and many ancient indigenous peoples of the Americas, performed methodical observations of the night sky. Astronomy has included disciplines as diverse as astrometry, celestial navigation, observational astronomy, the making of calendars, but professional astronomy is now considered to be synonymous with astrophysics. Professional astronomy is split into theoretical branches. Observational astronomy is focused on acquiring data from observations of astronomical objects, analyzed using basic principles of physics.
Theoretical astronomy is oriented toward the development of computer or analytical models to describe astronomical objects and phenomena. The two fields complement each other, with theoretical astronomy seeking to explain observational results and observations being used to confirm theoretical results. Astronomy is one of the few sciences in which amateurs still play an active role in the discovery and observation of transient events. Amateur astronomers have made and contributed to many important astronomical discoveries, such as finding new comets. Astronomy means "law of the stars". Astronomy should not be confused with astrology, the belief system which claims that human affairs are correlated with the positions of celestial objects. Although the two fields share a common origin, they are now distinct. Both of the terms "astronomy" and "astrophysics" may be used to refer to the same subject. Based on strict dictionary definitions, "astronomy" refers to "the study of objects and matter outside the Earth's atmosphere and of their physical and chemical properties," while "astrophysics" refers to the branch of astronomy dealing with "the behavior, physical properties, dynamic processes of celestial objects and phenomena."
In some cases, as in the introduction of the introductory textbook The Physical Universe by Frank Shu, "astronomy" may be used to describe the qualitative study of the subject, whereas "astrophysics" is used to describe the physics-oriented version of the subject. However, since most modern astronomical research deals with subjects related to physics, modern astronomy could be called astrophysics; some fields, such as astrometry, are purely astronomy rather than astrophysics. Various departments in which scientists carry out research on this subject may use "astronomy" and "astrophysics" depending on whether the department is affiliated with a physics department, many professional astronomers have physics rather than astronomy degrees; some titles of the leading scientific journals in this field include The Astronomical Journal, The Astrophysical Journal, Astronomy and Astrophysics. In early historic times, astronomy only consisted of the observation and predictions of the motions of objects visible to the naked eye.
In some locations, early cultures assembled massive artifacts that had some astronomical purpose. In addition to their ceremonial uses, these observatories could be employed to determine the seasons, an important factor in knowing when to plant crops and in understanding the length of the year. Before tools such as the telescope were invented, early study of the stars was conducted using the naked eye; as civilizations developed, most notably in Mesopotamia, Persia, China and Central America, astronomical observatories were assembled and ideas on the nature of the Universe began to develop. Most early astronomy consisted of mapping the positions of the stars and planets, a science now referred to as astrometry. From these observations, early ideas about the motions of the planets were formed, the nature of the Sun and the Earth in the Universe were explored philosophically; the Earth was believed to be the center of the Universe with the Sun, the Moon and the stars rotating around it. This is known as the geocentric model of the Ptolemaic system, named after Ptolemy.
A important early development was the beginning of mathematical and scientific astronomy, which began among the Babylonians, who laid the foundations for the astronomical traditions that developed in many other civilizations. The Babylonians discovered. Following the Babylonians, significant advances in astronomy were made in ancient Greece and the Hellenistic world. Greek astronomy is characterized from the start by seeking a rational, physical explanation for celestial phenomena. In the 3rd century BC, Aristarchus of Samos estimated the size and distance of the Moon and Sun, he proposed a model of the Solar System where the Earth and planets rotated around the Sun, now called the heliocentric model. In the 2nd century BC, Hipparchus discovered precession, calculated the size and distance of the Moon and inven
Acanthochronology is the interdisciplinary study of cactus spines or Euphorbia thorns grown in time ordered sequence. Physical, morphological or chemical characteristics and information about the relative order or absolute age of the spines or thorns is used to study past climate or plant physiology. For example, columnar cactus spines grow from the apex of the plant. After several weeks the spines have been moved to the side of the stem; the old spines remain in place for decades as new spines are created at the continually growing apex. The result is that along each external "rib" of the cactus is a series of spines arranged in the order they grew in – the oldest spines are at the bottom and the youngest spines are at the top; these spines can be dated using bomb-spike Carbon-14 and isotopes of carbon and oxygen may be used to infer past climate, plant stem growth or plant physiology. Alternatively, the width of small transverse bands in the spine may be used to infer daily information about cloud cover or plant productivity, although this remains to be tested.
It has been shown that regular waxy banding on the sides of a Costa Rican cactus indicate annual growth and can be used as temporal chronometers. This sub-discipline of paleoclimatology and ecophysiology is new. Acanthochronology is related to dendrochronology, dendroclimatology and isotope geochemistry and borrows many of the methods and techniques from these sub-disciplines of the Earth Sciences, it draws from the field of ecophysiology, a branch of Biology, to ascribe spine or thorn characteristics to particular environmental or physiological variables. The first peer-reviewed article to present and explain an isotope spine series was from a saguaro cactus in Tucson, Arizona; this and other work shows that radiocarbon and isotope time-series derived from spines can be used for demographic or palaeoclimate studies. Doménech-Carbó, Antonio. "Dating: An analytical task". Chemtexts. 1. Doi:10.1007/s40828-014-0005-6
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