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82-502: Relative to the stars, the Sun appears to drift eastward about one degree per day along a path called the ecliptic because there are 360 degrees in any complete revolution (circle), which takes about 365 days in the case of one revolution of the Earth around the Sun. Any given "distant" star in the belt of the ecliptic will be visible at night for only half of the year, when it will always remain below

164-473: A lunisolar calendar operating in accordance with the Egyptian civil calendar according to a 25 year cycle. The calendar seems to show its month beginning with the first visibility of the waxing crescent moon, but Parker displayed an error in the cycle of about a day in 500 years, using it to show the cycle was developed to correspond with the new moon around 357   BC. This date places it prior to

246-658: A "temple month" — were individually named and celebrated as stages in the life of the moon god, variously Thoth in the Middle Kingdom or Khonsu in the Ptolemaic era : "He ... is conceived ... on Psḏntyw ; he is born on Ꜣbd ; he grows old after Smdt ". The civil calendar was established at some early date in or before the Old Kingdom , with probable evidence of its use early in the reign of Shepseskaf ( c.  2510   BC, Dynasty IV ) and certain attestation during

328-459: A 30 day intercalary month every two to three years to accommodate the lunar year 's loss of about 11 days a year relative to the solar year and to maintain the placement of the heliacal rising of Sirius within its twelfth month . No evidence for such a month, however, exists in the present historical record. A second lunar calendar is attested by a demotic astronomical papyrus dating to sometime after 144 AD which outlines

410-645: A difference which causes Rolf Krauss to propose dating much of Egyptian history decades later than the present consensus. Following Alexander the Great 's conquest of the Persian Empire , the Macedonian Ptolemaic Dynasty came to power in Egypt , continuing to use its native calendars with Hellenized names. In 238 BC, Ptolemy III 's Canopus Decree ordered that every 4th year should incorporate

492-456: A further margin of error of about two decades. Although it is certain the Egyptian day began in the morning, another four years are shifted depending on whether the precise start occurred at the first light of dawn or at sunrise. It has been noted that there is no recognition in surviving records that Sirius's minor irregularities sometimes produce a triëteris or penteteris (three- or five-year periods of agreement with an Egyptian date) rather than

574-503: A method of telling the time at night based on the heliacal risings of 36 decan stars , one for each 10° segment of the 360° circle of the zodiac and corresponding to the ten-day "weeks" of their civil calendar. To the Māori of New Zealand , the Pleiades are called Matariki , and their heliacal rising signifies the beginning of the new year (around June). The Mapuche of South America called

656-490: A particular equinox, that is, the equinox of a particular date, known as an epoch ; the coordinates are referred to the direction of the equinox at that date. For instance, the Astronomical Almanac lists the heliocentric position of Mars at 0h Terrestrial Time , 4 January 2010 as: longitude 118°09′15.8″, latitude +1°43′16.7″, true heliocentric distance 1.6302454 AU, mean equinox and ecliptic of date. This specifies

738-556: A period of about 26,000 years, a process known as lunisolar precession , as it is due mostly to the gravitational effect of the Moon and Sun on Earth's equatorial bulge . Likewise, the ecliptic itself is not fixed. The gravitational perturbations of the other bodies of the Solar System cause a much smaller motion of the plane of Earth's orbit, and hence of the ecliptic, known as planetary precession . The combined action of these two motions

820-429: A periodic component to the position of the equinoxes; the positions of the celestial equator and (March) equinox with fully updated precession and nutation are called the true equator and equinox ; the positions without nutation are the mean equator and equinox . Obliquity of the ecliptic is the term used by astronomers for the inclination of Earth's equator with respect to the ecliptic, or of Earth's rotation axis to

902-403: A perpendicular to the ecliptic. It is about 23.4° and is currently decreasing 0.013 degrees (47 arcseconds) per hundred years because of planetary perturbations. The angular value of the obliquity is found by observation of the motions of Earth and other planets over many years. Astronomers produce new fundamental ephemerides as the accuracy of observation improves and as the understanding of

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984-523: A planet has a heliacal rising, there is a conjunction with the sun beforehand. Depending on the type of conjunction, there may be a syzygy , eclipse , transit , or occultation of the sun. The rising of a planet above the eastern horizon at sunset is called its acronycal rising , which for a superior planet signifies an opposition , another type of syzygy . When the Moon has an acronycal rising, it will occur near full moon and thus, two or three times

1066-472: A purely lunar calendar prior to the establishment of the solar civil calendar in which each month began on the morning when the waning crescent moon could no longer be seen. Until the closing of Egypt's polytheist temples under the Byzantines , the lunar calendar continued to be used as the liturgical year of various cults. The lunar calendar divided the month into four weeks, reflecting each quarter of

1148-551: A sixth day in its intercalary month, honoring him and his wife as gods equivalent to the children of Nut . The reform was resisted by the Egyptian priests and people and was abandoned. Egyptian scholars were involved with the establishment of Julius Caesar 's reform of the Roman calendar , although the Roman priests initially misapplied its formula and—by counting inclusively—added leap days every three years instead of every four. The mistake

1230-793: A time Egyptian culture was borrowing various objects and cultural features from the Fertile Crescent , leaving open the possibility that the main features of the calendar were borrowed in one direction or the other as well. The civil year comprised exactly 365 days, divided into 12 months of 30 days each and an intercalary month of five days, which were celebrated as the birthdays of the gods Osiris , Horus , Set , Isis , and Nephthys . The regular months were grouped into Egypt's three seasons, which gave them their original names, and divided into three 10-day periods known as decans or decades. In later sources, these were distinguished as "first", "middle", and "last". It has been suggested that during

1312-450: A time when Wep Renpet , its New Year , corresponded with Sirius's return to the night sky. Although this calendar's lack of leap years caused the event to shift one day every four years or so, astronomical records of this displacement led to the discovery of the Sothic cycle and, later, the establishment of the more accurate Julian and Alexandrian calendars . The Egyptians also devised

1394-418: A variety of 'fixed' dates for the rise of Sirius. His use of the year 139 seems questionable, as 136 seems to have been the start of the tetraëteris and the later date chosen to flatter the birthday of Censorinus's patron. Perfect observation of Sirius's actual behavior during the cycle—including its minor shift relative to the solar year—would produce a period of 1457 years; observational difficulties produce

1476-423: A year, a noticeable lunar eclipse . Cosmic(al) can refer to rising with sunrise or setting at sunset, or the first setting at morning twilight. Risings and settings are furthermore differentiated between apparent (the above discussed) and actual or true risings or settings. The use of the terms cosmical and acronycal is not consistent. The following table gives an overview of the different application of

1558-474: Is also used occasionally; the x -axis is directed toward the March equinox, the y -axis 90° to the east, and the z -axis toward the north ecliptic pole; the astronomical unit is the unit of measure. Symbols for ecliptic coordinates are somewhat standardized; see the table. Ecliptic coordinates are convenient for specifying positions of Solar System objects, as most of the planets' orbits have small inclinations to

1640-454: Is called general precession , and changes the position of the equinoxes by about 50 arc seconds (about 0.014°) per year. Once again, this is a simplification. Periodic motions of the Moon and apparent periodic motions of the Sun (actually of Earth in its orbit) cause short-term small-amplitude periodic oscillations of Earth's axis, and hence the celestial equator, known as nutation . This adds

1722-486: Is divided into 12 signs of 30° longitude, each of which approximates the Sun's motion in one month. In ancient times, the signs corresponded roughly to 12 of the constellations that straddle the ecliptic. These signs are sometimes still used in modern terminology. The " First Point of Aries " was named when the March equinox Sun was actually in the constellation Aries ; it has since moved into Pisces because of precession of

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1804-483: Is near an ascending or descending node at the same time it is at conjunction ( new ) or opposition ( full ). The ecliptic is so named because the ancients noted that eclipses only occur when the Moon is crossing it. The exact instants of equinoxes and solstices are the times when the apparent ecliptic longitude (including the effects of aberration and nutation ) of the Sun is 0°, 90°, 180°, and 270°. Because of perturbations of Earth's orbit and anomalies of

1886-401: Is the basis of the ecliptic coordinate system . The ecliptic is the apparent path of the Sun throughout the course of a year . Because Earth takes one year to orbit the Sun, the apparent position of the Sun takes one year to make a complete circuit of the ecliptic. With slightly more than 365 days in one year, the Sun moves a little less than 1° eastward every day. This small difference in

1968-543: The New Kingdom months, which in turn gave rise to the Hellenized names that were used for chronology by Ptolemy in his Almagest and by others. Copernicus constructed his tables for the motion of the planets based on the Egyptian year because of its mathematical regularity. A convention of modern Egyptologists is to number the months consecutively using Roman numerals . A persistent problem of Egyptology has been that

2050-571: The Nineteenth Dynasty and the Twentieth Dynasty the last two days of each decan were usually treated as a kind of weekend for the royal craftsmen, with royal artisans free from work. Dates were typically expressed in a YMD format , with a pharaoh 's regnal year followed by the month followed by the day of the month. For example, the New Year occurred on I Akhet 1. The importance of

2132-453: The Ptolemaic period and within the native Egyptian Dynasty XXX . Egypt's 1st Persian occupation , however, seems likely to have been its inspiration. This lunisolar calendar's calculations apparently continued to be used without correction into the Roman period , even when they no longer precisely matched the observable lunar phases. The days of the lunar month — known to the Egyptians as

2214-405: The dynamics increases, and from these ephemerides various astronomical values, including the obliquity, are derived. Until 1983 the obliquity for any date was calculated from work of Newcomb , who analyzed positions of the planets until about 1895: ε = 23°27′08.26″ − 46.845″ T − 0.0059″ T + 0.00181″ T where ε is the obliquity and T is tropical centuries from B1900.0 to

2296-452: The ecliptic , the small difference between the solar and sidereal years due to axial precession will cause their heliacal rising to recur about one sidereal year (about 365.2564 days) later, though this depends on its proper motion . For stars far from the ecliptic, the period is somewhat different and varies slowly, but in any case the heliacal rising will move all the way through the zodiac in about 26,000 years due to precession of

2378-400: The lunar phases . Because the exact time of morning considered to begin the Egyptian day remains uncertain and there is no evidence that any method other than observation was used to determine the beginnings of the lunar months prior to the 4th century BC, there is no sure way to reconstruct exact dates in the lunar calendar from its known dates. The difference between beginning the day at

2460-407: The mean equinox of 4 January 2010 0h TT as above , without the addition of nutation. Because the orbit of the Moon is inclined only about 5.145° to the ecliptic and the Sun is always very near the ecliptic, eclipses always occur on or near it. Because of the inclination of the Moon's orbit, eclipses do not occur at every conjunction and opposition of the Sun and Moon, but only when the Moon

2542-401: The solar year . Sirius itself, about 40° below the ecliptic , follows a Sothic year almost exactly matching that of the Sun, with its reappearance now occurring at the latitude of Cairo (ancient Heliopolis and Memphis ) on 19   July ( Julian ), only two or three days later than its occurrence in early antiquity. Following Censorinus and Meyer , the standard understanding

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2624-549: The Calendar of Lucky and Unlucky Days (on the back of the Teaching of Amenemope). The earliest calendars appear in the Middle Kingdom , but they do not become codified until the New Kingdom . It is unknown how staunchly these calendars were adhered to, as there are no references to decisions being made based on their horoscopes. Nevertheless, the different copies of the calendars are remarkably consistent with each other, with only 9.2% of

2706-526: The Earth–Moon center of mass , the apparent path of the Sun wobbles slightly, with a period of about one month . Because of further perturbations by the other planets of the Solar System , the Earth–Moon barycenter wobbles slightly around a mean position in a complex fashion. Because Earth's rotational axis is not perpendicular to its orbital plane , Earth's equatorial plane is not coplanar with

2788-570: The Pleiades Ngauponi which in the vicinity of the we tripantu (Mapuche new year) will disappear by the west, lafkenmapu or ngulumapu , appearing at dawn to the East, a few days before the birth of new life in nature. Heliacal rising of Ngauponi, i.e. appearance of the Pleiades by the horizon over an hour before the sun approximately 12 days before the winter solstice, announced we tripantu . When

2870-421: The Solar System orbit the Sun in nearly the same plane. This is likely due to the way in which the Solar System formed from a protoplanetary disk . Probably the closest current representation of the disk is known as the invariable plane of the Solar System . Earth's orbit, and hence, the ecliptic, is inclined a little more than 1° to the invariable plane, Jupiter's orbit is within a little more than ½° of it, and

2952-423: The Sun's position against the stars causes any particular spot on Earth's surface to catch up with (and stand directly north or south of) the Sun about four minutes later each day than it would if Earth did not orbit; a day on Earth is therefore 24 hours long rather than the approximately 23-hour 56-minute sidereal day . Again, this is a simplification, based on a hypothetical Earth that orbits at uniform speed around

3034-412: The Sun. The actual speed with which Earth orbits the Sun varies slightly during the year, so the speed with which the Sun seems to move along the ecliptic also varies. For example, the Sun is north of the celestial equator for about 185 days of each year, and south of it for about 180 days. The variation of orbital speed accounts for part of the equation of time . Because of the movement of Earth around

3116-502: The basis for the Egyptian calendar. Note that the names of the three natural seasons were incorporated into the Civil calendar year (see below), but as this calendar year is a wandering year , the seasons of this calendar slowly rotate through the natural solar year, meaning that Civil season Akhet/Inundation only occasionally coincided with the Nile inundation. The Egyptians appear to have used

3198-460: The beginning of their year, but more recent analysis has questioned whether the tablet's picture refers to Sirius at all. Similarly, based on the Palermo Stone , Alexander Scharff proposed that the Old Kingdom observed a 320-day year, but his theory has not been widely accepted. Some evidence suggests the early civil calendar had 360 days, although it might merely reflect the unusual status of

3280-537: The bright star Sirius in Ancient Egypt occurred not over a period of exactly one sidereal year but over a period called the " Sothic year " (from "Sothis", the name for the star Sirius). The Sothic year was about a minute longer than a Julian year of 365.25 days. Since the development of civilization , this has occurred at Cairo approximately on July 19 on the Julian calendar . Its returns also roughly corresponded to

3362-461: The calendar , the dates of these are not fixed. The ecliptic currently passes through the following thirteen constellations : There are twelve constellations that are not on the ecliptic, but are close enough that the Moon and planets can occasionally appear in them. The ecliptic forms the center of the zodiac , a celestial belt about 20° wide in latitude through which the Sun, Moon, and planets always appear to move. Traditionally, this region

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3444-459: The calendar is attested before Dynasty XVIII and the last date is now known to far predate early Egyptian civilization , it is typically credited to Dynasty II around the middle date. The classic understanding of the Sothic cycle relies, however, on several potentially erroneous assumptions. Following Scaliger , Censorinus's date is usually emended to 20   July but ancient authorities give

3526-445: The calendar to Egyptian religion is reflected in the use of the title "Lord of Years" ( Nb Rnpt ) for its various creator gods. Time was also considered an integral aspect of Maat , the cosmic order which opposed chaos, lies, and violence . The civil calendar was apparently established in a year when Sirius rose on its New Year ( I Akhet 1) but, because of its lack of leap years , it began to slowly cycle backwards through

3608-431: The calendrical date of the rise of Sirius have been used by Egyptologists to fix its calendar and other events dated to it, at least to the level of the four-Egyptian-year periods which share the same date for Sirius's return, known as "tetraëterides" or "quadrennia". For example, an account that Sothis rose on III Peret 1 —the 181st day of the year—should show that somewhere 720, 721, 722, or 723 years have passed since

3690-410: The celestial equator. Spherical coordinates , known as ecliptic longitude and latitude or celestial longitude and latitude, are used to specify positions of bodies on the celestial sphere with respect to the ecliptic. Longitude is measured positively eastward 0° to 360° along the ecliptic from the March equinox, the same direction in which the Sun appears to move. Latitude is measured perpendicular to

3772-718: The date in question. From 1984, the Jet Propulsion Laboratory's DE series of computer-generated ephemerides took over as the fundamental ephemeris of the Astronomical Almanac . Obliquity based on DE200, which analyzed observations from 1911 to 1979, was calculated: ε = 23°26′21.45″ − 46.815″ T − 0.0006″ T + 0.00181″ T where hereafter T is Julian centuries from J2000.0 . JPL's fundamental ephemerides have been continually updated. The Astronomical Almanac for 2010 specifies: ε = 23°26′21.406″ − 46.836769″ T − 0.0001831″ T + 0.00200340″ T − 0.576×10 ″ T − 4.34×10 ″ T These expressions for

3854-566: The day Seth was supposed to be born was considered particularly evil. The reformed Egyptian calendar continues to be used in Egypt as the Coptic calendar of the Egyptian Church and by the Egyptian populace at large, particularly the fellah , to calculate the agricultural seasons. It differs only in its era, which is dated from the ascension of the Roman emperor Diocletian . Contemporary Egyptian farmers, like their ancient predecessors, divide

3936-504: The day they were born. This could also be used to predict when or how they would die. For example, people born on the tenth day of the fourth month of Akhet were predicted to die of old age. The epagomenal days were added to the original 360 day calendar in order to synchronise the calendar with the approximate length of the solar year. Mythologically, these days allowed for the births of five children of Geb and Nut to occur and were considered to be particularly dangerous. In particular,

4018-426: The determinations of adversity or fortuitousness being due to a defined textual reason. The Calendars of Lucky and Unlucky Days seem to be based on scientific observation as well as myths. Periodicity has been established between phases of the moon as well as the brightening and dimming of the three-star system Algol as visible from earth. The calendars could also be used to predict someone's future depending on

4100-405: The ecliptic plane, but is inclined to it by an angle of about 23.4°, which is known as the obliquity of the ecliptic . If the equator is projected outward to the celestial sphere , forming the celestial equator , it crosses the ecliptic at two points known as the equinoxes . The Sun, in its apparent motion along the ecliptic, crosses the celestial equator at these points, one from south to north,

4182-412: The ecliptic, and therefore always appear relatively close to it on the sky. Because Earth's orbit, and hence the ecliptic, moves very little, it is a relatively fixed reference with respect to the stars. Because of the precessional motion of the equinox , the ecliptic coordinates of objects on the celestial sphere are continuously changing. Specifying a position in ecliptic coordinates requires specifying

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4264-436: The ecliptic, to +90° northward or −90° southward to the poles of the ecliptic, the ecliptic itself being 0° latitude. For a complete spherical position, a distance parameter is also necessary. Different distance units are used for different objects. Within the Solar System, astronomical units are used, and for objects near Earth , Earth radii or kilometers are used. A corresponding right-handed rectangular coordinate system

4346-434: The equinoxes . Egyptian calendar The ancient Egyptian calendar – a civil calendar – was a solar calendar with a 365-day year. The year consisted of three seasons of 120 days each, plus an intercalary month of five epagomenal days treated as outside of the year proper. Each season was divided into four months of 30 days. These twelve months were initially numbered within each season but came to also be known by

4428-406: The equinoxes . Because the heliacal rising depends on the observation of the object, its exact timing can be dependent on weather conditions. Heliacal phenomena and their use throughout history have made them useful points of reference in archeoastronomy . Some stars, when viewed from latitudes not at the equator , do not rise or set. These are circumpolar stars , which are either always in

4510-518: The festival beginning the next. Calendars that have survived from ancient Egypt often characterise the days as either lucky or unlucky. Of the calendars recovered, the Cairo calendar is one of the best examples. Discovered in modern-day Thebes , it dates from the Ramesside Period and acts as a guide to which days were considered lucky or unlucky. Other complete calendars include Papyrus Sallier IV, and

4592-411: The festivals which give their names to the months occur in the next month. Alan Gardiner proposed that an original calendar governed by the priests of Ra was supplanted by an improvement developed by the partisans of Thoth. Parker connected the discrepancy to his theories concerning the lunar calendar. Sethe , Weill , and Clagett proposed that the names expressed the idea that each month culminated in

4674-475: The first light of dawn or at sunrise accounts for an 11–14 year shift in dated observations of the lunar cycle. It remains unknown how the Egyptians dealt with obscurement by clouds when they occurred and the best current algorithms have been shown to differ from actual observation of the waning crescent moon in about one-in-five cases. Parker and others have argued for its development into an observational and then calculated lunisolar calendar which used

4756-410: The five epagomenal days as days "added on" to the proper year. With its interior effectively rainless for thousands of years, ancient Egypt was "a gift of the river" Nile , whose annual flooding organized the natural year into three broad natural seasons known to the Egyptians as: As early as the reign of Djer ( c.  3000   BC, Dynasty I ), yearly records were being kept of

4838-432: The flood's high-water mark. Otto E. Neugebauer noted that a 365-day year can be established by averaging a few decades of accurate observations of the Nile flood without any need for astronomical observations , although the great irregularity of the flood from year to year and the difficulty of maintaining a sufficiently accurate Nilometer and record in prehistoric Egypt has caused other scholars to doubt that it formed

4920-406: The horizon. During the other half of the year it will appear to be above the horizon but not visible because the sunlight is too bright during the day. The star's heliacal rising will occur when the Earth has moved to a point in its orbit where the star appears on the eastern horizon at dawn. Each day after the heliacal rising, the star will rise slightly earlier and remain visible for longer before

5002-423: The last apocatastasis. Following such a scheme, the record of Sirius rising on II Shemu 1 in 239   BC implies apocatastases on 1319 and 2779   BC ±3 years. Censorinus 's placement of an apocatastasis on 21   July AD   139 permitted the calculation of its predecessors to 1322, 2782, and 4242   BC. The last is sometimes described as "the first exactly dated year in history" but, since

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5084-453: The light from the rising sun overwhelms it. Over the following days the star will move further and further westward (about one degree per day) relative to the Sun, until eventually it is no longer visible in the sky at sunrise because it has already set below the western horizon. This is called the acronycal setting . The same star will reappear in the eastern sky at dawn approximately one year after its previous heliacal rising. For stars near

5166-416: The names of their principal festivals. Each month was divided into three 10-day periods known as decans or decades. It has been suggested that during the Nineteenth Dynasty and the Twentieth Dynasty the last two days of each decan were usually treated as a kind of weekend for the royal craftsmen, with royal artisans free from work. Because this calendrical year was nearly a quarter of a day shorter than

5248-408: The obliquity are intended for high precision over a relatively short time span, perhaps several centuries. J. Laskar computed an expression to order T good to 0.04″ /1000 years over 10,000 years. All of these expressions are for the mean obliquity, that is, without the nutation of the equator included. The true or instantaneous obliquity includes the nutation. Most of the major bodies of

5330-687: The onset of the annual flooding of the Nile , although the flooding is based on the tropical year and so would occur about three quarters of a day earlier per century in the Julian or Sothic year. (July 19, 1000 BC in the Julian Calendar is July 10 in the proleptic Gregorian Calendar . At that time, the sun would be somewhere near Regulus in Leo , where it is around August 21 in the 2020s.) The ancient Egyptians appear to have constructed their 365-day civil calendar at

5412-463: The other from north to south. The crossing from south to north is known as the March equinox , also known as the first point of Aries and the ascending node of the ecliptic on the celestial equator. The crossing from north to south is the September equinox or descending node . The orientation of Earth's axis and equator are not fixed in space, but rotate about the poles of the ecliptic with

5494-457: The other major planets are all within about 6°. Because of this, most Solar System bodies appear very close to the ecliptic in the sky. The invariable plane is defined by the angular momentum of the entire Solar System, essentially the vector sum of all of the orbital and rotational angular momenta of all the bodies of the system; more than 60% of the total comes from the orbit of Jupiter. That sum requires precise knowledge of every object in

5576-452: The possibility that the cycle's strict application was occasionally subject to political interference. The record and celebration of Sirius's rising would also vary by several days (equating to decades of the cycle) in eras when the official site of observation was moved from near Cairo . The return of Sirius to the night sky varies by about a day per degree of latitude , causing it to be seen 8–10 days earlier at Aswan than at Alexandria ,

5658-558: The proposal was resisted by the Egyptian priests and people and abandoned until the establishment of the Alexandrian or Coptic calendar by Augustus . The introduction of a leap day to the Egyptian calendar made it equivalent to the reformed Julian calendar , although by extension it continues to diverge from the Gregorian calendar at the turn of most centuries. This civil calendar ran concurrently with an Egyptian lunar calendar which

5740-555: The reign of Neferirkare (mid-25th century   BC, Dynasty V ). It was probably based upon astronomical observations of Sirius whose reappearance in the sky closely corresponded to the average onset of the Nile flood through the 5th and 4th millennium BC. A recent development is the discovery that the 30-day month of the Mesopotamian calendar dates as late as the Jemdet Nasr Period (late 4th-millennium   BC),

5822-576: The return of the Spring violets . — H.E. Winlock Current understanding of the earliest development of the Egyptian calendar remains speculative. A tablet from the reign of the First Dynasty pharaoh Djer ( c.  3000   BC) was once thought to indicate that the Egyptians had already established a link between the heliacal rising of Sirius ( Ancient Egyptian : Spdt or Sopdet , "Triangle"; Ancient Greek : Σῶθις , Sôthis ) and

5904-420: The same applies as to the other polar constellations in respect of the reverse tropic. Constellations containing stars that rise and set were incorporated into early calendars or zodiacs . The Sumerians , Babylonians , Egyptians , and Greeks all used the heliacal risings of various stars for the timing of agricultural activities. Because of its position about 40° off the ecliptic, the heliacal risings of

5986-771: The sky or never. For example, the North Star (Polaris) is not visible in Australia and the Southern Cross is not seen in Europe, because they always stay below the respective horizons. The term circumpolar is somewhat localised as between the Tropic of Cancer and the Equator, the Southern polar constellations have a brief spell of annual visibility (thus "heliacal" rising and "cosmic" setting) and

6068-468: The sky's distant background. The ecliptic forms one of the two fundamental planes used as reference for positions on the celestial sphere, the other being the celestial equator . Perpendicular to the ecliptic are the ecliptic poles , the north ecliptic pole being the pole north of the equator. Of the two fundamental planes, the ecliptic is closer to unmoving against the background stars, its motion due to planetary precession being roughly 1/100 that of

6150-411: The solar year, the Egyptian calendar lost about one day every four years relative to the Gregorian calendar . It is therefore sometimes referred to as the wandering year ( Latin : annus vagus ), as its months rotated about one day through the solar year every four years. Ptolemy III 's Canopus Decree attempted to correct this through the introduction of a sixth epagomenal day every four years but

6232-460: The system, making it a somewhat uncertain value. Because of the uncertainty regarding the exact location of the invariable plane, and because the ecliptic is well defined by the apparent motion of the Sun, the ecliptic is used as the reference plane of the Solar System both for precision and convenience. The only drawback of using the ecliptic instead of the invariable plane is that over geologic time scales, it will move against fixed reference points in

6314-403: The terms to the rising and setting instances. Ecliptic The ecliptic or ecliptic plane is the orbital plane of Earth around the Sun . From the perspective of an observer on Earth, the Sun's movement around the celestial sphere over the course of a year traces out a path along the ecliptic against the background of stars . The ecliptic is an important reference plane and

6396-463: The usual four-year periods and, given that the expected discrepancy is no more than 8 years in 1460, the cycle may have been applied schematically according to the civil years by Egyptians and the Julian year by the Greeks and Romans. The occurrence of the apocatastasis in the 2nd millennium BC so close to the great political and sun-based religious reforms of Amenhotep IV /Akhenaton also leaves open

6478-434: The year before a Julian leap year, when it occurs on 30   August instead. The calendars then resume their correspondence after 4   Phamenoth   / 29   February of the next year. For much of Egyptian history, the months were not referred to by individual names, but were rather numbered within the three seasons. As early as the Middle Kingdom , however, each month had its own name. These finally evolved into

6560-476: Was corrected by Augustus through omitting leap years for a number of cycles until AD   4. As the personal ruler of Egypt , he also imposed a reform of its calendar in 26 or 25   BC, possibly to correspond with the beginning of a new Callipic cycle , with the first leap day occurring on 6 Epag. in the year 22   BC. This "Alexandrian calendar" corresponds almost exactly to the Julian , causing 1   Thoth to remain at 29   August except during

6642-512: Was that, four years from the calendar's inception, Sirius would have no longer reappeared on the Egyptian New Year but on the next day ( I Akhet 2) ; four years later, it would have reappeared on the day after that; and so on through the entire calendar until its rise finally returned to I Akhet 1 1460 years after the calendar's inception, an event known as " apocatastasis ". Owing to the event's extreme regularity, Egyptian recordings of

6724-418: Was used for some religious rituals and festivals. Some Egyptologists have described it as lunisolar , with an intercalary month supposedly added every two or three years to maintain its consistency with the solar year, but no evidence of such intercalation before the 4th century BC has yet been discovered. Setting a calendar by the Nile flood would be about as vague a business as if we set our calendar by

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