Unit of time about as long the orbital period of the Moon
This article is about the general concept. For the internationally used calendar months, seeGregorian calendar § Months.
Amonth is a unit oftime, used withcalendars, that is approximately as long as a natural phase cycle of theMoon; the wordsmonth andMoon arecognates. The traditional concept of months arose with the cycle ofMoon phases; suchlunar months ("lunations") aresynodic months and last approximately 29.53days, making for roughly 12.37 such months in one Earth year. From excavatedtally sticks, researchers have deduced that people counted days in relation to the Moon's phases as early as thePaleolithic age. Synodic months, based on the Moon'sorbital period with respect to the Earth–Sun line, are still the basis of many calendars today and are used to divide theyear.
Calendars that developed from theRoman calendar system, such as the internationally usedGregorian calendar, divide the year into 12 months, each of which lasts between 28 and 31 days. The names of the months were Anglicized from various Latin names and events important to Rome, except for the months 9–12, which are named after the Latin numerals 7–10 (septem,octo,novem, anddecem) because they were originally the seventh through tenth months in the Roman calendar.[1] In the modern Gregorian calendar, the only month with a variable number of days is the second month, February, which has 29 days during aleap year and 28 days otherwise.
The following types of months are mainly of significance in astronomy. Most of them (but not the distinction between sidereal and tropical months) were first recognized inBabylonian lunar astronomy.
Thesidereal month is defined as the Moon'sorbital period in a non-rotating frame of reference (which on average is equal to itsrotation period in the same frame). It is about 27.32166 days (27 days, 7 hours, 43 minutes, 11.6 seconds). It is closely equal to the time it takes the Moon to twice pass a "fixed"star (different stars give different results because all have a very smallproper motion and are not really fixed in position).
Asynodic month is the most familiar lunar cycle, defined as the time interval between two consecutive occurrences of a particular phase (such as new moon or full moon) as seen by an observer on Earth. The mean length of the synodic month is 29.53059 days (29 days, 12 hours, 44 minutes, 2.8 seconds). Due to the eccentricity of the lunar orbit around Earth (and to a lesser degree, the Earth's elliptical orbit around the Sun), the length of a synodic month can vary by up to seven hours.
Thetropical month is the average time for the Moon to pass twice through the sameequinox point of the sky. It is 27.32158 days, very slightly shorter than the sidereal month (27.32166) days, because ofprecession of the equinoxes.
Ananomalistic month is the average time the Moon takes to go fromperigee to perigee—the point in the Moon's orbit when it is closest to Earth. An anomalistic month is about 27.55455 days on average.
Thedraconic month, draconitic month, or nodal month is the period in which the Moon returns to the samenode of its orbit; the nodes are the two points where the Moon's orbit crosses the plane of the Earth's orbit. Its duration is about 27.21222 days on average.
A synodic month is longer than a sidereal month because the Earth-Moon system is orbiting the Sun in the same direction as the Moon is orbiting the Earth.[citation needed] The Sun moves eastward with respect to the stars (as does the Moon) and it takes about 2.2 days longer for the Moon to return to the same apparent position with respect to the Sun.
An anomalistic month is longer than a sidereal month because the perigee moves in the same direction as the Moon is orbiting the Earth, one revolution in nine years. Therefore, the Moon takes a little longer to return to perigee than to return to the same star.
A draconic month is shorter than a sidereal month because the nodes move in the opposite direction as the Moon is orbiting the Earth, one revolution in 18.6 years. Therefore, the Moon returns to the same node slightly earlier than it returns to the same star.
At the simplest level, most well-known lunar calendars are based on the initial approximation that 2 lunations last 59solar days: a 30-dayfull month followed by a 29-dayhollow month — but this is only roughly accurate and regularly needsintercalation (correction) by aleap day.
Additionally, the synodic month does not fit easily into thesolar (or 'tropical') year, which makes accurate, rule-basedlunisolar calendars that combine the two cycles complicated. The most common solution to this problem is theMetonic cycle, which takes advantage of the fact that 235 lunations are approximately 19 tropical years (which add up to not quite 6,940 days): 12 years have 12 lunar months, and 7 years are 13 lunar months long. However, aMetonic calendar based year will drift against the seasons by about one day every 2 centuries. Metonic calendars include the calendar used in theAntikythera Mechanism about 21 centuries ago, and theHebrew calendar.
Alternatively in a purelunar calendar, years are defined as having always 12 lunations, so a year is 354 or 355 days long: theIslamic calendar is the prime example. Consequently, an Islamic year is about 11 days shorter than a solar year and cycles through the seasons in about 33 solar = 34 lunar years: theIslamic New Year has a differentGregorian calendar date in each (solar) year.
Purelysolar calendars often have months which no longer relate to the phase of the Moon, but are based only on the motion of the Sun relative to the equinoxes and solstices, or are purely conventional like in the widely usedGregorian calendar.
The complexity required in an accurate lunisolar calendar may explain why solar calendars have generally replaced lunisolar and lunar calendars for civil use in most societies.
However, the motion of the Moon in itsorbit is very complicated and its period is not constant. The date and time of this actual observation depends on the exact geographical longitude as well as latitude, atmospheric conditions, the visual acuity of the observers, etc. Therefore, the beginning and lengths of months defined by observation cannot be accurately predicted.
TheRoman calendar was reformed several times, the last three enduring reforms during historical times. The last three reformed Roman calendars are called theJulian,Augustan, andGregorian; all had the same number of days in their months. Despite other attempts, the names of the months after theAugustan calendar reform have persisted, and the number of days in each month (except February) have remained constant since before theJulian reform. TheGregorian calendar, like theRoman calendars before it, has twelve months, whoseAnglicized names are:
On top of the knuckles (yellow): 31 days Between the knuckles (blue): 30 days February (red) has 28 or 29 days.The white keys of the musical keyboard correspond to months with 31 day months. (F corresponds to January.)
The famousmnemonicThirty days hath September is a common way of teaching the lengths of the months in the English-speaking world. The knuckles of the four fingers of one's hand and the spaces between them can be used to remember the lengths of the months. By making a fist, each month will be listed as one proceeds across the hand. All months landing on a knuckle are 31 days long and those landing between them are 30 days long, with variable February being the remembered exception. When the knuckle of the index finger is reached (July), go over to the first knuckle on the other fist, held next to the first (or go back to the first knuckle) and continue with August. This physical mnemonic has been taught to primary school students for many decades, if not centuries.[6][7]
This cyclical pattern of month lengths matches themusical keyboard alternation of wide white keys (31 days) and narrow black keys (30 days). The noteF corresponds toJanuary, the noteF♯ corresponds toFebruary, the exceptional 28–29 day month, and so on.
The Romans divided their months into three parts, which they called thecalends, thenones, and theides. Their system is somewhat intricate. Theides occur on the thirteenth day in eight of the months, but in March, May, July, and October, they occur on the fifteenth. Thenones always occur 8 days (one Roman 'week') before the ides, i.e., on the fifth or the seventh. Thecalends are always the first day of the month,[a] and before Julius Caesar's reform fell sixteen days (two Roman weeks) after the ides (except the ides of February and the intercalary month).
Relations between dates, weekdays, and months in the Gregorian calendar
These are also the names used in theIndian national calendar for the newly redefined months. Purushottam Maas or Adhik Maas (translit.adhika = 'extra',māsa = 'month') is an extra month in theHindu calendar that is inserted to keep thelunar andsolar calendars aligned. "Purushottam" is an epithet ofVishnu, to whom the month is dedicated.
The names in the solar calendar are just the names of thezodiac sign in which the sun travels. They are
TheBaháʼí calendar is the calendar used by theBaháʼí Faith. It is a solar calendar with regular years of 365 days, andleap years of 366 days. Years are composed of 19 months of 19 days each (361 days), plus an extra period of "Intercalary Days" (4 in regular and 5 in leap years).[8] The months are named after the attributes of God. Days of the year begin and end at sundown.[8]
TheIranian / Persian calendar, currently used inIran, also has 12 months. ThePersian names are included in the parentheses. It begins on the northern Spring equinox.
Different from the Hindu calendar, the Khmer calendar consists of both a lunar calendar and a solar calendar. The solar is used more commonly than the lunar calendar.
The Khmer lunar calendar most often contains 12 months; however, the eighth month is repeated (as a "leap month") every two or three years, making 13 months instead of 12.[10] Each lunar month has 29 or 30 days. The year normally has then 354 or 384 days (when an intercalary month is added), but the calendar follows the rules of the Gregorian calendar to determine leap years and add a lead day to one month, so the Khmer lunar year may have a total of 354, 355, 384 or 385 days.
Pingelapese, a language fromMicronesia, also uses a lunar calendar. There are 12 months associated with their calendar. The Moon first appears in March,[clarification needed] they name this monthKahlek. This system has been used for hundreds of years and throughout many generations. This calendar is cyclical and relies on the position and shape of the Moon.[11]
The old Icelandic calendar is not in official use anymore, but some Icelandic holidays and annual feasts are still calculated from it. It has 12 months, broken down into two groups of six often termed "winter months" and "summer months". The calendar is peculiar in that the months always start on the sameweekday rather than on the samedate. Hence Þorri always starts on a Friday sometime between January 22 and January 28(Old style: January 9 to January 15), Góa always starts on a Sunday between February 21 and February 27(Old style: February 8 to February 14).
Skammdegi ("Short days")
Gormánuður (mid-October – mid-November, "slaughter month" or "Gór's month")
Einmánuður (mid-March – mid-April, "lone" or "single month")
Náttleysi ("Nightless days")
Harpa (mid-April – mid-May, Harpa is a female name, probably a forgotten goddess, first day of Harpa is celebrated asSumardagurinn fyrsti – first day of summer)
Skerpla (mid-May – mid-June, another forgotten goddess)
Like the Old Norse calendar, theAnglo-Saxons had their own calendar before they wereChristianized which reflected native traditions and deities. These months were attested byBede in his worksOn Chronology andThe Reckoning of Time written in the 8th century.[14] HisOld English month names are probably written as pronounced in Bede's nativeNorthumbrian dialect. The months were named after the Moon; the new moon marking the end of an old month and start of a new month; thefull moon occurring in the middle of the month, after which the whole month took its name.
TheColigny calendar (Gaulish/Celtic) is an Iron Age Metonic lunisolar calendar, with 12 lunar months of either 29 or 30 days. The lunar month is calculated to a precision of within 24 hours of the lunar phase, achieved by a particular arrangement of months, and the month of EQUOS having a variable length of 29 or 30 days to adjust for any lunar slippage. This setup means the calendar could stay precisely aligned to its lunar phase indefinitely.
The lunar month is divided into two halves, the first of 15 days and the second of 14 or 15 days. The month is calculated to start at the first quarter moon, with the full moon at the centre of the first half-month and the dark moon at the centre of the second half-month. The calendar does not rely on unreliable visual sightings.
An intercalary lunar month is inserted before every 30 lunar months to keep in sync with the solar year. Every 276 years this adds one day to the solar point, so if for example the calendar was 1,000 years old, it would only have slipped by less than 4 days against the solar year.
Nagyszombati kalendárium (in Latin:Calendarium Tyrnaviense) from 1579.Historically Hungary used a 12-month calendar that appears to have been zodiacal in nature[15] but eventually came to correspond to the Gregorian months as shown below:[16]
Boldogasszony hava (January, 'month of the happy/blessed lady')
Böjtelő hava (February, 'month of early fasting/Lent' or 'month before fasting/Lent')
Böjtmás hava (March, 'second month of fasting/Lent')
Szent György hava (April, 'Saint George's month')
Pünkösd hava (May, 'Pentecost month')
Szent Iván hava (June, 'Saint John [the Baptist]'s month')
The ancient civil Egyptian calendar had a year that was 365 days long and was divided into 12 months of 30 days each, plus 5 extra days (epagomenes) at the end of the year.[18] The months were divided into 3 "weeks" of ten days each. Because the ancient Egyptian year was almost a quarter of a day shorter than the solar year and stellar events "wandered" through the calendar, it is referred to as Annus Vagus or "Wandering Year".
This calendar was proposed during the French Revolution, and used by the French government for about twelve years from late 1793. There were twelve months of 30 days each, grouped into three ten-day weeks calleddécades. The five or six extra days needed to approximate the tropical year were placed after the months at the end of each year. A period of four years ending on a leap day was to be called aFranciade. It began at the autumn equinox:
Ojibwe month names[b] are based on the key feature of the month. Consequently, months between various regions have different names based on the key feature of each month in their particular region. In the Eastern Ojibwe, this can be seen in when thesucker makes its run, which allows the Ojibwe to fish for them. Additionally, Rhodes[20] also informs of not only the variability in the month names, but how in Eastern Ojibwe these names were originally applied to thelunar months the Ojibwe originally used, which was alunisolar calendar, fixed by the date ofAkiinaaniwan (typically December 27) that marks when sunrise is the latest in the Northern Hemisphere.
^More precisely, thecalends were when the name of a month first began being used when referring to dates. Instead of counting the number of days elapsed, the Romans used acountdown to number their dates. See the articleRoman calendar for a more detailed explanation.
^abDue to Eastern Ojibwe is avowelsyncope dialect, the elided vowels (and the occasionally elided consonants) have been added back in the table below, shown in brackets.
^Rhodes, Richard A., ed. (1993) [1985].Eastern Ojibwa-Chippewa-Ottawa Dictionary. Trends in Linguistics. Berlin, DE; New York, NY: Walter de Gruyter.ISBN978-3-11-013749-1.
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