A month is a unit of time, used with calendars, which was first used and invented in Mesopotamia, as a natural period related to the motion of the Moon; month and Moon are cognates. The traditional concept arose with the cycle of moon phases; such months (lunations) are synodic months and last approximately 29.53 days. From excavated tally sticks, researchers have deduced that people counted days in relation to the Moon's phases as early as the Paleolithic age. Synodic months, based on the Moon's orbital period, are still the basis of many calendars today, and are used to divide the year.
Types of months
The following types of months are mainly of significance in astronomy, most of them (but not the distinction between sidereal and tropical months) first recognized in Babylonian lunar astronomy.
The period of the Moon's orbit as defined with respect to the celestial sphere (of the fixed stars, nowadays the International Celestial Reference Frame (ICRF)) is known as a sidereal month because it is the time it takes the Moon to return to a given position among the stars (Latin: sidera): 27.321661 days (27 d 7 h 43 min 11.5 s). This type of month has been observed among cultures in the Middle East, India, and China in the following way: they divided the sky into 27 or 28 lunar mansions, one for each day of the month, identified by the prominent star(s) in them.
It is customary to specify positions of celestial bodies with respect to the vernal equinox. Because of precession, this point moves back slowly along the ecliptic. Therefore it takes the Moon less time to return to an ecliptic longitude of zero than to the same point amidst the fixed stars: 27.321582 days (27 d 7 h 43 min 4.7 s). This slightly shorter period is known as tropical month; cf. the analogous tropical year of the Sun.
The Moon's orbit approximates an ellipse rather than a circle. However, the orientation (as well as the shape) of this orbit is not fixed. In particular, the position of the extreme points (the line of the apsides: perigee and apogee), makes a full circle (lunar precession) in about nine years. It takes the Moon longer to return to the same apsis because it moved ahead during one revolution. This longer period is called the anomalistic month, and has an average length of 27.554551 days (27 d 13 h 18 min 33.2 s). The apparent diameter of the Moon varies with this period, and therefore this type has some relevance for the prediction of eclipses (see Saros), whose extent, duration, and appearance (whether total or annular) depend on the exact apparent diameter of the Moon. The apparent diameter of the full moon varies with the full moon cycle which is the beat period of the synodic and anomalistic month, and also the period after which the apsides point to the Sun again.
Sometimes written 'draconitic' month, and also called the nodical month. The orbit of the moon lies in a plane that is tilted with respect to the plane of the ecliptic: it has an inclination of about five degrees. The line of intersection of these planes defines two points on the celestial sphere: the ascending node, when the moon's path crosses the ecliptic as the moon moves into the northern hemisphere, and descending node when the moon's path crosses the ecliptic as the moon moves into the southern hemisphere. The draconic or nodical month is the average interval between two successive transits of the moon through its ascending node. Because of the sun's gravitational pull on the moon, the moon's orbit gradually rotates westward on its axis, which means the nodes gradually rotate around the earth. As a result, the time it takes the moon to return to the same node is shorter than a sidereal month. It lasts 27.212220 days (27 d 5 h 5 min 35.8 s). The plane of the moon's orbit precesses over a full circle in about 18.6 years.
Because the moon's orbit is inclined with respect to the ecliptic, the sun, moon, and earth are in line only when the moon is at one of the nodes. Whenever this happens a solar or lunar eclipse is possible. The name "draconic" refers to a mythical dragon, said to live in the nodes and eat the sun or moon during an eclipse.
This is the average period of the Moon's revolution with respect to the line joining the Sun and Earth. The synodic month is the period of the Moon's phases, because the Moon's appearance depends on the position of the Moon with respect to the Sun as seen from the Earth.
While the moon is orbiting the Earth, the Earth is progressing in its orbit around the Sun. After completing a sidereal month the Moon must move a little further to reach the new position having the same angular distance from the Sun. This longer period is called the synodic month (Greek: σὺν ὁδῴ, sun hodō, meaning "with the way [of the Sun]").
Because of perturbations in the orbits of the Earth and Moon, the actual time between lunations may range from about 29.18 to about 29.93 days. The long-term average duration is 29.530589 days (29 d 12 h 44 min 2.9 s). The synodic month is used to calculate eclipse cycles.
Month type Length in days anomalistic 27.554549878 − 0.000000010390 × Y sidereal 27.321661547 + 0.000000001857 × Y tropical 27.321582241 + 0.000000001506 × Y draconic 27.212220817 + 0.000000003833 × Y synodic 29.530588853 + 0.000000002162 × Y
Note: In this table, time is expressed in Ephemeris Time (more precisely Terrestrial Time) with days of 86,400 SI seconds. Y is years since the epoch (2000), expressed in Julian years of 365.25 days. For calendrical calculations, one would probably use days measured in the time scale of Universal Time, which follows the somewhat unpredictable rotation of the Earth, and progressively accumulates a difference with ephemeris time called ΔT.
At the simplest level, all lunar calendars are based on the approximation that 2 lunations last 59 days: a 30 day full month followed by a 29 day hollow month — but this is only marginally accurate and quickly needs correction by using larger cycles, or the equivalent of leap days.
Second, the synodic month does not fit easily into the year, which makes constructing accurate, rule-based lunisolar calendars difficult. The most common solution to this problem is the Metonic cycle, which takes advantage of the fact that 235 lunations are approximately 19 tropical years (which add up to not quite 6940 days). However, a Metonic calendar (such as the Hebrew calendar) will drift against the seasons by about 1 day every 200 years.
The problems of creating reliable lunar calendars may explain why solar calendars, having months which no longer relate to the phase of the Moon, and being based only on the motion of the Sun against the sky, have generally replaced lunar calendars for civil use in most societies.
Months in various calendars
Beginning of the lunar month
However, the motion of the Moon in its orbit 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 in these calendars can not be accurately predicted.
While some like the Karaites Jews still rely on actual moon observations, most people use the Gregorian solar calendar.
Like the Hindu calendar, the Khmer calendar consists of both a lunar calendar and a solar calendar.
The solar calendar is used more commonly than the lunar calendar. There are 12 months and the numbers of days follow the Julian and Gregorian calendar.
Khmer Name Transliteration Julian and Gregorian Name # មករា Makara January # កម្ភៈ Kompeak February # មិនា or មីនា Mik Nea or Me Na March # មេសា Mesa April # ឧសភា Uk Sak Phea May # មិថុនា Mik Thok Na June # កក្កដា Kak Ka Da July # សីហា Seiha August # កញ្ញា Kagna September # តុលា Tola October # វិច្ឆិកា Vicheka November # ធ្នូ Thnu December
The Lunar Calendar
The lunar calendar contains 12 months, though, however, the eighth month is divided into 2 months in the calendar a few years ago making 13 months instead if 12 months.
- វិសាខ/ ពិសាខ
- ឤស឵ធ, divided into:
Julian and Gregorian calendars
Chronology Alphabetic Days 1 January 31 days 2 February 28 days, 29 in leap years 3 March 31 days 4 April 30 days 5 May 31 days 6 June 30 days 7 July 31 days 8 August 31 days 9 September 30 days 10 October 31 days 11 November 30 days 12 December 31 days
The mean month length of the Gregorian calendar is 30.436875 days.
Months existing in the Roman calendar in the past include:
- Mercedonius, an occasional month after February to realign the calendar.
- Quintilis, renamed to July in honour of Julius Caesar.
- Sextilis, renamed to August in honour of Augustus.
Also, note that the latter 10 months of the year form a pair of 31-30-31-30-31-day 5-month cycles.
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 not. When the knuckle of the index finger is reached (July), go back to the first knuckle (or over to the first knuckle on the other fist, held next to the first) and continue with August. This physical mnemonic has been taught to primary school students for many decades.
This cyclical pattern of month lengths matches the musical keyboard alternation of white and black keys (with the note 'F' correlating to the month of January).
Calends, nones, and ides
The ides occur on the thirteenth day in eight of the months, but in March, May, July, and October, they occur on the fifteenth. The nones always occur 8 days before the ides, i.e., on the fifth or the seventh. The calends are always the first day of the month.
No. Name Punjabi Days Julian Months 1 Chet ਚੇਤ 31 14 March – 13 April 2 Vaisakh ਵੈਸਾਖ 31 14 April – 14 May 3 Jeth ਜੇਠ 31 15 May – 14 June 4 Harh ਹਾੜ 31 15 June – 15 July 5 Sawan ਸਾਵਣ 31 16 July – 15 August 6 Bhadon ਭਾਦੋਂ 30 16 August – 14 September 7 Assu ਅੱਸੂ 30 15 September – 14 October 8 Katak ਕੱਤਕ 30 15 October – 13 November 9 Maghar ਮੱਘਰ 30 14 November – 13 December 10 Poh ਪੋਹ 30 14 December – 12 January 11 Magh ਮਾਘ 30 13 January – 11 February 12 Phagun ਫੱਗਣ 30/31 12 February – 13 March
There are also twelve months in the Islamic calendar. They are named as follows:
- Muharram ul Haram (or shortened to Muharram) محرّم
- Safar صفر
- Rabi`-ul-Awwal (Rabi' I) ربيع الأول
- Rabi`-ul-Akhir (or Rabi` al-Tיhaany) (Rabi' II) ربيع الآخر أو ربيع الثاني
- Jumaada-ul-Awwal (Jumaada I) جمادى الأول
- Jumaada-ul-Akhir (or Jumaada al-Thaany) (Jumaada II) جمادى الآخر أو جمادى الثاني
- Rajab رجب
- Sha'aban شعبان
- Ramadhan رمضان
- Shawwal شوّال
- Dhul Qadah (or Thou al-Qi`dah) ذو القعدة
- Dhul Hijja (or Thou al-Hijjah) ذو الحجة
See the article Islamic calendar for more information on the Islamic calendar.
The Bahá'í calendar is the calendar used by the Bahá'í Faith. It is a solar calendar with regular years of 365 days, and leap 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). The months are named after the attributes of God. Days of the year begin and end at sundown.
The Hebrew calendar has 12 or 13 months.
- Nisan, 30 days ניסן
- Iyyar, 29 days אייר
- Sivan, 30 days סיון
- Tammuz, 29 days תמוז
- Av, 30 days אב
- Elul, 29 days אלול
- Tishri, 30 days תשרי
- Heshvan, 29/30 days חשון
- Kislev, 29/30 days כסלו
- Tevet, 29 days טבת
- Shevat, 30 days שבט
- Adar 1, 30 days, intercalary month אדר א
- Adar 2, 29 days אדר ב
Adar 1 is only added 7 times in 19 years. In ordinary years, Adar 2 is simply called Adar.
French Republican calendar
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 called décades. 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 a Franciade. It began at the autumn equinox:
- Farvardin (31 days, فروردین)
- Ordibehesht (31 days, اردیبهشت)
- Khordad (31 days, خرداد)
- Tir (31 days, تیر)
- Mordad (31 days, مرداد)
- Shahrivar (31 days, شهریور)
- Mehr (30 days, مهر)
- Aban (30 days, آبان)
- Azar (30 days, آذر)
- Dey (30 days, دی)
- Bahman (30 days, بهمن)
- Esfand (29 days- 30 days in leap year, اسفند)
Reformed Bengali calendar
The Bangla Calendar, used in Bangladesh, follows solar months and it has six seasons. The months and seasons in the calendar are:
- Baishakh- 31 days
- Jyoishtho- 31 days
- Aashar- 31 days
- Shrabon- 31 days
- Bhadro- 31 days
- Aashwin- 30 days
Hemanta (Late Autumn)
- Kartik- 30 days
- Aghrahayon- 30 days
- Falgoon- 30 days
- Chaitra- 30 days (31 days in leap year)
The Tongan Calendar are based on the cycles of the moon around the earth in one year.The following months are:
- Liha Mu'a
- Liha Mui
- Vai Mu'a
- Vai Mui
- Faka'afu Mo'ui
- Faka'afu Mate
- Hilinga Kelekele
- Hilinga Mea'a
The Hindu Calendar has various systems of naming the months. The months in the lunar calendar are:
Sanskrit name Bengali name Tamil name # Chaitra (चैत्र) Chaitra (চৈত্র) Chitirai (சித்திரை) # Vaishaakha (वैशाख) *Baishakha (বৈশাখ) Vaikasi (வைகாசி) # Jyaishtha (ज्येष्ठ) Jyaishtha (জ্যৈষ্ঠ) Aani (ஆனி) # Aashaadha (आषाढ़) Ashadha (আষাঢ়) Aadi (ஆடி) # Shraavana (श्रावण) Shravana (শ্রাবণ) Aavani (ஆவணி) # Bhaadrapada (भाद्रपद) Bhadra (ভাদ্র) Purratasi (புரட்டாசி) # Aashvayuja (अश्विन) Ashwina (আশ্বিন) Aiypasi (ஐப்பசி) # Kaartika (कार्तिक) Kartika (কার্তিক) Kaarthigai (கார்த்திகை) # Maargashiirsha (मार्गशीर्ष) Agrahayana (অগ্রহায়ণ) Maargazhi (மார்கழி) # Pausha (पौष) Pausha (পৌষ) Thai (தை) # Maagha (माघ) Magha (মাঘ) Maasi (மாசி) # Phaalguna (फाल्गुन) Phalguna (ফাল্গুন) Panguni (பங்குனி)
In Bengali reckoning, Baishakha is the first month.
These are also the names used in the Indian national calendar for the newly redefined months.
The names in the solar calendar are just the names of the zodiac sign in which the sun travels. They are
The Sinhala calendar is the Buddhist calendar in Sri Lanka with Sinhala names. Each full moon Poya day marks the start of a Buddhist lunar month. The first month is Bak. The Sinhala and Tamil New Year Day is the start of the Hindu solar calendar (usually 14 April), an event unrelated to the Buddhist calendar.
- Duruthu (දුරුතු)
- Navam (නවම්)
- Medin (මැදින්)
- Bak (බක්)
- Vesak (වෙසක්)
- Poson (පොසොන්)
- Esala (ඇසල)
- Nikini (නිකිණි)
- Binara (බිනර)
- Vap (වප්)
- Il (iL) (ඉල්)
- Unduvap (උඳුවප්)
Icelandic/Old Norse calendar
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 same weekday rather than on the same date. Hence Þorri always starts on a Friday sometime between January 19 and January 25 (Old style: January 9 to January 15), Góa always starts on a Sunday between February 18 and February 24 (Old style: February 8 to February 14).
- Skammdegi ("Short days")
- Gormánuður (mid October - mid November, "slaughter month" or "Gór's month")
- Ýlir (mid November - mid December, "Yule month")
- Mörsugur (mid December - mid January, "fat sucking month")
- Þorri (mid January - mid February, "frozen snow month")
- Góa (mid February - mid March, "Góa's month, see Nór")
- 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 as Sumardagurinn fyrsti - first day of summer)
- Skerpla (mid May - mid June, another forgotten goddess)
- Sólmánuður (mid June - mid July, "sun month")
- Heyannir (mid July - mid August, "hay business month")
- Tvímánuður (mid August - mid September, "two" or "second month")
- Haustmánuður (mid September - mid October, "autumn month")
Old English calendar
Like the Old Norse calendar, the Anglo-Saxons had their own calendar before they were Christianized which reflected native traditions and deities. These months were attested by Bede in his work On Chronology written in the 8th century. His months are probably those as written in the Northumbrian dialect of Old English which he was familiar with. The months were so named after the moon; the new moon marking the end of an old month and start of a new month; the full moon occurring in the middle of the month, after which the month was named.
- Æfterra-ġēola mōnaþ (January, 'After-Yule month’)
- Sol-mōnaþ (February, 'Sol month')
- Hreþ-mōnaþ (March, 'Hreth month’)
- Ēostur-mōnaþ (April, 'Ēostur month’)
- Ðrimilce-mōnaþ (May, 'Three-milkings month’)
- Ærra-Liþa (June, 'Ere-Litha')
- Æftera-Liþa (July, 'After-Litha')
- Wēod-mōnaþ (August, 'Weed month’)
- Hāliġ-mōnaþ or Hærfest-mōnaþ (September, 'Holy month’ or 'Harvest month')
- Winter-fylleþ (October, 'Winter-filleth’)
- Blōt-mōnaþ (November, 'Blót month’)
- Ærra-ġēola mōnaþ (December, 'Ere-Yule')
Old Hungarian calendar
- 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’)
- Szent Jakab hava (July, ‘Saint James’ month’)
- Kisasszony hava (August, ‘month of the Virgin’)
- Szent Mihály hava (September, ‘Saint Michael’s month’)
- Mindszent hava (October, ‘all saints' month’)
- Szent András hava (November, ‘Saint Andrew’s month’)
- Karácsony hava (December, ‘month of Yule/Christmas’)
Old Egyptian calendar
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. 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".
- K'aliiyee = Going North - referring to the Sun returning to its usual place in the sky
- Buxwlaks = Needles Blowing About - February is usually a very windy month in the Nass River Valley
- Xsaak = To Eat Oolichans - Oolichans are harvested during this month
- Mmaal = Canoes - The river has defrosted, hence canoes are used once more
- Yansa'alt = Leaves are Blooming - Warm weather has arrived and leaves on the trees begin to bloom
- Miso'o = Sockeye - majority of Sockeye Salmon runs begin this month
- Maa'y = Berries - berry picking season
- Wii Hoon = Great Salmon - referring to the abundance of Salmon that are now running
- Genuugwwikw = Trail of the Marmot - Marmots, Ermines and animals as such are hunted
- Xlaaxw = To Eat Trout - trout are mostly eaten this time of year
- Gwilatkw = To Blanket - The earth is "blanketed" with snow
- Luut'aa = Sit In - the Sun "sits" in one spot for a period of time
Gregorian month Arabic month January يناير كانون الثاني Kanoon Al-Thani February فبراير شباط Shbaat March مارس اذار Athar April ابريل نيسان Nissan May مايو أيّار Ayyar June يونيو حزيران Hzeiran July يوليو تمّوز Tammuz August أغسطس اَب Aab September سبتمبر أيلول Aylool October أكتوبر تشرين الأول Tishreen Al-Awwal November نوفمبر تشرين الثاني Tishreen Al-Thani December ديسمبر كانون الأول Kanoon Al-Awwal
- Table of lunar month correspondences
- Maya calendar
- Chinese calendar
- Egyptian Calendar
- List of months by year: 2000–2050
- ^ a b Linton, Christopher M. (2004). From Eudoxus to Einstein: a history of mathematical astronomy. Cambridge University Press. p. 7. "In medieval times, the part of the Moon's orbit south of the ecliptic was known as the 'dragon' (which devoured the Moon during eclipses) and from this we get the terminology 'dragon's head' for the ascending node and 'dragon's tail' for the descending node. … The periods between successive nodes has, over time, been termed the dracontic, draconic and draconitic month, the words deriving from the Greek for 'dragon'."
- ^ Derived from ELP2000-85: M. Chapront-Touzé, J. Chapront (1991): Lunar tables and programs from 4000 B. C. to A. D. 8000. Willmann-Bell, Richmond VA; ISBN 0-943396-33-6
- ^ Days in each Month - Mnemonics to improve memory
- ^ The Boy Mechanic: A Handy Calendar (1913) from Project Gutenberg
- ^ "What is the Sikh Nanakshahi calendar". allaboutsikhs.com. Archived from the original on 2008-05-10. http://web.archive.org/web/20080510183956/http://www.allaboutsikhs.com/sikh-way-of-life/the-sikh-nanakshahi-calendar-3.html. Retrieved 2008-05-09.
- ^ a b Esslemont, J. E. (1980). Bahá'u'lláh and the New Era (5th ed.). Wilmette, Illinois, USA: Bahá'í Publishing Trust. pp. 178–179. ISBN 0877431604. http://reference.bahai.org/en/t/je/BNE/.
- ^ Sri Lanka – Festival Calendar
- ^ The Significance of Poya
- ^ The Old English Calendar
- ^ The Calendar by Marsigli: the ancient Hungarian Calendar
- ^ Hónapok nevei
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