A month is a unit of time, used with calendars, which is approximately as long 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 with respect to the Earth-Sun line, are still the basis of many calendars today, and are used to divide the year.
Types of months in astronomy
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 sidereal month is defined as the Moon's orbital period in a non-rotating frame of reference (which on average is equal to its rotation 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 pass twice a "fixed" star (different stars give different results because all have a very small proper motion and are not really fixed in position).
- A synodic 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.
- The tropical month is the average time for the Moon to pass twice through the same equinox point of the sky. It is 27.32158 days, very slightly shorter than the sidereal month (27.32166) days, because of precession of the equinoxes.
- An anomalistic month is the average time the Moon takes to go from perigee 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.
- The draconic month, draconitic month, or nodal month is the period in which the Moon returns to the same node 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. 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 59 days: a 30-day full month followed by a 29-day hollow month — but this is only roughly accurate, and eventually needs correction by using larger cycles, or the equivalent of leap days. Additionally, the synodic month does not fit easily into the year, which makes accurate, rule-based lunisolar calendars complicated. 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 will drift against the seasons by about 1 day every 200 years. Metonic calendars include the calendar used in the Antikythera Mechanism about 2000 years ago, and the Hebrew calendar.
The complexity required in an accurate lunisolar calendar may explain why solar calendars (which 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) 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 defined by observation cannot be accurately predicted.
Pingelapese, a language from Micronesia, also uses a lunar calendar. There are 12 months associated with their calendar. The moon first appears in March, they name this month Kahlek. 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.
Julian and Gregorian calendars
|2||February||28 days, 29 in leap years|
The mean month length of the Gregorian calendar is 30.436875 days.
Months existing in the Roman calendar in the past include:
- the mensis intercalaris, 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 any five consecutive months (not including February) contain 153 days.
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 (one Roman week) before the ides, i.e., on the fifth or the seventh. The calends are always the first day of the month, and before Julius Caesar's reform fell sixteen days (two Roman weeks) after the ides (except the ides of February and the intercalary month).
The Hebrew calendar has 12 or 13 months.
- Nisan, 30 days ניסן
- Iyar, 30 days אייר
- Sivan, 30 days סיון
- Tammuz, 29 days תמוז
- Av, 30 days אב
- Elul, 29 days אלול
- Tishri, 30 days תשרי
- Marcheshvan, 29/30 days מַרְחֶשְׁוָן
- Kislev, 30/29 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.
There are also twelve months in the Islamic calendar. They are named as follows:
- Muharram (Restricted/sacred) محرّم
- Safar (Empty/Yellow) صفر
- Rabī' al-Awwal/Rabi' I (First Spring) ربيع الأول
- Rabī’ ath-Thānī/Rabi` al-Aakhir/Rabi' II (Second spring or Last spring) ربيع الآخر أو ربيع الثاني
- Jumada al-Awwal/Jumaada I (First Freeze) جمادى الأول
- Jumada ath-Thānī or Jumādā al-Thānī/Jumādā II (Second Freeze or Last Freeze) جمادى الآخر أو جمادى الثاني
- Rajab (To Respect) رجب
- Sha'bān (To Spread and Distribute) شعبان
- Ramadān (Parched Thirst) رمضان
- Shawwāl (To Be Light and Vigorous) شوّال
- Dhu al-Qi'dah (The Master of Truce) ذو القعدة
- Dhu al-Hijjah (The Possessor of Hajj) ذو الحجة
See Islamic calendar for more information on the Islamic calendar.
|Gregorian month||Arabic month|
|January||يناير||كانون الثاني||Kanun Al-Thani|
|October||أكتوبر||تشرين الأول||Tishrin Al-Awwal|
|November||نوفمبر||تشرين الثاني||Tishrin Al-Thani|
|December||ديسمبر||كانون الأول||Kanun Al-Awwal|
The Hindu calendar has various systems of naming the months. The months in the lunar calendar are:
|Sanskrit name||Tamil name||Telugu name||Nepali name|
|1||Caitra (चैत्र)||Chitirai (சித்திரை)||Chaithramu (చైత్రము)||Chaitra (चैत्र/चैत)|
|2||Vaiśākha (वैशाख)||Vaikasi (வைகாசி)||Vaisaakhamu (వైశాఖము)||Baisakh (बैशाख)|
|3||Jyeṣṭha (ज्येष्ठ)||Aani (ஆனி)||Jyeshttamu (జ్యేష్ఠము)||Jesth (जेष्ठ/जेठ)|
|4||Ashadha (आषाढ)||Aadi (ஆடி)||Aashaadhamu (ఆషాఢము)||Aasad (आषाढ/असार)|
|5||Śrāvaṇa (श्रावण)||Aavani (ஆவணி)||Sraavanamu (శ్రావణము)||Srawan (श्रावण/साउन)|
|6||Bhadrapada (भाद्रपद)||Purratasi (புரட்டாசி)||Bhaadhrapadamu (భాద్రపదము)||Bhadau (भाद्र|भदौ)|
|7||Āśvina (अश्विन)||Aiypasi (ஐப்பசி)||Aasveeyujamu (ఆశ్వయుజము)||Asoj (आश्विन/असोज)|
|8||Kārtika (कार्तिक)||Kaarthigai (கார்த்திகை)||Kaarthikamu (కార్తీకము)||Kartik (कार्तिक)|
|9||Mārgaśīrṣa (मार्गशीर्ष)||Maargazhi (மார்கழி)||Maargaseershamu (మార్గశిరము)||Mangsir (मार्ग/मंसिर)|
|10||Pauṣa (पौष)||Thai (தை)||Pushyamu (పుష్యము)||Push (पौष/पुष/पूस)|
|11||Māgha (माघ)||Maasi (மாசி)||Maaghamu (మాఘము)||Magh (माघ)|
|12||Phālguna (फाल्गुन)||Panguni (பங்குனி)||Phaalgunamu (ఫాల్గుణము)||Falgun (फाल्गुन/फागुन)|
These are also the names used in the Indian national calendar for the newly redefined months. Purushottam Maas or Adhik Maas (translit. adhika = 'extra', māsa = 'month') is an extra month in the Hindu calendar that is inserted to keep the lunar and solar calendars aligned. "Purushottam" is an epithet of Vishnu, to whom the month is dedicated.
The names in the solar calendar are just the names of the zodiac sign in which the sun travels. They are
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.
Iranian calendar (Persian calendar)
- 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
|No.||Name||Bengali season||Days||Julian months|
|1||Boishakh (বৈশাখ)||Grishmo (গ্রীষ্ম)||31||14 April - May|
|2||Joishtho (জ্যৈষ্ঠ)||Grishmo (গ্রীষ্ম)||31||May - June|
|3||Asharh (আষাঢ়)||Borsha (বর্ষা)||31||June - July|
|4||Shrabon (শ্রাবণ)||Borsha (বর্ষা)||31||July - August|
|5||Bhadro (ভাদ্র)||Shorot (শরৎ)||31||August - September|
|6||Aashin (আশ্বিন)||Shorot (শরৎ)||30||September - October|
|7||Kartik (কার্তিক)||Hemonto(হেমন্ত)||30||October - November|
|8||Ogrohayon (অগ্রহায়ণ)||Hemonto(হেমন্ত)||30||November - December|
|9||Poush (পৌষ)||Sheet (শীত)||30||December - January|
|10||Magh (মাঘ)||Sheet (শীত)||30||January - February|
|11||Falgun (ফাল্গুন)||Boshonto (বসন্ত)||30 (31 in leap years)||February - March|
|12||Choitro (চৈত্র)||Boshonto (বসন্ত)||30||March - April|
The months in the Nanakshahi calendar are:
|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|
Like the Hindu calendar, the Khmer calendar consists of both a lunar calendar and a solar calendar.
The Khmer 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|
|3||មិនា or មីនា||Mik Nea or Me Na||March|
|5||ឧសភា||Uk Sak Phea||May|
|6||មិថុនា||Mik Thok Na||June|
|7||កក្កដា||Kak Ka Da||July|
- វិសាខ/ ពិសាខ
- ឤសាឍ, or in the case of a year with a leap-month:
|English name||Thai name||Abbr.||Transcription||Sanskrit word||Zodiac sign|
|February||กุมภาพันธ์||ก.พ.||kumphaphan||kumbha "pitcher, water-pot"||Aquarius|
|March||มีนาคม||มี.ค.||minakhom||mīna "(a specific kind of) fish"||Pisces|
|June||มิถุนายน||มิ.ย.||mithunayon||mithuna "a pair"||Gemini|
|December||ธันวาคม||ธ.ค.||thanwakhom||dhanu "bow, arc"||Sagittarius|
The Tongan calendar is based on the cycles of the moon around the earth in one year. The months are:
- Liha Mu'a
- Liha Mui
- Vai Mu'a
- Vai Mui
- Faka'afu Mo'ui
- Faka'afu Mate
- Hilinga Kelekele
- Hilinga Mea'a
Kollam era (Malayalam) calendar
|Malayalam name||Transliteration||Concurrent Gregorian months||Sanskrit word and meaning||Zodiac sign|
|ധനു||dha-nu||December–January||dhanu "bow, arc"||Sagittarius|
|കുംഭം||kum-bha-m||February–March||kumbha "pitcher, water-pot"||Aquarius|
|മീനം||mee-na-m||March–April||mīna "(a specific kind of) fish"||Pisces|
|ഇടവം||i-Ta-va-m||May - June||vṛṣabha "bull"||Taurus|
|മിഥുനം||mi-thu-na-m||June–July||mithuna "a pair"||Gemini|
- Duruthu (දුරුතු)
- Navam (නවම්)
- Mædin (මැදින්)
- Bak (බක්)
- Vesak (වෙසක්)
- Poson (පොසොන්)
- Æsala (ඇසල)
- Nikini (නිකිණි)
- Binara (බිනර)
- Vap (වප්)
- Il (iL) (ඉල්)
- Unduvap (උඳුවප්)
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 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")
- 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 Georgian calendar
|Month||Georgian Month Name||Transliteration||Georgian Other Names||Transliteration|
|January||აპნისი, აპანი||Apnisi, Apani|
|June||მარიალისა||Marialisa||თიბათვე, ივანობისთვე||Tibatve, Ivanobistve|
|July||თიბისა||Tibisa||მკათათვე, კვირიკობისთვე||Mkatatve, Kvirikobistve|
|November||ტირისკონი||Tiriskoni||გიორგობისთვე, ჭინკობისთვე||Giorgobistve, Chinkobistve|
*NOTE: New Year in ancient Georgia started from September.
Old Swedish calendar
- Torsmånad (January, 'Torre's month' (ancient god))
- Göjemånad (February, 'Goe's month' (ancient goddess))
- Vårmånad (March, 'Spring month')
- Gräsmånad (April, 'Grass month')
- Blomstermånad (May, 'Bloom month')
- Sommarmånad (June, 'Summer month')
- Hömånad (July, 'Hay month')
- Skördemånad, Rötmånad (August, 'Harvest month' or 'Rotten month')
- Höstmånad (September, 'Autumn month')
- Slaktmånad (October, 'Slaughter month')
- Vintermånad (November, 'Winter month')
- Julmånad (December, 'Christmas 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 works On Chronology and The Reckoning of Time 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')
- Hrēð-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')
- Weōd-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
Nagyszombati kalendárium (in Latin: Calendarium Tyrnaviense) from 1579. Historically Hungary used a 12-month calendar that appears to have been zodiacal in nature but eventually came to correspond to the Gregorian months as shown below:
- 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".
The Nisga'a calendar coincides with the Gregorian calendar with each month referring to the type of harvesting that is done during the month.
- 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
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:
Purushottam Maas or Adhik Maas is an extra month in the Hindu calendar that is inserted to keep the lunar and solar calendars aligned. "Purushottam" is an epithet of Vishnu, to whom the month is dedicated.
|Look up month in Wiktionary, the free dictionary.|
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