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.
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.
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.
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.
|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:
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).
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.
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:
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||Āṣāḍha (आषाढ)||Aadi (ஆடி)||Aashaadhamu (ఆషాఢము)||Aasad (आषाढ/असार)|
|5||Śrāvaṇa (श्रावण)||Aavani (ஆவணி)||Sraavanamu (శ్రావణము)||Srawan (श्रावण/साउन)|
|6||Bhādrapada (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.
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.
|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|
|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|
The Khmer lunar calendar contains 12 months; however, the eighth month is repeated (as a "leap-month") every two or three years, making 13 months instead of 12.
|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:
|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|
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).
|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.
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.
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:
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.
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:
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