Month

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.

  1. 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).
  2. 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.
  3. 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.
  4. 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.
  5. 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.

Other Languages
Afrikaans: Maand
Alemannisch: Monat
አማርኛ: ወር
Ænglisc: Mōnaþ
العربية: شهر
aragonés: Mes
ܐܪܡܝܐ: ܝܪܚܐ
armãneashti: Mesu
অসমীয়া: মাহ
asturianu: Mes
Avañe'ẽ: Jasy (ára)
авар: МоцІ
Aymar aru: Phaxsi
azərbaycanca: Ay (zaman)
تۆرکجه: آی(زامان)
বাংলা: মাস
Bân-lâm-gú: Goe̍h
башҡортса: Ай
беларуская: Месяц (каляндарны)
беларуская (тарашкевіца)‎: Месяц (каляндар)
भोजपुरी: महीना
български: Месец
Boarisch: Monat
བོད་ཡིག: ཟླ།
bosanski: Mjesec (period)
brezhoneg: Miz
català: Mes
Чӑвашла: Уйăх (вăхăт)
Chavacano de Zamboanga: Mes
Chi-Chewa: Mwezi
chiShona: Mwedzi (Mwaka)
chiTumbuka: Mwezi
Cymraeg: Mis
dansk: Måned
davvisámegiella: Mánotbadji
Deitsch: Munet
Deutsch: Monat
ދިވެހިބަސް: މަސް
dolnoserbski: Mjasec
Ελληνικά: Μήνας
emiliàn e rumagnòl: Més
эрзянь: Ков
español: Mes
Esperanto: Monato
estremeñu: Mes
euskara: Hilabete
فارسی: ماه (زمان)
Fiji Hindi: Mahina
français: Mois
furlan: Mês
Gaeilge:
Gaelg: Mee
Gagauz: Aylar
galego: Mes
ગુજરાતી: મહિનો
客家語/Hak-kâ-ngî: Ngie̍t
한국어: 달 (시간)
հայերեն: Ամիս
हिन्दी: मास
hornjoserbsce: Měsac
Ido: Monato
Bahasa Indonesia: Bulan (penanggalan)
interlingua: Mense
Iñupiak: Tatqiat
Ирон: Мæй
íslenska: Mánuður
italiano: Mese
עברית: חודש
Basa Jawa: Sasi
kalaallisut: Ukiup qaammatai
ಕನ್ನಡ: ತಿಂಗಳು
къарачай-малкъар: Ай (заман)
ქართული: თვე
kernowek: Mis
Kinyarwanda: Ukwezi
Kirundi: Ukwezi
Kiswahili: Mwezi (wakati)
Kreyòl ayisyen: Mwa
kurdî: Meh
Kwanyama: Omwedi
кырык мары: Тӹлзӹ (календарь)
Ladino: Mez
лакку: Барз
ລາວ: ເດືອນ
Latina: Mensis
latviešu: Mēnesis
Lëtzebuergesch: Mount
lietuvių: Mėnuo
Limburgs: Maondj
Livvinkarjala: Kuu
la .lojban.: masti
lumbaart: Més
magyar: Hónap
मैथिली: महिना
македонски: Месец
മലയാളം: മാസം
मराठी: महिना
მარგალური: თუთა
مصرى: شهر
Bahasa Melayu: Bulan (takwim)
Mìng-dĕ̤ng-ngṳ̄: Nguŏk
монгол: Сар (тоолол)
Nederlands: Maand
नेपाली: महिना
日本語: 月 (暦)
нохчийн: Бутт
Nordfriisk: Kalendermoune
norsk: Måned
norsk nynorsk: Månad
Nouormand: Mais
Novial: Mensu
occitan: Mes
олык марий: Тылзе
ଓଡ଼ିଆ: ମାସ
oʻzbekcha/ўзбекча: Oy (vaqt birligi)
ਪੰਜਾਬੀ: ਮਹੀਨਾ
پنجابی: مہینا
Tok Pisin: Mun (taim)
Plattdüütsch: Maand (Tiet)
polski: Miesiąc
português: Mês
qırımtatarca: Ay (zaman)
Ripoarisch: Moohnd (Kalländo)
română: Lună (timp)
Runa Simi: Killa pacha
русский: Месяц
саха тыла: Ый (кэм)
संस्कृतम्: आङ्ग्लमासाः
Sängö: Nze tî ngu
sardu: Mesi
Scots: Month
Sesotho sa Leboa: Kgwedi
shqip: Muaji
Simple English: Month
سنڌي: مهينو
SiSwati: Ínyangá
slovenščina: Mesec
ślůnski: Mjeśůnc
Soomaaliga: Bilaha
کوردی: مانگەکان
Sranantongo: Mun (ten)
српски / srpski: Месеци
srpskohrvatski / српскохрватски: Mjesec (jedinica)
Basa Sunda: Bulan (wayah)
suomi: Kuukausi
svenska: Månad
தமிழ்: மாதம்
Taqbaylit: Ayyur
tarandíne: Mise
татарча/tatarça: Ай
తెలుగు: నెల
ไทย: เดือน
ᏣᎳᎩ: ᏏᏅᏙ
Türkçe: Ay (zaman)
українська: Місяць (час)
اردو: مہینا
Vahcuengh: Ndwen
vepsän kel’: Ku
Tiếng Việt: Tháng
walon: Moes
West-Vlams: Moand
Winaray: Bulan (oras)
Wolof: Weer
ייִדיש: מאנאט
Yorùbá: Oṣù
粵語: 月份
Zazaki: Aşme
žemaitėška: Mienou
中文:
डोटेली: महिना
ГӀалгӀай: Бутт