The time for the Moon to orbit the Earth, relative to a reference point. Lunar motion is very complex. The Moon orbits the Earth in 27·32 days (relative to the stars), passing through the familiar cycle of lunar phases. The lunar month of 29·53 days is the interval between successive new Moons. Twelve lunar months is less than one solar year, so the calendar months are arbitrarily longer than lunar months.
such months (lunations) are synodic months and last ~29.53 days. Moreover, many cultures (most notably those using the ancient Hebrew (Jewish) calendar and the Islamic calendar) start a month with the first appearance of the thin crescent of the new moon after sunset over the western horizon.Sidereal month
The actual period of the Moon's orbit as measured in a fixed frame of reference is known as a sidereal month, because it is the time it takes the Moon to return to the same position on the celestial sphere among the fixed stars (Latin: sidus): 27.321 661 days (27 d 7 h 43 min 11.5 s) or about 27 ⅓ days. This type of month has appeared among cultures in the Middle East, India, and China in the following way: they divided the sky in 27 or 28 lunar mansions, characterized by asterisms (apparent groups of stars), one for each day that the Moon follows its track among the stars. Therefore it takes the Moon less time to return to an ecliptic longitude of zero than to the same point amidst the fixed stars: 27.321 582 days (27 d 7 h 43 min 4.7 s). This longer period is called the anomalistic month, and has an average length of 27.554 551 days (27 d 13 h 18 min 33.2 s), or about 27 1/2 days. 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.
Draconic 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. Hence the time it takes the Moon to return to the same node is again shorter than a sidereal month: this is called the draconic, nodical, or draconitic month. It lasts 27.212 220 days (27 d 5 h 5 min 35.8 s), or about 27 1/5 days.
Synodic month
The cause of moon phases is that from the Earth we see the part of the Moon that is illuminated by the Sun from different angles as the Moon traverses its orbit. Because of the perturbations of the orbits of the Earth and Moon, the actual time between lunations may range from about 29.27 to about 29.83 days. The long-term average duration is 29.530 588 days (29 d 12 h 44 min 2.8 s), or about 29 ½ days.
Month lengths
Here is a list of the average length of the various astronomical lunar months . These are not constant, so a first-order (linear) approximation of the secular change is provided:
Valid for the epoch J2000.0 (1 Jan. 2000 12:00 TT):
| sidereal month | 27.321661547 + 0.000000001857×y days |
| tropical month | 27.321582241 + 0.000000001506×y days |
| anomalistic month | 27.554549878 − 0.000000010390×y days |
| draconic month | 27.212220817 + 0.000000003833×y days |
| synodic month | 29.530588853 + 0.000000002162×y days |
Note: time expressed in Ephemeris Time (more precisely Terrestrial Time) with days of 86400 SI seconds.
Calendrical consequences
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. 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 more predictable motion of the sun against the sky, have generally replaced lunar calendars for civil use in most societies.
Months in various calendars
Julian and Gregorian calendars
The Gregorian calendar, like the Julian calendar before it, has twelve months:
January, 31 days February, 28 days, 29 in leap years, or 30 on certain occasions in related calendars March, 31 days April, 30 days May, 31 days June, 30 days July, 31 days August, 31 days September, 30 days October, 31 days November, 30 days December, 31 days One of Wikipedia's sister projects, Wiktionary, provides translations of each of the Gregorian/Julian calendar months into a dozen or more languages. Month-by-month links are provided here: January, February, March, April, May, June, July, August, September, October, November, December.Months existing in the Roman calendar in the past include:
Mercedonius, an occasional month after February to realign the calendar.The famous mnemonic Thirty days hath September is the most common way of teaching the lengths of the months. All months landing on a knuckle are 31 days long and those landing between them are not (it's up to you to figure out February). 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. It began at the autumn equinox:
Autumn: Vendémiaire Brumaire Frimaire Winter: Nivôse Pluviôse Ventôse Spring: Germinal Floréal Prairial Summer: Messidor Thermidor FructidorIslamic calendar
There are also twelve months in the Islamic calendar.
Hebrew Calendar
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 שבת Adar 1, 30 days, intercalary month אדר א Adar 2, 29 days אדר בAdar 1 is only added 7 times in 19 years.
Hindu Calendar
The Hindu Calendar has various systems of naming the months. The months in the lunar calendar are:
Chaitra Vaishaakha Jyaishtha Aashaadha Shraavana Bhaadrapada Aashvayuja Kaartika Maargashiirsha Pausha Maagha Phaalguna ColanaThese are also the names used in the Indian national calendar for the newly redefined months. They are
Mesha Vrishabha Mithuna Kataka Simha Kanyaa Tulaa Vrishcika Dhanus Makara Kumbha MiinaIranian/Persian calendar
The Iranian / Persian calendar, currently used in Iran and Afghanistan, also has 12 months.
Farvardin (فروردین), 31 days Ordibehesht (اردیبهشت), 31 days Khordad (خرداد), 31 days Tir (تیر), 31 days Mordad (مرداد), 31 days Shahrivar (شهریور), 31 days Mehr (مهر), 30 days Aban (آبان), 30 days Azar (آذر), 30 days Dey (دی), 30 days Bahman (بهمن), 30 days Esfand (اسفند), 29 days, 30 in leap yearsIcelandic/Old Norse calendar
The old Icelandic calendar is not in official use anymore, but some holidays and annual feasts are still calculated according to it in Iceland. It has 12 months, broken down into two groups of six often termed "winter months" and "summer months".
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")
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