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[link]

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.

Sidereal month[link]

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.

Tropical month[link]

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.

Anomalistic month[link]

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.

Draconic month[link]

Sometimes written 'draconitic' month,^[1] 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.^[1]

Synodic month[link]

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 lengths[link]

Here is a list of the average length of the various astronomical lunar months.^[2] These are not constant, so a first-order (linear) approximation of the secular change is provided:

Valid for the epoch J2000.0 (1 January 2000 12:00 TT):

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.

Calendrical consequences[link]

For more details on this topic, see lunar calendar and lunisolar calendar.

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. One exception is the Hebrew calendar, which was calculated very precisely over a thousand years ago, based on a lunation of 29 days, 12 hours, 44 minutes and 3⅓ seconds, which differs from modern calculations by only one day in 15000 years.

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[link]

Beginning of the lunar month[link]

The Hellenic calendars, the Hebrew Lunisolar calendar and the Islamic Lunar calendar started the month with the first appearance of the thin crescent of the new moon.

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 Jewish Karaites still rely on actual moon observations, most people use the Gregorian solar calendar.

Khmer calendar[link]

Like the Hindu calendar, the Khmer calendar consists of both a lunar calendar and a solar calendar.

Solar calendar[link]

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.

The lunar calendar[link]

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 of 12 months.

• មិគសិរ​
• បុស
• មាគ
• ផល្គុន
• ចេត្រ
• វិសាខ/​​​ ពិសាខ
• ជេស្ឋ
• ឤស឵ធ, divided into:
  • បឋមសាធ
  • ទុតិយាសាធ
• ស្រាពណ៍
• ភទ្របទ
• អស្សុច
• កត្តិក

Julian and Gregorian calendars[link]

The Gregorian calendar, like the Julian calendar before it, has twelve months:

On top of the knuckles (yellow): 31 days
Between the knuckles (blue): 30 days
February (red) has 28 or 29 days.

The white keys of the musical keyboard correlate to months with 31 days. ('F' correlates to January.)

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.

The famous mnemonic Thirty days hath September is the most^{[citation needed]} common way of teaching the lengths of the months in the English-speaking world.

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.^[3][4]

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[link]

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.

Nanakshahi calendar[link]

The months in the Nanakshahi calendar are:^[5]

Islamic calendar[link]

There are also twelve months in the Islamic calendar. They are named as follows:

1. Muharram (Forbidden) محرّم
2. Safar (Empty/Yellow) صفر
3. Rabī' al-Awwal/Rabi' I (First Spring) ربيع الأول
4. Rabī’ ath-Thānī/Rabi` al-Aakhir/Rabi' II (Second spring) ربيع الآخر أو ربيع الثاني
5. Jumada al-Awwal/Jumaada I (First Freeze) جمادى الأول
6. Jumada ath-Thānī/or Jumādā al-Thānī)/Jumādā II (Second Freeze) جمادى الآخر أو جمادى الثاني
7. Rajab (To Respect) رجب
8. Sha'bān (To Spread and Distribute) شعبان
9. Ramadān (Parched Thirst) رمضان
10. Shawwāl (To Be Light and Vigorous) شوّال
11. Dhu al-Qi'dah (The Master of Truce) ذو القعدة
12. Dhu al-Hijjah (The Possessor of Hajj) ذو الحجة

See Islamic calendar for more information on the Islamic calendar.

Bahá'í calendar[link]

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).^[6] The months are named after the attributes of God. Days of the year begin and end at sundown.^[6]

Hebrew Calendar[link]

The Hebrew calendar has 12 or 13 months.

1. Nisan, 30 days ניסן
2. Iyyar, 29 days אייר
3. Sivan, 30 days סיון
4. Tammuz, 29 days תמוז
5. Av, 30 days אב
6. Elul, 29 days אלול
7. Tishri, 30 days תשרי
8. Heshvan, 29/30 days חשון
9. Kislev, 29/30 days כסלו
10. Tevet, 29 days טבת
11. Shevat, 30 days שבט
12. Adar 1, 30 days, intercalary month אדר א
13. 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[link]

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:

• Autumn:

1. Vendémiaire
2. Brumaire
3. Frimaire

• Winter:

1. Nivôse
2. Pluviôse
3. Ventôse

• Spring:

1. Germinal
2. Floréal
3. Prairial

• Summer:

1. Messidor
2. Thermidor
3. Fructidor

Iranian calendars (Persian calendars)[link]

The Iranian / Persian calendar, currently used in Iran and Afghanistan, also has 12 months. The Persian names are included in the parentheses.

1. Farvardin (31 days, فروردین)
2. Ordibehesht (31 days, اردیبهشت)
3. Khordad (31 days, خرداد)
4. Tir (31 days, تیر)
5. Mordad (31 days, مرداد)
6. Shahrivar (31 days, شهریور)
7. Mehr (30 days, مهر)
8. Aban (30 days, آبان)
9. Azar (30 days, آذر)
10. Dey (30 days, دی)
11. Bahman (30 days, بهمن)
12. Esfand (29 days- 30 days in leap year, اسفند)

Reformed Bengali calendar[link]

The Bangla Calendar, used in Bangladesh, follows solar months and it has six seasons. The months and seasons in the calendar are:

Grishho (Summer)

1. Baishakh- 31 days
2. Jyoishtho- 31 days

Borsha (Rainy)

1. Aashar- 31 days
2. Shrabon- 31 days

Sharat (Autumn)

1. Bhadro- 31 days
2. Aashwin- 30 days

Hemanta (Late Autumn)

1. Kartik- 30 days
2. Aghrahayon- 30 days

Sheeth (Winter)

1. Poush- 30 days
2. Magh- 30 days

Bashanta (Spring)

1. Falgoon- 30 days
2. Chaitra- 30 days (31 days in leap year)

Thai calendar[link]

Tongan calendar[link]

The Tongan calendar are based on the cycles of the moon around the earth in one year.The following months are:

1. Liha Mu'a
2. Liha Mui
3. Vai Mu'a
4. Vai Mui
5. Faka'afu Mo'ui
6. Faka'afu Mate
7. Hilinga Kelekele
8. Hilinga Mea'a
9. 'Ao'ao
10. Fu'ufu'unekinanga
11. 'Uluenga
12. Tanumanga
13. 'O'oamofanongo

Hindu calendar[link]

The Hindu calendar has various systems of naming the months. The months in the lunar calendar are:

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

1. Mesha
2. Vrishabha
3. Mithuna
4. Kataka
5. Simha
6. Kanyaa
7. Tulaa
8. Vrishcika
9. Dhanus
10. Makara
11. Kumbha
12. Miina

Sinhala calendar[link]

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.^[7] The first month is Bak.^[8] 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.

1. Duruthu (දුරුතු)
2. Navam (නවම්)
3. Medin (මැදින්)
4. Bak (බක්)
5. Vesak (වෙසක්)
6. Poson (පොසොන්)
7. Esala (ඇසල)
8. Nikini (නිකිණි)
9. Binara (බිනර)
10. Vap (වප්)
11. Il (iL) (ඉල්)
12. Unduvap (උඳුවප්)

Germanic calendar[link]

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")

1. Gormánuður (mid October - mid November, "slaughter month" or "Gór's month")
2. Ýlir (mid November - mid December, "Yule month")
3. Mörsugur (mid December - mid January, "fat sucking month")
4. Þorri (mid January - mid February, "frozen snow month")
5. Góa (mid February - mid March, "Góa's month, see Nór")
6. Einmánuður (mid March - mid April, "lone" or "single month")

• Náttleysi ("Nightless days")

1. 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)
2. Skerpla (mid May - mid June, another forgotten goddess)
3. Sólmánuður (mid June - mid July, "sun month")
4. Heyannir (mid July - mid August, "hay business month")
5. Tvímánuður (mid August - mid September, "two" or "second month")
6. Haustmánuður (mid September - mid October, "autumn month")

Old Georgian calendar[link]

*NOTE: New Year in ancient Georgia started from September.

Old English calendar[link]

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.^[9] 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.

1. Æfterra-ġēola mōnaþ (January, 'After-Yule month’)
2. Sol-mōnaþ (February, 'Sol month')
3. Hreþ-mōnaþ (March, 'Hreth month’)
4. Ēostur-mōnaþ (April, 'Ēostur month’)
5. Ðrimilce-mōnaþ (May, 'Three-milkings month’)
6. Ærra-Liþa (June, 'Ere-Litha')
7. Æftera-Liþa (July, 'After-Litha')
8. Wēod-mōnaþ (August, 'Weed month’)
9. Hāliġ-mōnaþ or Hærfest-mōnaþ (September, 'Holy month’ or 'Harvest month')
10. Winter-fylleþ (October, 'Winter-filleth’)
11. Blōt-mōnaþ (November, 'Blót month’)
12. Ærra-ġēola mōnaþ (December, 'Ere-Yule')

Old Hungarian calendar[link]

Nagyszombati kalendárium (in Latin: Calendarium Tyrnaviense) from 1579. Historically Hungary used a 12-month calendar that appears to have been zodiacal in nature^[10] but eventually came to correspond to the Gregorian months as shown below^[11]:

1. Boldogasszony hava (January, ‘month of the happy/blessed lady’)
2. Böjtelő hava (February, ‘month of early fasting/Lent’ or ‘month before fasting/Lent’)
3. Böjtmás hava (March, ‘second month of fasting/Lent’)
4. Szent György hava (April, ‘Saint George’s month)
5. Pünkösd hava (May, ‘Pentecost month’)
6. Szent Iván hava (June, ‘Saint John [the Baptist]’s month’)
7. Szent Jakab hava (July, ‘Saint James’ month’)
8. Kisasszony hava (August, ‘month of the Virgin’)
9. Szent Mihály hava (September, ‘Saint Michael’s month’)
10. Mindszent hava (October, ‘all saints' month’)
11. Szent András hava (November, ‘Saint Andrew’s month’)
12. Karácsony hava (December, ‘month of Yule/Christmas’)

Old Egyptian calendar[link]

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".

1. Thout
2. Paopi
3. Hathor
4. Koiak
5. Tooba
6. Emshir
7. Paremhat
8. Paremoude
9. Pashons
10. Paoni
11. Epip
12. Mesori

Nisga'a calendar[link]

The Nisga'a Calendar coincides with the Gregorian Calendar with each month referring to the type of Harvesting that is done during the month.^{[citation needed]}

1. K'aliiyee = Going North - referring to the Sun returning to its usual place in the sky
2. Buxwlaks = Needles Blowing About - February is usually a very windy month in the Nass River Valley
3. Xsaak = To Eat Oolichans - Oolichans are harvested during this month
4. Mmaal = Canoes - The river has defrosted, hence canoes are used once more
5. Yansa'alt = Leaves are Blooming - Warm weather has arrived and leaves on the trees begin to bloom
6. Miso'o = Sockeye - majority of Sockeye Salmon runs begin this month
7. Maa'y = Berries - berry picking season
8. Wii Hoon = Great Salmon - referring to the abundance of Salmon that are now running
9. Genuugwwikw = Trail of the Marmot - Marmots, Ermines and animals as such are hunted
10. Xlaaxw = To Eat Trout - trout are mostly eaten this time of year
11. Gwilatkw = To Blanket - The earth is "blanketed" with snow
12. Luut'aa = Sit In - the Sun "sits" in one spot for a period of time

Arabic calendar[link]

See also[link]

Look up month in Wiktionary, the free dictionary.

• Maya calendar
• Chinese calendar
• Egyptian Calendar
• List of months by year: 2000–2050

References[link]

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