Month

The following types of months are mainly of significance in astronomy. Most of them (but not the distinction between sidereal and tropical months) were 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 twice pass 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.^{[citation needed]} 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 solar days: a 30-day full month followed by a 29-day hollow month — but this is only roughly accurate and regularly needs intercalation (correction) by a leap day.

Additionally, the synodic month does not fit easily into the solar (or 'tropical') year, which makes accurate, rule-based lunisolar calendars that combine the two cycles 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 6,940 days): 12 years have 12 lunar months, and 7 years are 13 lunar months long. However, a Metonic calendar based year will drift against the seasons by about one day every 2 centuries. Metonic calendars include the calendar used in the Antikythera Mechanism about 21 centuries ago, and the Hebrew calendar.

Alternatively in a pure lunar calendar, years are defined as having always 12 lunations, so a year is 354 or 355 days long: the Islamic calendar is the prime example. Consequently, an Islamic year is about 11 days shorter than a solar year and cycles through the seasons in about 33 solar = 34 lunar years: the Islamic New Year has a different Gregorian calendar date in each (solar) year.

Purely solar calendars often 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, or are purely conventional like in the widely used Gregorian calendar.

The complexity required in an accurate lunisolar calendar may explain why solar calendars have generally replaced lunisolar and lunar calendars for civil use in most societies.

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 defined by observation cannot be accurately predicted.

While some like orthodox Islam and the Jewish Karaites still rely on actual moon observations, reliance on astronomical calculations and tabular methods is increasingly common in practice.^[2][3]

There are 12 months and an additional leap year month in the Ahom sexagenary calendar known as Lak-ni.^[4][5] The first month is Duin Shing.^[4]

The Roman calendar was reformed several times, the last three enduring reforms during historical times. The last three reformed Roman calendars are called the Julian, Augustan, and Gregorian; all had the same number of days in their months. Despite other attempts, the names of the months after the Augustan calendar reform have persisted, and the number of days in each month (except February) have remained constant since before the Julian reform. The Gregorian calendar, like the Roman calendars before it, has twelve months, whose Anglicized names are:

Order	Name	Number of days
1	January	31
2	February	28 29 in leap years
3	March	31
4	April	30
5	May	31
6	June	30
7	July formerly Quinctilis	31
8	August formerly Sextilis	31
9	September	30
10	October	31
11	November	30
12	December	31

The famous mnemonic Thirty days hath September is a common way of teaching the lengths of the months in the English-speaking world. 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 30 days long, with variable February being the remembered exception. When the knuckle of the index finger is reached (July), go over to the first knuckle on the other fist, held next to the first (or go back to the first knuckle) and continue with August. This physical mnemonic has been taught to primary school students for many decades, if not centuries.^[6][7]

This cyclical pattern of month lengths matches the musical keyboard alternation of wide white keys (31 days) and narrow black keys (30 days). The note F corresponds to January, the note F^♯ corresponds to February, the exceptional 28–29 day month, and so on.

The mean month-length in the Gregorian calendar is 30.436875 days.

Any five consecutive months that do not include February contain 153 days.

Months in the pre-Julian Roman calendar included:

• Intercalaris an intercalary month occasionally embedded into February, to realign the calendar.
• Quintilis, later renamed to Julius in honour of Julius Caesar.
• Sextilis, later renamed to Augustus in honour of Augustus.

The Romans divided their months into three parts, which they called the calends, the nones, and the ides. Their system is somewhat intricate. 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,^[a] and before Julius Caesar's reform fell sixteen days (two Roman weeks) after the ides (except the ides of February and the intercalary month).

Within a month, the following dates fall on the same day of the week:

• 01, 08, 15, 22, and 29 (e.g., in January 2022, all these dates fell on a Saturday)
• 02, 09, 16, 23, and 30 (e.g., in January 2022, all these dates fell on a Sunday)
• 03, 10, 17, 24, and 31 (e.g., in January 2022, all these dates fell on a Monday)
• 04, 11, 18, and 25 (e.g., in January 2022, all these dates fell on a Tuesday)
• 05, 12, 19, and 26 (e.g., in January 2022, all these dates fell on a Wednesday)
• 06, 13, 20, and 27 (e.g., in January 2022, all these dates fell on a Thursday)
• 07, 14, 21, and 28 (e.g., in January 2022, all these dates fell on a Friday)

Some months have the same date/weekday structure.

In a non-leap year:

• January/October (e.g., in 2022, they began on a Saturday)
• February/March/November (e.g., in 2022, they began on a Tuesday)
• April/July (e.g., in 2022, they began on a Friday)
• September/December (e.g., in 2022, they began on a Thursday)
• 1 January and 31 December fall on the same weekday (e.g. in 2022 on a Saturday)

In a leap year:

• February/August (e.g., in 2020, they began on a Saturday)
• March/November (e.g., in 2020, they began on a Sunday)
• January/April/July (e.g., in 2020, they began on a Wednesday)
• September/December (e.g., in 2020, they began on a Tuesday)
• 29 February (the leap day) falls on the same weekday like 1, 8, 15, 22 February and 1 August (see above; e.g. in 2020 on a Saturday)

The Hebrew calendar has 12 or 13 months.

1. Nisan, 30 days ניסן
2. Iyar, 30 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. Marcheshvan, 29/30 days מַרְחֶשְׁוָן
9. Kislev, 30/29 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.

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

1. Muharram (Restricted/sacred) 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 or Last 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 or Last 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.

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

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

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

The Baháʼí calendar is the calendar used by the Baháʼí 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).^[8] The months are named after the attributes of God. Days of the year begin and end at sundown.^[8]

The Iranian / Persian calendar, currently used in Iran, also has 12 months. The Persian names are included in the parentheses. It begins on the northern Spring equinox.

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)

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

The months in the Nanakshahi calendar are:^[9]

Different from the Hindu calendar, the Khmer calendar consists of both a lunar calendar and a solar calendar. The solar is used more commonly than the lunar calendar.

The Khmer lunar calendar most often contains 12 months; however, the eighth month is repeated (as a "leap month") every two or three years, making 13 months instead of 12.^[10] Each lunar month has 29 or 30 days. The year normally has then 354 or 384 days (when an intercalary month is added), but the calendar follows the rules of the Gregorian calendar to determine leap years and add a lead day to one month, so the Khmer lunar year may have a total of 354, 355, 384 or 385 days.

The Tongan calendar is based on the cycles of the Moon around the Earth in one year. The 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

Pingelapese, a language from Micronesia, also uses a lunar calendar. There are 12 months associated with their calendar. The Moon first appears in March,^{[clarification needed]} 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.^[11]

The Sinhalese calendar is the Buddhist calendar in Sri Lanka with Sinhala names. Each full moon Poya day marks the start of a Buddhist lunar month.^[12] The first month is Bak.^[13]

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

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")

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

1. Torsmånad (January, 'Torre's month' (ancient god))
2. Göjemånad (February, 'Goe's month' (ancient goddess))
3. Vårmånad (March, 'Spring month')
4. Gräsmånad (April, 'Grass month')
5. Blomstermånad (May, 'Bloom month')
6. Sommarmånad (June, 'Summer month')
7. Hömånad (July, 'Hay month')
8. Skördemånad, Rötmånad (August, 'Harvest month' or 'Rot month')
9. Höstmånad (September, 'Autumn month')
10. Slaktmånad (October, 'Slaughter month')
11. Vintermånad (November, 'Winter month')
12. Julmånad (December, 'Christmas month')

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.^[14] His Old English month names are probably written as pronounced in Bede's native Northumbrian dialect. The months were 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 whole month took its name.

Old English month names from Bede's The Reckoning of Time

Year order	Northumbrian Old English	Modern English transliteration	Roman equivalent
1	Æfterra-ġēola mōnaþ	"After-Yule month"	January
2	Sol-mōnaþ	"Sol month"	February
3	Hrēð-mōnaþ	"Hreth month"	March
4	Ēostur-mōnaþ	"Ēostur month"	April
5	Ðrimilce-mōnaþ	"Three-milkings month"	May
6	Ærra-Liþa	"Ere-Litha"	June
7	Æftera-Liþa	"After-Litha"	July
8	Weōd-mōnaþ	"Weed month"	August
9	Hāliġ-mōnaþ or Hærfest-mōnaþ	"Holy month" or "Harvest month"	September
10	Winter-fylleþ	"Winter-filleth"	October
11	Blōt-mōnaþ	"Blót month"	November
12	Ærra-ġēola mōnaþ	"Ere-Yule"	December

When an intercalary month was needed, a third Litha month was inserted in mid-summer.^[14]

The Coligny calendar (Gaulish/Celtic) is an Iron Age Metonic lunisolar calendar, with 12 lunar months of either 29 or 30 days. The lunar month is calculated to a precision of within 24 hours of the lunar phase, achieved by a particular arrangement of months, and the month of EQUOS having a variable length of 29 or 30 days to adjust for any lunar slippage. This setup means the calendar could stay precisely aligned to its lunar phase indefinitely.

The lunar month is divided into two halves, the first of 15 days and the second of 14 or 15 days. The month is calculated to start at the first quarter moon, with the full moon at the centre of the first half-month and the dark moon at the centre of the second half-month. The calendar does not rely on unreliable visual sightings.

An intercalary lunar month is inserted before every 30 lunar months to keep in sync with the solar year. Every 276 years this adds one day to the solar point, so if for example the calendar was 1,000 years old, it would only have slipped by less than 4 days against the solar year.

Nagyszombati kalendárium (in Latin: Calendarium Tyrnaviense) from 1579. Historically Hungary used a 12-month calendar that appears to have been zodiacal in nature^[15] but eventually came to correspond to the Gregorian months as shown below:^[16]

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')

1. Leden – derives from 'led' (ice)
2. Únor – derives from 'nořit' (to dive, referring to the ice sinking into the water due to melting)
3. Březen – derives from 'bříza' (birch)
4. Duben – derives from 'dub' (oak)
5. Květen – derives from 'květ' (flower)
6. Červen – derives from 'červená' (red – for the color of apples and tomatoes)
7. Červenec – is the second 'červen' (formerly known as 2nd červen)
8. Srpen – derives from old Czech word 'sirpsti' (meaning to reflect, referring to the shine on the wheat)
9. Září – means 'to shine'
10. Říjen – derives from 'jelení říje', which refers to the estrous cycle of female elk
11. Listopad – falling leaves
12. Prosinec – derives from old Czech 'prosiněti', which means to shine through (refers to the sun light shining through the clouds)^[17]

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.^[18] 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

The Nisga'a calendar coincides with the Gregorian calendar with each month referring to the type of harvesting that is done during the month.^[19]

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

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

Ojibwe month names^[b] are based on the key feature of the month. Consequently, months between various regions have different names based on the key feature of each month in their particular region. In the Eastern Ojibwe, this can be seen in when the sucker makes its run, which allows the Ojibwe to fish for them. Additionally, Rhodes^[20] also informs of not only the variability in the month names, but how in Eastern Ojibwe these names were originally applied to the lunar months the Ojibwe originally used, which was a lunisolar calendar, fixed by the date of Akiinaaniwan (typically December 27) that marks when sunrise is the latest in the Northern Hemisphere.

Roman Month	Month in Eastern Ojibwe[b]	English translation	Original order in the Ojibwa year	Starting at the first full moon after:
January in those places that have a sucker run during that time	n[a]mebin-giizis	sucker moon	1	Akiinaaniwan on 27 December
	n[a]meb[i]ni-giizis
February	[o]naab[a]ni-giizis	Crust-on-the-snow moon	2	25 January
March	zii[n]z[i]baak[wa]doke-giizis	Sugaring moon	3	26 February
April in those places that have a sucker run during that time	n[a]mebin-giizis	sucker moon	4	25 March
	n[a]meb[i]ni-giizis
April in those places that do not have a sucker run during that time	waawaas[a]gone-giizis	Flower moon
May in those places that have an April sucker run
May in those places that have a January sucker run	g[i]tige-giizis	Planting moon	5	24 April
June in those places that have an April sucker run
June in those places that have a January sucker run	[o]deh[i]min-giizis	Strawberry moon	6	23 May
July	miin-giizis	Blueberry moon	7	22 June
August	[o]dat[a]gaag[o]min-giizis	Blackberry moon	8	20 July
September	m[an]daamin-giizis	Corn moon	9	18 August
October	b[i]naakwe-giizis	Leaves-fall moon	10	17 September
	b[i]naakwii-giizis	Harvest moon
November	g[a]shkadin-giizis	Freeze-up moon	11	16 October
December	g[i]chi-b[i]boon-giizis	Big-winter moon	12	15 November
January in those places that do not have a sucker run during that time	[o]shki-b[i]boon-gii[zi]soons	Little new-winter moon	13 (leap month)	only used if the new moon after g[i]chi-b[i]boon-giizis occurs before Akiinaaniwan on 27 December.

Look up month in Wiktionary, the free dictionary.

• Assyrian calendar
• Chinese calendar
• Egyptian calendar
• Ethiopian calendar
• French Republican calendar
• Kurdish calendar
• Lunar month
• Maya calendar
• Month of year