Месяц: от лунных циклов до современного календаря

Unit of time about as long the orbital period of the Moon
the general concept
A month is a unit of time, used with calendars, that is approximately as long as a natural orbital period of the Moon; the words month and Moon are cognates. The traditional concept of months arose with the cycle of Moon phases; such lunar months ("lunations") are synodic months and last approximately 29.53 days, making for roughly 12.37 such months in one Earth year. 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. Calendars that developed from the Roman calendar system, such as the internationally used Gregorian calendar, divide the year into 12 months, each of which lasts between 28 and 31 days. The names of the months were Anglicized from various Latin names and events important to Rome, except for the months 9–12, which are named after the Latin numerals 7–10 (septem, octo, novem, and decem) because they were originally the seventh through tenth months in the Roman calendar. In the modern Gregorian calendar, the only month with a variable number of days is the second month, February, which has 29 days during a leap year and 28 days otherwise.

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. 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). 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. 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. 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. 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.

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.

There are 12 months and an additional leap year month in the Ahom sexagenary calendar known as Lak ni. The first month is Duin Shing. No. Name Ahom script Gregorian Month 1 Duin Shing 𑜓𑜢𑜤𑜃𑜫 𑜋𑜢𑜂𑜫 November December 2 Duin Kam 𑜓𑜢𑜤𑜃𑜫 𑜀𑜪 December January 3 Duin Tsam 𑜓𑜢𑜤𑜃𑜫 𑜏𑜪 January February 4 Duin Shi 𑜓𑜢𑜤𑜃𑜫 𑜏𑜢 February March 5 Duin Ha 𑜓𑜢𑜤𑜃𑜫 𑜑𑜡 March April 6 Duin Rok 𑜓𑜢𑜤𑜃𑜫 𑜍𑜤𑜀𑜫 April May 7 Duin Shit 𑜓𑜢𑜤𑜃𑜫 𑜋𑜢𑜄𑜫 May June 8 Duin paet 𑜓𑜢𑜤𑜃𑜫 𑜆𑜢𑜤𑜄𑜫 June July 9 Duin kauo 𑜓𑜢𑜤𑜃𑜫 𑜀𑜰𑜫 July August 10 Duin sip 𑜓𑜢𑜤𑜃𑜫 𑜏𑜢𑜆𑜫 August September 11 Duin tsip it 𑜓𑜢𑜤𑜃𑜫 𑜏𑜢𑜆𑜫 𑜒𑜪𑜄𑜫 September October 12 Duin sip song 𑜓𑜢𑜤𑜃𑜫 𑜏𑜢𑜆𑜫 𑜁 October November

The mean month length in the Gregorian calendar is 30.436875 days. Any five consecutive months, that do not include February, contain 153 days.

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). The months are named after the attributes of God. Days of the year begin and end at sundown. No. Name Punjabi Days Julian months 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

English name Thai name Abbr. Transcription Sanskrit word Zodiac signJanuary มกราคม ม.ค. mokarakhom makara "sea monster" CapricornFebruary กุมภาพันธ์ ก.พ. kumphaphan kumbha "pitcher, water pot" AquariusMarch มีนาคม มี.ค. minakhom mīna "(a specific kind of) fish" PiscesApril เมษายน เม.ย. mesayon meṣa "ram" AriesMay พฤษภาคม พ.ค. phruetsaphakhom vṛṣabha "bull" TaurusJune มิถุนายน มิ.ย. mithunayon mithuna "a pair" GeminiJuly กรกฎาคม ก.ค. karakadakhom karkaṭa "crab" CancerAugust สิงหาคม ส.ค. singhakhom siṃha "lion" LeoSeptember กันยายน ก.ย. kanyayon kanyā "girl" VirgoOctober ตุลาคม ต.ค. tulakhom tulā "balance" LibraNovember พฤศจิกายน พ.ย. phruetsachikayon vṛścika "scorpion" ScorpioDecember ธันวาคม ธ.ค. thanwakhom dhanu "bow, arc" Sagittarius

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.