The month is a unit of time, used with calendars, which was first used and inventedas some 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.
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.
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: sidus): 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.
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.
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.
Sometimes written 'draconitic' month, 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.
This is the average period of the Moon's revolution with respect to the sun. The synodic month is responsible for 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. This means that after completing a sidereal month the Moon must move a little farther to reach the new position of the Earth with respect to 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 29.27 to about 29.83 days. The long-term average duration is 29.530589 days (29 d 12 h 44 min 2.9 s). The synodic month is used in the Metonic cycle.
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:
|anomalistic month||27.554549878 − 0.000000010390 × y days|
|sidereal month||27.321661547 + 0.000000001857 × y days|
|tropical month||27.321582241 + 0.000000001506 × 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 86,400 SI seconds. y is years since the epoch (2000), expressed in Julian years of 365.25 days. Note that 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.
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.
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.
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.
|2||February||28 days, 29 in leap years|
The average month in the Gregorian calendar has a length of 30.4167 days or 4.345 weeks in a non-leap year and 30.5 days or 4.357 weeks in a leap year, or 30.436875 days in a mean Gregorian month overall (365.2425 ÷ 12).
Months existing in the Roman calendar in the past include:
Also, note that the latter 10 months of the year form a pair of 31-30-31-30-31-day 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.
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.
The Baha’i year consists of 19 months of 19 days each (361 days), with the addition of “Intercalary Days” (four in ordinary and five in leap years) between the eighteenth and nineteenth months to adjust the calendar to the solar year. The months are named after the attributes of God. The Baha’i New Year coincides with the March equinox (March 21). The Baha’i Era commenced with the year of the Bab’s declaration (1844 A.D.). Days of the year begin and end at sundown, for example, March 21st starts at sundown on the 20th and ends on the 21st at sundown.
The Hebrew calendar has 12 or 13 months.
Adar 1 is only added 7 times in 19 years. In ordinary years, Adar 2 is simply called Adar.
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:
There are also twelve months in the Islamic calendar. They are named as follows:
See the article Islamic calendar for more information on the Islamic calendar.
The Bangla Calendar follows solar months and it has six seasons. The months and seasons in the calendar are:
Hemanta (Late Autumn)
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.
The names in the solar calendar are just the names of the zodiac sign in which the sun travels. They are
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. The first month is Bak. 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.
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).
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 . 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.
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".
The Nisga'a Calendar coincides with the Gregorian Calendar with each month referring to the type of Harvesting that is done during the month.
MONTH (a common Teutonic word, cf. Ger. Mond, Du. maand, Dan. maaned, &c., and cognate with Lat. mensis, Gr. µ7v, &c., in other branches of the Indo-Germanic family; all ultimately from the root seen in the word for the moon in nearly all those languages), originally the period between two returns of the new moon; generally called a lunar and sometimes a synodic or illuminative month. The anomalistic month is the mean time taken by the moon in passing from one perigee to the next; the sidereal month is the mean time in which the moon makes a circuit among the stars; the tropical month is the mean time in which the moon traverses 360° of longitude; the nodical or draconic month is the mean time taken by the moon in passing from one rising node to the next; the solar month is one-twelfth of a tropical year. The lengths of the various months are: synodic = 29.53059 days; anomalistic = 2 7.554 60, sidereal = 27 32166, tropical = 27.32156, nodical = 27.21222, solar= 30.43685. (For the calendar months see Calendar.) In law a month may mean either a lunar month, that is, a period of twenty-eight days, or a calendar month. At common law, "month" generally means a lunar month, although in mercantile matters it has been generally understood to mean a calendar month, but there is no general exception giving it that meaning in commercial documents. It can only have that meaning where according to the ordinary rules of construction a secondary meaning can be admitted (Bruner v. Moore, 1904, 1 Ch. 305). In bills of exchange or promissory notes month means a calendar month (Bills of Exchange Act, 1882, s. 14 ). Where a servant is engaged subject to a month's notice or payment of a month's wages month is interpreted as a calendar month (Gordon v. Potter, 1 F. & F. 644). In acts of parliament passed before the year 1850 month, unless otherwise specially interpreted, means lunar month, but in all acts passed since that date, month, unless words be added showing that lunar month was intended, means calendar month (Interpretation Act 1889, s. 3). In the rules of the supreme court and in the county court rules month means a calendar month. In mercantile contracts in computing the period of a month the day from which the time is to begin to run is excluded, but in sentences of imprisonment the day on which the sentence begins is included, so that the numerically corresponding day in the month in which the sentence expires would be excluded.
Among the Egyptians the month of thirty days each was in use long before the time of the Exodus, and formed the basis of their calculations. From the time of the institution of the Mosaic law the month among the Jews was lunar. The cycle of religious feasts depended on the moon. The commencement of a month was determined by the observation of the new moon. The number of months in the year was usually twelve (1 Kg 4:7; 1Chr 27:1ff); but every third year an additional month (we-Adar) was inserted, so as to make the months coincide with the seasons.
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A month is an amount of time used with calendars. It is about one twelfth of a year. It is a little longer than a natural period of the moon 29.53 days. In some cultures, the idea for months came from moon phases. People have counted days based on moon phases for a very long time. Some calendars are still strongly based on the moon, or "lunar calendars".
|Months of the Year|