This article deals with the definitions of a 'month' that are mainly of significance in astronomy. For other definitions, including a description of a month in the calendars of different cultures around the world, see: month.
In Shona, Middle-Eastern, and European traditions, the month starts when the young crescent moon first becomes visible, at evening, after conjunction with the Sun one or two days before that evening (e.g., in the Islamic calendar). In ancient Egypt the lunar month began on the day when the waning moon could no longer be seen just before sunrise. Others run from full moon to full moon.
Yet others use calculation, of varying degrees of sophistication, e.g., the Hebrew calendar or the ecclesiastical lunar calendar. Calendars count integer days, so months may be 29 or 30 days in length, in some regular or irregular sequence. Lunar cycles are prominent, and calculated with great precision, in the ancient Hindu 'Panchang' Calendar, widely used in the Indian sub continent. In India the month from conjunction to conjunction is divided into thirty parts known as tithis. A tithi is between 19 to 26 hours long. The date is named after the tithi ruling at sunrise. When the tithi is shorter than the day, the tithi may jump. This case is called xaya or loap. Conversely a tithi may 'stall' as well, that is - the same tithi is associated with two consecutive days. This is known as vriddhi.
In English common law, a "lunar month" traditionally meant exactly 28 days or four weeks, thus a contract for 12 months ran for exactly 48 weeks. In the United Kingdom, the lunar month was formally replaced by the calendar month for deeds and other written contracts by the Law of Property Act 1925 and for all other legal purposes by the Interpretation Act 1978.
Most of the following types of lunar month, except the distinction between the sidereal and tropical months, were first recognized in Babylonian lunar astronomy.
The period of the Moon's orbit as defined with respect to the celestial sphere of apparently fixed stars (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 similar position among the stars (Latin: sidera): 27.321661 days (27 d 7 h 43 min 11.6 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.
The synodic month (Greek: συνοδικός, synodikós, meaning "pertaining to a synod, i.e., a meeting"; in this case, of the Sun and the Moon) is the average period of the Moon's orbit with respect to the line joining the Sun and Earth. This is the period of the lunar 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, appearing to move with respect to the stars since the previous month. Therefore, the synodic month takes 2.2 days longer than the sidereal month. Thus, about 13.37 sidereal months, but about 12.37 synodic months, occur in a Gregorian year.
Since Earth's orbit around the Sun is elliptical and not circular, the speed of Earth's progression around the Sun varies during the year. Thus, the angular rate is faster nearer periapsis and slower near apoapsis. The same is so for the Moon's orbit around the Earth. Because of these variations in angular rate, the actual time between lunations may vary from about 29.18 to about 29.93 days. The long-term average duration is 29.530587981 days (29 d 12 h 44 min 2.8016 s). The synodic month is used to calculate eclipse cycles.
It is customary to specify positions of celestial bodies with respect to the vernal equinox. Because of Earth's precession of the equinoxes, this point moves back slowly along the ecliptic. Therefore, it takes the Moon less time to return to an ecliptic longitude of 0° than to the same point amid the fixed stars: 27.321582 days (27 d 7 h 43 min 4.7 s). This slightly shorter period is known as the tropical month; compare the analogous tropical year.
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), rotates once (apsidal precession) in about 3,233 days (8.85 years). It takes the Moon longer to return to the same apsis because it has 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, so 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, as well as the period after which the apsides point to the Sun again.
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 or draconitic month is also known as a nodal or nodical month. The name draconic refers to a mythical dragon, said to live in the lunar nodes and eat the Sun or Moon during an eclipse. A solar or lunar eclipse is possible only when the Moon is at or near either of the two points where its orbit crosses the ecliptic plane; i.e., the satellite is at or near either of its orbital nodes.
The orbit of the Moon lies in a plane that is inclined about 5.14° with respect to the ecliptic plane. The line of intersection of these planes passes through the two points at which the Moon's orbit crosses the ecliptic plane: the ascending node, where the Moon enters the Northern Celestial Hemisphere, and the descending node, where the Moon moves into the Southern.
The draconic or nodical month is the average interval between two successive transits of the Moon through the same node. Because of the torque exerted by the Sun's gravity on the angular momentum of the Earth–Moon system, the plane of the Moon's orbit gradually rotates westward, which means the nodes gradually rotate around Earth. As a result, the time it takes the Moon to return to the same node is shorter than a sidereal month, lasting 27.212220 days (27 d 5 h 5 min 35.8 s). The line of nodes of the Moon's orbit precesses 360° in about 6,798 days (18.6 years).
A draconic month is shorter than a sidereal month because the nodes precess in the opposite direction to that in which the Moon is orbiting Earth, one rotation every 18.6 years. Therefore, the Moon returns to the same node slightly earlier than it returns to meet the same reference star.
Regardless of the culture, all lunar months approximate the mean length of the synodic month, the average period the Moon takes to cycle through its phases (new, first quarter, full, last quarter) and back again: 29–30 days. The Moon completes one orbit around Earth every 27.3 days (a sidereal month), but due to Earth's orbital motion around the Sun, the Moon does not yet finish a synodic cycle until it has reached the point in its orbit where the Sun is in the same relative position.
|Month type||Length in days|
|anomalistic||27.554549878 − 0.000000010390 × Y|
|sidereal||27.321661547 + 0.000000001857 × Y|
|tropical||27.321582241 + 0.000000001506 × Y|
|draconic||27.212220817 + 0.000000003833 × Y|
|synodic||29.530588853 + 0.000000002162 × Y|
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 calendric 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.
Apart from the long term (millennial) drift in these values, all these periods vary continually around their mean values because of the complex orbital effects of the Sun and planets affecting its motion.
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In medieval times, the part of the Moon's orbit south of the ecliptic was known as the 'dragon' (which devoured the Moon during eclipses) and from this we get the terminology 'dragon's head' for the ascending node and 'dragon's tail' for the descending node. … The periods between successive nodes has, over time, been termed the dracontic, draconic and draconitic month, the words deriving from the Greek for 'dragon'.
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The nodical month is the time in which the Moon accomplishes a revolution with respect to her nodes, the line of which is also movable.
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