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.
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As a result, the time it takes the Moon to return to the same node is shorter than a sidereal month, lasting days (27 d 5 h 5 m 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.
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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.
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The Moon's appearance is considerably more complex. Its motion, like the Sun, is between two limits—known as lunistices rather than solstices. However, its travel between lunistices is considerably faster. It takes a sidereal month to complete its cycle rather than the year-long trek of the Sun.
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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.
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Each month began with a new moon; the weeks were nine days long The month names on this artifact are expressed in symbols, based on natural phenomena and agricultural cycles.Cosmology of the ancient Balts (i.e. the sidereal month was divided into three parts). The Julian calendar was used in the Grand Duchy of Lithuania; the Gregorian calendar was adopted by the Polish-Lithuanian Commonwealth in 1586, a few years after its promulgation in 1582 by Pope Gregory XIII.
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The periods are derived from polynomial expressions for Delaunay's arguments used in lunar theory, as listed in Table 4 of Chapront, Chapront-Touzé & Francou (2002): W1 is the ecliptic longitude of the Moon w.r.t. the fixed ICRS equinox: its period is the sidereal month. If we add the rate of precession to the sidereal angular velocity, we get the angular velocity w.r.t. the equinox of the date: its period is the tropical month, which is rarely used.
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The 28 Lunar Mansions, or more precisely lodgings () are the Chinese and East Asian form of the lunar stations. They can be considered as the equivalent to the Western zodiac, although the 28 stations reflect the movement of the Moon through a sidereal month rather than the Sun in a tropical year. In their final form, they embodied the astral forms of the Four Symbols: two real and two legendary animals important in traditional Chinese culture, such as feng shui.
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A lunar station, also called a lunar mansion or lunar house, is a segment of the ecliptic through which the Moon passes in its orbit around the Earth. The concept was used by several ancient cultures as part of their calendrical system. In general, though not always, the zodiac is divided into 27 or 28 segments relative to the fixed stars – one for each day of the lunar month. (A sidereal month lasts about 27.3 days.) The Moon's position is charted with respect to those fixed segments.
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The method relies on the relatively quick movement of the moon across the background sky, completing a circuit of 360 degrees in 27.3 days (the sidereal month), or 13.2 degrees per day. In one hour it will move approximately half a degree, roughly its own angular diameter, with respect to the background stars and the Sun. Using a sextant, the navigator precisely measures the angle between the moon and another body. That could be the Sun or one of a selected group of bright stars lying close to the Moon's path, near the ecliptic.
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The Moon is an exceptionally large natural satellite relative to Earth: Its diameter is more than a quarter and its mass is 1/81 of Earth's. It is the largest moon in the Solar System relative to the size of its planet, though Charon is larger relative to the dwarf planet Pluto, at 1/9 Pluto's mass. The Earth and the Moon's barycentre, their common center of mass, is located (about a quarter of Earth's radius) beneath Earth's surface. The Earth revolves around the Earth-Moon barycentre once a sidereal month, with 1/81 the speed of the Moon, or about per second.
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The Celts used periods of darkness such as night and winter to begin their calculations of time. This meant that the first period of time in a "week" was a night, followed by a day. Further, they also counted the ending night period, giving rise to periods of time with more nights than days. In Irish, the term nómad is used to signify a small number of days and is exactly the length of the nine night week as in co cend nomaide - a period of time with nine nights divided nicely into a sidereal month of 27 nights.
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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 (): days (27 d 7 h 43 m 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.
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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.
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The annual path that the Sun appears to follow against the background of relatively fixed stars is known as the ecliptic. Since the Moon's orbit is inclined 5.14° to the ecliptic, the Moon will always remain within about 5° north or south of the ecliptic. For half of a sidereal month (with respect to the stars), the Moon is either north or south of the ecliptic. The two points where the Moon's orbit intersects that of Earth are called the lunar nodes; at the ascending node, the Moon moves north of the ecliptic, while at the descending node, the satellite moves south.
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The nodal period (or draconic period) of a satellite is the time interval between successive passages of the satellite through either of its orbital nodes, typically the ascending node. This type of orbital period applies to artificial satellites, like those that monitor weather on Earth, and natural satellites like the Moon. It is distinct from the sidereal period, which measures the period with respect to reference stars seemingly fixed onto a spherical background, since the location of a satellite's nodes precess over time. For example, the nodal period of the Moon is 27.2122 days (one draconic month), while its sidereal period is 27.3217 days (one sidereal month).
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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 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 pass twice a "fixed" star (different stars give different results because all have a very small proper motion and are not really fixed in position).
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The main reason is that during the time that the Moon has completed an orbit around the Earth, the Earth (and Moon) have completed about of their orbit around the Sun: the Moon has to make up for this in order to come again into conjunction or opposition with the Sun. Secondly, the orbital nodes of the Moon precess westward in ecliptic longitude, completing a full circle in about 18.60 years, so a draconic month is shorter than a sidereal month. In all, the difference in period between synodic and draconic month is nearly days. Likewise, as seen from the Earth, the Sun passes both nodes as it moves along its ecliptic path.
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Given the date of an eclipse, one saros later a nearly identical eclipse can be predicted. During this 18-year period, about 40 other solar and lunar eclipses take place, but with a somewhat different geometry. One saros equaling 18.03 years is not equal to a perfect integer number of lunar orbits (Earth revolutions with respect to the fixed stars of 27.32166 days sidereal month), therefore, even though the relative geometry of the Earth–Sun–Moon system will be nearly identical after a saros, the Moon will be in a slightly different position with respect to the stars for each eclipse in a saros series. The axis of rotation of the Earth–Moon system exhibits a precession period of 18.59992 years.
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The twenty-eight mansions of the Chinese astronomy The Twenty-Eight Mansions (), ', ' Gary D. Thompson chapter 11-24 or 'Richard Hinckley Allen in Star Names: Their Lore and Meaning are part of the Chinese constellations system. They can be considered as the equivalent to the zodiacal constellations in Western astronomy, though the Twenty-eight Mansions reflect the movement of the Moon through a sidereal month rather than the Sun in a tropical year. The lunar mansion system was in use in other parts of East Asia, such as ancient Japan; the Bansenshukai, written by Fujibayashi Yasutake, mentions the system several times and includes an image of the twenty-eight mansions. Another similar system, called Nakshatra, is used in traditional Indian astronomy.
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This longer period is called the anomalistic month and has an average length of days (27 d 13 h 18 m 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.
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Apsidal precession occurs when the direction of the major axis of the Moon's elliptic orbit rotates once every 8.85 years. The second kind of precession of the Moon's orbit is that of the major axis of the Moon's elliptic orbit (the line of the apsides from perigee to apogee), which precesses eastward by 360° in approximately 8.85 years. This is the reason that an anomalistic month (the period the Moon moves from the perigee to the apogee and to the perigee again) is longer than the sidereal month (the period the Moon takes to complete one orbit with respect to the fixed stars). This apsidal precession completes one rotation in the same time as the number of sidereal months exceeds the number of anomalistic months by exactly one, after about 3,233 days (8.85 years).
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A lunar standstill is the gradually varying range between the northern and the southern limits of the Moon's declination, or the lunistices, over the course of one-half a sidereal month (about two weeks), or 13.66 days. (Declination is a celestial coordinate measured as the angle from the celestial equator, analogous to latitude.) One major, or one minor, lunar standstill occurs every 18.6 years due to the precessional cycle of the lunar nodes at that rate. At a major lunar standstill, the Moon's range of declination, and consequently its range of azimuth at moonrise and moonset, reaches a maximum. As a result, viewed from the middle latitudes, the Moon's altitude at upper culmination (the daily moment when the object appears to contact the observer's meridian) changes in just two weeks - from highest to lowest above the horizon due north or south, depending on the observer's hemisphere.
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The modelled rotational period of the Moon pointer (averaged over a year) is 27.321 days, compared to the modern length of a lunar sidereal month of 27.321661 days. As mentioned, the pin/slot driving of the k1/k2 gears varies the displacement over a year's time, and the mounting of those two gears on the e3 gear supplies a precessional advancement to the ellipticity modelling with a period of 8.8826 years, compared with the current value of precession period of the moon of 8.85 years. The system also models the phases of the Moon. The Moon pointer holds a shaft along its length, on which is mounted a small gear named r, which meshes to the Sun pointer at B0 (the connection between B0 and the rest of B is not visible in the original mechanism, so whether b0 is the current date/mean Sun pointer or a hypothetical true Sun pointer is not known).
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