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Alunar node is either of the twoorbital nodes of theMoon; that is, the two points at which theorbit of the Moonintersects theecliptic. Theascending (ornorth) node is where the Moon moves into the northernecliptic hemisphere, while thedescending (orsouth) node is where the Moon enters the southern ecliptic hemisphere.
Theline of nodes, which is also the intersection between the two respective planes, rotates (precesses) with a period of 18.6 years or 19.5°year-1. When viewed from the celestial north, the nodes move clockwise around Earth, I.e. with aretrograde motion (opposite to Earth's own spin and its revolution around the Sun). So the time from one node crossing to the next (seeeclipse season) is approximately a half-year minus half of 19.1 days -- or about 173 days.
Because theorbital plane of the Moonprecesses in space, the lunar nodesalso precess around the ecliptic, completing one revolution (called adraconic period ornodal period) in 18.612958 years (6,798.383 days). (This is not the same as asaros of 18.03 years) The same cycle measured against an inertial frame of reference, such asInternational Celestial Reference System (ICRS), acoordinate system relative to thefixed stars, is 18.599525 years.
Alunar eclipse can occur only when thefull Moon is near either lunar node (within 11° 38' ecliptic longitude), while asolar eclipse can occur only when thenew Moon is near either lunar node (within 17° 25').
Both solar eclipses of July 2000 (on the1st and31st days) occurred around the time when the Moon was at its ascending node. Ascending-node eclipses recur after onedraconic year on average, which is about 0.94901Gregorian year, as do descending-node eclipses.
AnEclipse of the Moon or Sun can occur when the nodes align with the Sun, roughly every 173.3 days. Lunar orbit inclination also determines eclipses; shadows cross when nodes coincide with full and new moon when the Sun, Earth, and Moon align in three dimensions.
In effect, this means that the "tropical year" on the Moon is only 347 days long. This is called thedraconic year (oreclipse year). The "seasons" on the Moon fit into this period. For about half of this draconic year, the Sun is north of the lunar equator (but at most 1.543°), and for the other half, it is south of the lunar equator. Obviously, the effect of these seasons is minor compared to the difference between lunar night and lunar day. At the lunar poles, instead of usual lunar days and nights of about 15 Earth days, the Sun will be "up" for 173 days as it will be "down"; polar sunrise and sunset takes 18 days each year. "Up" here means that the centre of the Sun is above the horizon. Lunar polar sunrises and sunsets occur around the time of eclipses (solar or lunar). For example, at theSolar eclipse of March 9, 2016, the Moon was near its descending node, and the Sun was near the point in the sky where the equator of the Moon crosses the ecliptic. When the Sun reaches that point, the center of the Sun sets at the lunar north pole and rises at the lunar south pole.
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The nodes are called by different names in different cultures of the world.
In medievalArabic texts, alongside the sevenclassical planets, it was believed that an eighth pseudo-planet was the cause of solar and lunar eclipses, termedal-Tinnīn (the Dragon) oral-Jawzahr (fromClassical PersianGawzahr).[1] The planet was split into two parts representing the lunar nodes, termed the Head (ra’s) and Tail (dhanab) of the mythological dragon.[2] Similarly, the nodes are termedrosh ha-teli u-zenavo (ראש התלי וזנבו)[verification needed] inHebrew, andcaput draconis (head of the dragon) orcauda draconis (tail of the dragon) inLatin.[3] The ascending node is referred to as thedragon's head with theastronomical orastrological symbol of ☊ and the descending node is known as thedragon's tail with the symbol ☋.
InHindu astronomy, the nodes are considered with theseven planets to form the nineNavagrahas; the ascending node ☊ is calledRahu and the descending node ☋ is calledKetu.[4]Rāhu (Sanskrit: राहु, ) is one of the nine major celestial bodies (navagraha) in Hindu texts and the king of meteors. It represents the ascension of the Moon in its precessional orbit around the Earth, also referred as the north lunar node, and along withKetu, is a "shadow planet" that causes eclipses. Despite having no physical existence, Rahu has been allocated the status of the planet by ancient seers owing to its strong influence in astrology. Rahu is usually paired withKetu, another shadow planet. The time of day considered to be under the influence of Rahu is calledRāhu kāla and is considered inauspicious.
InTibetan astrology (partially based on theKalachakra Tantra) these nodes are respectively namedRahu andKalagni.[5]
Every 18.6 years, the angle between the Moon's orbit and Earth'sequator reaches a maximum of 28°36′, the sum of Earth'sequatorial tilt (23°27′) and the Moon'sorbital inclination (5°09′) to theecliptic. This is calledmajorlunar standstill. Around this time, the Moon'sdeclination will vary from −28°36′ to +28°36′. Conversely, 9.3 years later, the angle between the Moon's orbit and Earth's equator reaches its minimum of 18°20′. This is called aminor lunar standstill. The last lunar standstill was a minor standstill in October 2015. At that time the descending node was lined up with the equinox (the point in the sky havingright ascension zero anddeclination zero). The nodes are moving west by about 19° per year. The Sun crosses a given node about 20 days earlier each year.
When the inclination of the Moon's orbit to the Earth's equator is at its minimum of 18°20′, the centre of the Moon's disk will be above thehorizon every day from latitudes less than 70°43' (90° − 18°20' – 57' parallax) north or south. When the inclination is at its maximum of 28°36', the centre of the Moon's disk will be above the horizon every day only from latitudes less than 60°27' (90° − 28°36' – 57' parallax) north or south.
Athigher latitudes, there will be a period of at least one day each month when the Moon does not rise, but there will also be a period of at least one day each month when the Moon does not set. This is similar to theseasonal behaviour of the Sun, but with a period of 27.2 days instead of 365 days. Note that a point on the Moon can actually be visible when it is about 34arc minutes below the horizon, due toatmospheric refraction.
Because of the inclination of the Moon's orbit with respect to the Earth's equator, the Moon is above the horizon at theNorth andSouth Pole for almost two weeks every month, even though the Sun is below the horizon for six months at a time. The period from moonrise to moonrise at the poles is atropical month, about 27.3 days, quite close to the sidereal period. When the Sun is the furthest below the horizon (winter solstice), the Moon will be full when it is at its highest point. When the Moon is inGemini it will be above the horizon at the North Pole, and when it is inSagittarius it will be up at the South Pole.
The Moon's light is used byzooplankton in the Arctic when the Sun is below the horizon for months and must have been helpful to the animals that lived in Arctic and Antarctic regions when the climate was warmer.
The Moon's orbit is inclined about 5.14° to theecliptic; hence, the Moon can be up to about 5° north or south of the ecliptic. The ecliptic is inclined about 23.44° to thecelestial equator, whose plane isperpendicular to the rotational axis of Earth. As a result, once during the 18.6-year nodal period (when the ascending node of the Moon's orbit coincides with thevernal equinox), the Moon'sdeclination reaches a maximum and minimum (northern and southern extremes): about 28.6° from the celestial equator. Therefore, the moonrise or moonsetazimuth has its northern- and southernmost points on the horizon; the Moon atculmination has its lowest and highestaltitude (when the body transits themeridian); and first sightings of thenew moon potentially have their latest times. Furthermore,occultations by the Moon of thePleiades star cluster, which is over 4° north of the ecliptic, occur during a comparatively brief period once every nodal period.
Theprecession of the lunar nodes has a small effect on Earth'stides –atmospheric,oceanic, orcrustal.[6][7]The U.S.National Oceanic and Atmospheric Administration (NOAA) determinesmean lower low water (MLLW) at a location by averaging the height of the lowest tide recorded at that location each day during a 19-year recording period, known as the National Tidal Datum Epoch.[8] The 19-year recording period is the nearest full-year count to the 18.6-year cycle of the lunar nodes.[9]
In conjunction withsea level rise caused byglobal warming, lunar nodal precession is predicted to contribute to a rapid rise in the frequency ofcoastal flooding throughout the 2030s.[10]