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A lunar node is either of the two orbital nodes of the
Moon The Moon is Earth's only natural satellite. It is the fifth largest satellite in the Solar System and the largest and most massive relative to its parent planet, with a diameter about one-quarter that of Earth (comparable to the width of ...
, that is, the two points at which the orbit of the Moon intersects the ecliptic. The ''ascending'' (or ''north'') node is where the Moon moves into the northern ecliptic hemisphere, while the ''descending'' (or ''south'') node is where the Moon enters the southern ecliptic hemisphere.


Eclipses

A
lunar eclipse A lunar eclipse occurs when the Moon moves into the Earth's shadow. Such alignment occurs during an eclipse season, approximately every six months, during the full moon phase, when the Moon's orbital plane is closest to the plane of the Ear ...
can occur only when the
full Moon The full moon is the lunar phase when the Moon appears fully illuminated from Earth's perspective. This occurs when Earth is located between the Sun and the Moon (when the ecliptic longitudes of the Sun and Moon differ by 180°). This mea ...
is near either lunar node (within 11° 38' ecliptic longitude), while a
solar eclipse A solar eclipse occurs when the Moon passes between Earth and the Sun, thereby obscuring the view of the Sun from a small part of the Earth, totally or partially. Such an alignment occurs during an eclipse season, approximately every six mon ...
can occur only when the
new Moon In astronomy, the new moon is the first lunar phase, when the Moon and Sun have the same ecliptic longitude. At this phase, the lunar disk is not visible to the naked eye, except when it is silhouetted against the Sun during a solar ecl ...
is near either lunar node (within 17° 25'). Both solar eclipses of July 2000 (on the 1st and 31st days) occurred around the time when the Moon was at its ascending node. Ascending-node eclipses recur after one draconic year on average, which is about 0.94901
Gregorian year Gregorian may refer to: *The thought or ideology of Pope Gregory I or Pope Gregory VII (also called ''Gregorianism'') *Things named for Pope Gregory I: **Gregorian chant, the central tradition of Western plainchant, a form of monophonic, unaccom ...
, as do descending-node eclipses.


Precession

Because the orbital plane of the Moon precesses in space, the lunar nodes also precess around the ecliptic, completing one revolution (called a ''draconic'' or '' nodal period'') in . (This is not the same length as a saros.) The same cycle measured against an inertial frame of reference, such as
International Celestial Reference System The International Celestial Reference System (ICRS) is the current standard celestial reference system adopted by the International Astronomical Union (IAU). Its origin is at the barycenter of the Solar System, with axes that are intended to "s ...
(ICRS), a
coordinate system In geometry, a coordinate system is a system that uses one or more numbers, or coordinates, to uniquely determine the position of the points or other geometric elements on a manifold such as Euclidean space. The order of the coordinates is sig ...
relative to the fixed stars, is 18.599525 years.


Names and symbols

The nodes are called by different names in different cultures of the world. In medieval texts, the nodes are referred to as ''ras wa dhanav al-tinnîn'' in
Arabic Arabic (, ' ; , ' or ) is a Semitic language spoken primarily across the Arab world.Semitic languages: an international handbook / edited by Stefan Weninger; in collaboration with Geoffrey Khan, Michael P. Streck, Janet C. E.Watson; Walter ...
. ''rosh ha-teli u-zenavo'' (ראש ה תלי וזנבו) in
Hebrew Hebrew (; ; ) is a Northwest Semitic language of the Afroasiatic language family. Historically, it is one of the spoken languages of the Israelites and their longest-surviving descendants, the Jews and Samaritans. It was largely preserved ...
, and ''caput draconis'' (head of the dragon) or ''cauda draconis'' (tail of the dragon) in
Latin Latin (, or , ) is a classical language belonging to the Italic languages, Italic branch of the Indo-European languages. Latin was originally a dialect spoken in the lower Tiber area (then known as Latium) around present-day Rome, but through ...
. The ascending node is referred to as the dragon's head with the astronomical or astrological symbol of ☊ and the descending node is known as the dragon's tail with the symbol ☋. In Hindu astronomy, the nodes are considered with the seven planets among the nine
Navagrahas Navagraha are nine heavenly bodies and deities that influence human life on Earth according to Hinduism and Hindu astrology. The term is derived from ''nava'' ( sa, नव "nine") and ''graha'' ( sa, ग्रह "planet, seizing, laying hold of, ...
; the ascending node ☊ is called ''
Rahu Rāhu ( Sanskrit: राहु, 16px, ☊) 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 th ...
'' and the descending node ☋ is called '' Ketu''. In
Tibetan astrology Tibetan astrology () is a traditional discipline of the Tibetan peoples that has influence from both Chinese astrology and Indian astrology. Tibetan astrology is one of the 'Ten Sciences' (Wylie: rig-pa'i gnas bcu; Sanskrit: daśavidyā) in the e ...
(partially based on the Kalachakra Tantra) these nodes are respectively named ''Rahu'' and '' Kalagni''.


Declination extremes

The Moon's orbit is inclined about 5.14° to the ecliptic; hence, the Moon can be up to about 5° north or south of the ecliptic. The ecliptic is inclined about 23.44° to the celestial equator, whose plane is
perpendicular In elementary geometry, two geometric objects are perpendicular if they intersect at a right angle (90 degrees or π/2 radians). The condition of perpendicularity may be represented graphically using the '' perpendicular symbol'', ⟂. It c ...
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 the vernal equinox), the Moon's
declination In astronomy, declination (abbreviated dec; symbol ''δ'') is one of the two angles that locate a point on the celestial sphere in the equatorial coordinate system, the other being hour angle. Declination's angle is measured north or south of th ...
reaches a maximum and minimum (northern and southern extremes): about 28.6° from the celestial equator. Therefore, the moonrise or moonset
azimuth An azimuth (; from ar, اَلسُّمُوت, as-sumūt, the directions) is an angular measurement in a spherical coordinate system. More specifically, it is the horizontal angle from a cardinal direction, most commonly north. Mathematical ...
has its northern- and southernmost points on the horizon; the Moon at
culmination In observational astronomy, culmination is the passage of a celestial object (such as the Sun, the Moon, a planet, a star, constellation or a deep-sky object) across the observer's local meridian. These events were also known as meridian tran ...
has its lowest and highest
altitude Altitude or height (also sometimes known as depth) is a distance measurement, usually in the vertical or "up" direction, between a reference datum and a point or object. The exact definition and reference datum varies according to the context ...
(when the body transits the meridian); and first sightings of the
new moon In astronomy, the new moon is the first lunar phase, when the Moon and Sun have the same ecliptic longitude. At this phase, the lunar disk is not visible to the naked eye, except when it is silhouetted against the Sun during a solar ecl ...
potentially have their latest times. Furthermore,
occultation An occultation is an event that occurs when one object is hidden from the observer by another object that passes between them. The term is often used in astronomy, but can also refer to any situation in which an object in the foreground blocks ...
s by the Moon of the
Pleiades The Pleiades (), also known as The Seven Sisters, Messier 45 and other names by different cultures, is an asterism and an open star cluster containing middle-aged, hot B-type stars in the north-west of the constellation Taurus. At a distance ...
star cluster, which is over 4° north of the ecliptic, occur during a comparatively brief period once every nodal period.


Effect on tides

The precession of the lunar nodes has a small effect on Earth's tides
atmospheric An atmosphere () is a layer of gas or layers of gases that envelop a planet, and is held in place by the gravity of the planetary body. A planet retains an atmosphere when the gravity is great and the temperature of the atmosphere is low. A ...
, oceanic, or crustal. The U.S.
National Oceanic and Atmospheric Administration The National Oceanic and Atmospheric Administration (abbreviated as NOAA ) is an United States scientific and regulatory agency within the United States Department of Commerce that forecasts weather, monitors oceanic and atmospheric conditi ...
(NOAA) determines mean 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. The 19-year recording period is the nearest full-year count to the 18.6-year cycle of the lunar nodes. In conjunction with
sea level rise Globally, sea levels are rising due to human-caused climate change. Between 1901 and 2018, the globally averaged sea level rose by , or 1–2 mm per year on average.IPCC, 2019Summary for Policymakers InIPCC Special Report on the Ocean and Cr ...
caused by
global warming In common usage, climate change describes global warming—the ongoing increase in global average temperature—and its effects on Earth's climate system. Climate change in a broader sense also includes previous long-term changes to ...
, lunar nodal precession is predicted to contribute to a rapid rise in the frequency of coastal flooding throughout the 2030s.


See also

*
Eclipse An eclipse is an astronomical event that occurs when an astronomical object or spacecraft is temporarily obscured, by passing into the shadow of another body or by having another body pass between it and the viewer. This alignment of three c ...
* Lunar standstill * Orbit of the Moon


References


External links


Sun and Moon Polar Applet
showing moonrise/moonset azimuths

What are the standstills of the Moon? Retrieved 2016-08-26.



about inertial frame of reference for measuring movements of celestial bodies.

{{DEFAULTSORT:Lunar Node Orbit of the Moon Technical factors of Western astrology Technical factors of Hindu astrology