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An orbital node is either of the two points where an
orbit In celestial mechanics, an orbit is the curved trajectory of an object such as the trajectory of a planet around a star, or of a natural satellite around a planet, or of an artificial satellite around an object or position in space such as ...
intersects a plane of reference to which it is inclined. A non-inclined orbit, which is contained in the reference plane, has no nodes.


Planes of reference

Common planes of reference include the following: * For a geocentric orbit,
Earth Earth is the third planet from the Sun and the only astronomical object known to harbor life. While large volumes of water can be found throughout the Solar System, only Earth sustains liquid surface water. About 71% of Earth's sur ...
's equatorial plane. In this case, non-inclined orbits are called ''equatorial''. * For a heliocentric orbit, the ecliptic or
invariable plane The invariable plane of a planetary system, also called Laplace's invariable plane, is the plane passing through its barycenter (center of mass) perpendicular to its angular momentum vector. In the Solar System, about 98% of this effect is con ...
. In this case, non-inclined orbits are called ''ecliptic''. * For an orbit outside the
Solar System The Solar System Capitalization of the name varies. The International Astronomical Union, the authoritative body regarding astronomical nomenclature, specifies capitalizing the names of all individual astronomical objects but uses mixed "Solar ...
, the plane through the primary perpendicular to a line through the observer and the primary (called the ''
plane of the sky Plane(s) most often refers to: * Aero- or airplane, a powered, fixed-wing aircraft * Plane (geometry), a flat, 2-dimensional surface Plane or planes may also refer to: Biology * Plane (tree) or ''Platanus'', wetland native plant * ''Planes' ...
'').


Node distinction

If a reference direction from one side of the plane of reference to the other is defined, the two nodes can be distinguished. For geocentric and heliocentric orbits, the ascending node (or north node) is where the orbiting object moves north through the plane of reference, and the descending node (or south node) is where it moves south through the plane.ascending node
entry in ''The Encyclopedia of Astrobiology, Astronomy, and Spaceflight'', David Darling, on line, accessed May 17, 2007.
In the case of objects outside the Solar System, the ascending node is the node where the orbiting secondary passes away from the observer, and the descending node is the node where it moves towards the observer., p. 137. The position of the node may be used as one of a set of parameters, called '' orbital elements'', which describe the orbit. This is done by specifying the longitude of the ascending node (or, sometimes, the longitude of the node.) The line of nodes is the intersection of the object's orbital plane with the plane of reference. It passes through the two nodes.


Symbols and nomenclature

The symbol of the ascending node is (
Unicode Unicode, formally The Unicode Standard,The formal version reference is is an information technology standard for the consistent encoding, representation, and handling of text expressed in most of the world's writing systems. The standard, ...
: U+260A, ☊), and the symbol of the descending node is (
Unicode Unicode, formally The Unicode Standard,The formal version reference is is an information technology standard for the consistent encoding, representation, and handling of text expressed in most of the world's writing systems. The standard, ...
: U+260B, ☋). In medieval and early modern times the ascending and descending nodes were called the "dragon's head" (
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 ...
: ,
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 ...
: ) and "dragon's tail" ( la, cauda draconis), respectively. These terms originally referred to the times when 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 ...
crossed the apparent path of the sun in the sky. Also, corruptions of the Arabic term such as ''ganzaar'', ''genzahar'', ''geuzaar'' and ''zeuzahar'' were used in the medieval West to denote either of the nodes. The Greek terms and were also used for the ascending and descending nodes, giving rise to the
English English usually refers to: * English language * English people English may also refer to: Peoples, culture, and language * ''English'', an adjective for something of, from, or related to England ** English national ...
words ''anabibazon'' and ''catabibazon''.


Lunar nodes

For the orbit of the Moon around
Earth Earth is the third planet from the Sun and the only astronomical object known to harbor life. While large volumes of water can be found throughout the Solar System, only Earth sustains liquid surface water. About 71% of Earth's sur ...
, the plane is taken to be the ecliptic, not the equatorial plane. The
gravitational In physics, gravity () is a fundamental interaction which causes mutual attraction between all things with mass or energy. Gravity is, by far, the weakest of the four fundamental interactions, approximately 1038 times weaker than the str ...
pull of the Sun upon the Moon causes its nodes to gradually precess westward, completing a cycle in approximately 18.6 years.


See also

* Eclipse * Euler angles * Longitude of the ascending node


References

{{DEFAULTSORT:Orbital Node Technical factors of astrology Orbits