Anorbital node is either of the two points where anorbitintersects aplane of reference to which it is inclined.^[1] Anon-inclined orbit, which iscontained in the reference plane, has no nodes.

Commonplanes of reference include the following:

• For ageocentric orbit,Earth'sequatorial plane. In this case, non-inclined orbits are calledequatorial.^[2]
• For aheliocentric orbit, theecliptic orinvariable plane. In this case, non-inclined orbits are calledecliptic.^[2]
• For an orbit outside theSolar System, the plane through theprimary perpendicular to a line through the observer and the primary (called theplane of the sky).^[3]

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, theascending node (ornorth node) is where the orbiting object moves north through the plane of reference, and thedescending node (orsouth node) is where it moves south through the plane.^[4] 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.^{[5], p. 137.}

The position of the node may be used as one of a set of parameters, calledorbital elements, which describe the orbit. This is done by specifying thelongitude of the ascending node (or, sometimes, thelongitude of the node.)

Theline of nodes is the straight line resulting from the intersection of the object's orbital plane with the plane of reference; it passes through the two nodes.^[2]

The symbol of the ascending node is  (Unicode: U+260A, ☊), and the symbol of the descending node is  (Unicode: U+260B, ☋).

In medieval and early modern times, the ascending and descending nodes of the Moon in the ecliptic plane were called the "dragon's head" (Latin:caput draconis,Arabic:رأس الجوزهر) and "dragon's tail" (Latin:cauda draconis), respectively.^{[6]: p.141,  [7]: p.245} These terms originally referred to the times when theMoon crossed the apparent path of the sun in the sky (as in asolar eclipse). Also, corruptions of the Arabic term such asganzaar,genzahar,geuzaar andzeuzahar were used in the medieval West to denote either of the nodes.^{[8]: pp.196–197,  [9]: p.65,  [10]: pp.95–96}

TheKoine Greek termsαναβιβάζων andκαταβιβάζων were also used for the ascending and descending nodes, giving rise to the English termsanabibazon andcatabibazon.^{[11][12]:  ¶27}

For theorbit of the Moon aroundEarth, theplane is taken to be theecliptic, not theequatorial plane. Thegravitational pull of theSun upon the Moon causesits nodes to graduallyprecess westward, completing a cycle in approximately 18.6 years.^[1][13]

The image of the ascending and descending orbital nodes as the head and tail of a dragon, 180 degrees apart in the sky, goes back to the Chaldeans; it was used by the Zoroastrians, and then by Arabic astronomers and astrologers. In Middle Persian, its head and tail were respectively calledgōzihr sar andgōzihr dumb; in Arabic,al-ra's al-jawzihr andal-dhanab al-jawzihr — or in the case of the Moon, ___ al-tennin.^[14] Among the arguments against astrologers made byIbn Qayyim al-Jawziyya (1292–1350), in hisMiftah Dar al-SaCadah: "Why is it that you have given an influence toal-Ra's [the head] andal-Dhanab [the tail], which are two imaginary points [ascending and descending nodes]?"^[15]

• Eclipse
• Euler angles
• Longitude of the ascending node