Orbital node

The ascending node.

An orbital node is one of the two points where an orbit crosses a plane of reference to which it is inclined.^[1] An orbit which is contained in the plane of reference (called non-inclined) has no nodes.

Planes of reference

Common planes of reference include:

• For a geocentric orbit, the Earth's equatorial plane. In this case, non-inclined orbits are called equatorial.^[2]
• For a heliocentric orbit, the ecliptic. In this case, non-inclined orbits are called ecliptic.^[2]
• For an orbit outside the Solar System, the plane through the primary perpendicular to a line through the observer and the primary (called the plane of the sky).^{[3], chap. 17.}

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.^[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, 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.^[2]

Symbols and nomenclature

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 were called the dragon's head (Latin: caput draconis, Arabic: ra's al-jauzahar) and dragon's tail (Latin: cauda draconis), respectively.^{[6], p. 141; [7], p. 245.} These terms originally referred to the times when the moon 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.^{[8], pp. 196–197; [9], p. 65; [10], pp. 95–96.} The Greek terms αναβιβάζων and καταβιβάζων were also used for the ascending and descending nodes, giving rise to the English words anabibazon and catabibazon.^{[11]; [12], ¶27.}

Earth orbit nodes

For the orbit of the Earth around the Sun is important the line of nodes formed by the ecliptic and the equator. There are two special days in which this line points to the sun. For these days, night and day take the same time. These two points in the sun orbit are named equinoxes.

Lunar nodes

Main article: Lunar node

For the orbit of the Moon around the Earth, the reference plane is taken to be the ecliptic, not the equatorial plane. The gravitational pull of the Sun upon the Moon causes its nodes, called the lunar nodes, to precess gradually westward, performing a complete circle in approximately 18.6 years.^[1][13]

See also

• Euler angles

References

1. ^ ^{a b} node, entry in The Columbia Encyclopedia, 6th ed., New York: Columbia University Press, 2001–04. Accessed on line May 17, 2007.
2. ^ ^{a b c} line of nodes, entry in The Encyclopedia of Astrobiology, Astronomy, and Spaceflight, David Darling, on line, accessed May 17, 2007.
3. ^ Celestial Mechanics, Jeremy B. Tatum, on line, accessed May 17, 2007.
4. ^ ascending node, entry in The Encyclopedia of Astrobiology, Astronomy, and Spaceflight, David Darling, on line, accessed May 17, 2007.
5. ^ The Binary Stars, R. G. Aitken, New York: Semi-Centennial Publications of the University of California, 1918.
6. ^ Survey of Islamic Astronomical Tables, E. S. Kennedy , Transactions of the American Philosophical Society, new series, 46, #2 (1956), pp. 123–177.
7. ^ Cyclopædia, or, An universal dictionary of arts and sciences, Ephraim Chambers, London: Printed for J. and J. Knapton [and 18 others], 1728, vol. 1.
8. ^ Planetary Latitudes, the Theorica Gerardi, and Regiomontanus, Claudia Kren, Isis, 68, #2 (June 1977), pp. 194–205.
9. ^ Prophatius Judaeus and the Medieval Astronomical Tables, Richard I. Harper, Isis 62, #1 (Spring, 1971), pp. 61–68.
10. ^ Lexicographical Gleanings from the Philobiblon of Richard de Bury, Andrew F. West, Transactions of the American Philological Association (1869-1896), 22 (1891), pp. 93–104.
11. ^ anabibazon, entry in Webster's third new international dictionary of the English language unabridged: with seven language dictionary, Chicago: Encyclopaedia Britannica, 1986. ISBN 0-85229-503-0.
12. ^ New thoughts on the genesis of the mysteries of Mithras, Roger Beck, Topoi 11, #1 (2001), pp. 59–76.
13. ^ Introduction to Astronomy 250, Coordinates, Seasons, Eclipses, lecture notes, Astronomy 250, Marcia Rieke, University of Arizona, accessed on line May 17, 2007.

See also

• Eclipse
• Longitude of the ascending node