Lunar node
From Wikipedia, the free encyclopedia
The lunar nodes are the orbital nodes of the Moon, that is, the points where the orbit of the Moon crosses the ecliptic (which is the apparent path of the Sun across the heavens against the background stars). The ascending node is where the moon crosses to the north of the ecliptic. The descending node is where it crosses to the south.
Eclipses occur only near the lunar nodes: Solar eclipses occur when the passage of the Moon through a node coincides with the new moon; lunar eclipses occur when it coincides with the full moon. The Moon's distance to the nodes will be less than about 1.5°.[1]
The lunar nodes precess rather quickly around the ecliptic, completing a revolution (called a draconitic or nodical period, the period of nutation) in 6793.5 days or 18.5996 years (please be aware that this is not the saros eclipse cycle).
Contents |
[edit] Names and symbols
The nodes are called by different names in different areas of the world.
Since the ascending node is the point of intersection between the ecliptic and the plane of the lunar orbit where the Moon is ascending from the South to the North, it is sometimes called the North node. In ancient European texts, it is referred to as the dragon's head (Caput Draconis, or Anabibazon). The symbol of the ascending node is 
, the astronomical and astrological symbol for the Dragon's head. Similarly the descending node is the point where the Moon is descending from North to South, and is sometimes referred to as South node. It is also known as the dragon's tail (Cauda Draconis, or Catabibazon), and its symbol is the inversion of that of the ascending node: 
. Note that the so-called North node may in fact lie South of the South node in the course of the nodal cycle.
In Hindu astrology (or Jyotish), the ascending node is called Rahu and the descending node is called Ketu.
[edit] Extreme declinations
The lunar orbit is inclined by about 5 degrees on the ecliptic: hence the Moon can be up to about 5 degrees North of the ecliptic, and as much South of the ecliptic. The ecliptic is inclined by about 23.4° on the celestial equator, the plane that is perpendicular to the rotation axis of the Earth. As a consequence, once during the 18.6-year nodal period, when the ascending node of the Moon's orbit coincides with the vernal equinox, then the Moon reaches extreme northern and southern declinations. Then it also has its extreme northern and southern azimuth points of rising and setting on the horizon; its extreme lowest and highest altitude when crossing the meridian; and potentially extreme late first sightings of the new moon. Furthermore, occultations by the Moon of the bright star group the Pleiades, which are over 4° North of the ecliptic, occur during a comparatively brief period once every nodal period.
[edit] Astrological significance
The lunar nodes are of major astrological significance in Vedic astrology, and are considered to a limited degree in Western astrology. Usually only the north node is marked in horoscopes, as the south node is by definition always located at the opposite point in the astrological chart. In Vedic Astrology, the north node is called Rahu and the south node Ketu, and both are marked in the chart.
Their significance can vary widely between approaches. In general, the north node is viewed a point of opportunity for growth and development and self help, while the south node is thought to represent karmic repression or tendencies that restrict growth. The north node carries the positive and beneficial tone of Jupiter, while the south node expresses the restrictions and obstacles of Saturn. Another view is that the north node represents positive objectives and the south node denotes the easy way out with little opportunity for growth. The two nodes together are most commonly referred to simply as the nodal axis, the lunar nodes, or the moon's nodes.
In Vedic astrology, Rahu (north node) is considered to be similar to Saturn, and Ketu similar to Mars. Rahu signified materialism and desires, whereas Ketu signifies obstacles in the material realm but spiritual tendencies.
[edit] See also
[edit] References
- ^ André Danjon. Astronomie Générale. 2nd Edition, Paris, 1980.
[edit] External links
- Sun and Moon Polar Applet, showing moonrise/moonset azimuths
- Astronomy Answers: What are the standstills of the Moon?
