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The lunar nodes are the orbital nodes of the Moon, that is, the points where the orbit of the Moon crosses the ecliptic. The ascending or north node is where the moon crosses from south of the ecliptic to north of the ecliptic. The descending or south node is where it crosses from north of the ecliptic to south of the ecliptic.
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 passage coincides with the full moon. A lunar eclipse may occur if there is a full moon within 11° 38' (Celestial Longitude), of a node, and a solar eclipse may occur if there is a new moon within 17° 25' of a node.
The plane of the lunar orbit precesses in space and hence the lunar nodes precess around the ecliptic, completing a revolution (called a draconic or nodal period) in 6798.3835 days or 18.612958 years (note that this is not the same length as a saros). The same cycle, measured against an inertial frame of reference such as ICRS (relative to the stars) is 18.599525 years.
The two solar eclipses in July 2000 (Solar eclipse of July 1, 2000 and Solar eclipse of July 31, 2000) were around the time that the moon was at its ascending node. Ascending-node eclipses recur after one draconic year on average, which is about 0.94901 Gregorian years, and the same goes for descending-node eclipses.
Names and symbols
The nodes are called by different names in different areas of the world.
In ancient European texts, the ascending node is referred to as the dragon's head (Caput Draconis, or Anabibazon). Its symbol is , the astronomical and astrological symbol for the dragon's head. By the same analogy, the descending node is also known as the dragon's tail (Cauda Draconis, or Catabibazon), and its symbol is the inversion of that of the ascending node: .
The lunar orbit is inclined by about 5.1 degrees to the ecliptic: hence the Moon can be up to about 5 degrees north or south of the ecliptic. The ecliptic is inclined by about 23.4° to the celestial equator, the plane that is perpendicular to the rotational 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 star group the Pleiades, which are over 4° North of the ecliptic, occur during a comparatively brief period once every nodal period.
- Sun and Moon Polar Applet, showing moonrise/moonset azimuths
- Astronomy Answers: What are the standstills of the Moon? Retrieved 2016-08-26.
- Eclipses, Cosmic Clockwork of the Ancients
- Lunar and solar periods, about inertial frame of reference for measuring movements of celestial bodies.
- Mean lunar and solar periods