Descending node
The descending node is the point in the orbit of an object where it crosses the plane of reference from North to South in the direction of motion. Its symbol is ☋, the astronomical and astrological symbol for the Dragon's tail (Cauda draconis). (Note that Fig. 2, below, has incorrect symbols for the argument of periapsis and the longitude of the ascending node)
The plane of reference for the orbit of Earth's Moon is the ecliptic, thus the Moon's descending node is one of only two places where a lunar or solar eclipse can occur.
The line of nodes is the intersection of the object's orbital plane with the plane of reference, and runs between the ascending and descending nodes.
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Orbit.png
Orbit.png
Fig. 1: The line of nodes is the green line in this diagram.
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Orbits-OrbitalParameters-001.PNG
Orbits-OrbitalParameters-001.PNG
Fig. 2: Descending node and other orbital elements.