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The longitude of the ascending node, also known as the right ascension of the ascending node, is one of the orbital elements used to specify the orbit of an object in space. Denoted with the symbol Ω , it is the angle from a specified reference direction, called the origin of longitude , to the direction of the ascending node (☊), as ...
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
Longitude of the ascending node (Ω) — horizontally orients the ascending node of the ellipse (where the orbit passes from south to north through the reference plane, symbolized by ☊) with respect to the reference frame's vernal point (symbolized by ♈︎). This is measured in the reference plane, and is shown as the green angle Ω in the ...
A lunar node is either of the two orbital nodes of the Moon, that is, the two points at which the orbit of the Moon intersects the ecliptic. The ascending (or north) node is where the Moon moves into the northern ecliptic hemisphere, while the descending (or south) node is where the Moon enters the southern ecliptic hemisphere.
This ground track is shifted east or west depending on the longitude of the ascending node, which can vary over time due to perturbations of the orbit. If the period of the satellite is slightly longer than an integer fraction of a day, the ground track will shift west over time; if it is slightly shorter, the ground track will shift east.
Right ascension and declination as seen on the inside of the celestial sphere.The primary direction of the system is the March equinox, the ascending node of the ecliptic (red) on the celestial equator (blue).
Since the line of ascending node is the line of intersection between the orbital plane and the reference plane, it is perpendicular to both the normal vectors of the reference plane and the orbital plane (or ). Therefore, the ascending node vector can be defined by the cross product of these two vectors.
The period of the lunar nodal precession is defined as the time it takes the ascending node to move through 360° relative to the vernal equinox (autumnal equinox in the Southern Hemisphere). It is about 18.6 years and the direction of motion is westward, i.e., in the direction opposite to the Earth's orbit around the Sun.