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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
The longitude of the ascending node, Ω, the inclination, i, and the argument of periapsis, ω, or the longitude of periapsis, ϖ, specify the orientation of the orbit in its plane. Either the Mean longitude at epoch, L 0, the mean anomaly at epoch, M 0, or the time of periapsis passage, T 0, are used to specify a known point in the orbit. The ...
The true longitude l can be calculated as follows: [1] [2] [3] l = ν + ϖ. where: ν is the orbit's true anomaly, ϖ ≡ ω + Ω is the longitude of orbit's periapsis, ω is the argument of periapsis, and; Ω is the longitude of the orbit's ascending node,
Diagram of an orbit. The plane of the orbit is in yellow, the reference plain is in gray, and the reference direction (vernal point) is the arrow in red.Also labeled are the mean anomaly (M) in pink, the true anomaly in red, the argument of periapsis (ω) and periapsis in purple, the longitude of ascending node (Ω) in green, and the inclination (i) in dark green.
For a typical prograde orbit around Earth (that is, in the direction of primary body's rotation), the longitude of the ascending node decreases, that is the node precesses westward. If the orbit is retrograde, this increases the longitude of the ascending node, that is the node precesses eastward.
In the case of equatorial orbits (which have no ascending node), the argument is strictly undefined. However, if the convention of setting the longitude of the ascending node Ω to 0 is followed, then the value of ω follows from the two-dimensional case: ω = a t a n 2 ( e y , e x ) {\displaystyle \omega =\mathrm {atan2} \left(e_{y},e_{x}\right)}
In celestial mechanics, the orbital plane of reference (or orbital reference plane) is the plane used to define orbital elements (positions). The two main orbital elements that are measured with respect to the plane of reference are the inclination and the longitude of the ascending node.