Longitude of the Ascending Node

The longitude of the ascending node is an angle that defines where the orbit of a celestial body crosses the reference plane. It is one of six standard parameters used to describe an orbit in space.

This angle is measured from a fixed reference direction to the ascending node, the point where the body crosses the reference plane while moving northward. The measurement is given in degrees, from 0 to 360.

Definition

Two intersecting planes form a line of nodes. This line touches the orbit at two points: the ascending node and the descending node. The longitude of the ascending node is the angle between the reference direction and the ascending node, measured within the reference plane.

Reference planes

For bodies orbiting the Sun, the reference plane is typically the ecliptic, and the reference direction is the vernal equinox. For satellites orbiting Earth, the reference plane is the celestial equator.

Relationship to other parameters

The longitude of the ascending node alone is insufficient to describe an orbit. It is used together with inclination, which shows how the orbit tilts relative to the reference plane, and argument of periapsis, which indicates the direction of the orbit within its plane. These three angles describe the orientation of the orbit in space; the size and shape of the orbit are specified by the semi-major axis and eccentricity.

Variation over time

The longitude of the ascending node does not remain constant when perturbative forces are present, such as the equatorial bulge of Earth or gravitational effects from other bodies. This causes the line of nodes to rotate gradually, a phenomenon known as nodal precession. Some satellites are designed to maintain an orbit that precesses in synchronization with the Sun.