Commonly used reference planes and origins of longitude include: • For
geocentric orbits (e.g., artificial satellites around earth),
Earth's
equatorial plane as the reference plane, and the
First Point of Aries (FPA) as the origin of longitude. In this case, the longitude is also called the
right ascension of the ascending node (
RAAN). The angle is measured eastwards (or, as seen from the
north,
counterclockwise) from the FPA to the node. An alternative orbital element to the RAAN is the
local time of the ascending node (
LTAN), defined as the
local mean time at which the spacecraft crosses the equator traveling northward. Similar definitions exist for satellites around other planets (see
planetary coordinate systems). • For
heliocentric orbits, the
ecliptic as the reference plane, and the FPA as the origin of longitude. The angle is measured counterclockwise (as seen from north of the ecliptic) from the
First Point of Aries to the node. • For orbits outside the
Solar System, the plane tangent to the
celestial sphere at the point of interest (called the
plane of the sky) as the reference plane, and north (i.e. the
perpendicular projection of the direction from the observer to the
north celestial pole onto the plane of the sky) as the origin of longitude. The angle is measured eastwards (or, as seen by the observer, counterclockwise) from north to the node., pp. 40, 72, 137; , chap. 17. In the case of a
binary star known only from visual observations, it is not possible to tell which node is ascending and which is descending. In this case the orbital parameter which is recorded is simply labeled
longitude of the node, ☊, and represents the longitude of whichever node has a longitude between 0 and 180 degrees., chap. 17;, p. 72. ==Calculation from state vectors==