The longitude that a planet has at a particular date and time. It is the angle measured along the ecliptic from the vernal equinox eastwards to the ascending node (i.e. to the longitude of the ascending node), and then eastwards along the orbital plane to the planet’s position at that instant. In the diagram, the longitude at the epoch is the angle γ‎SN plus the angle NSP. See also elements, orbital.

longitude at the epoch:

• N ascending node

• P planet

• S Sun

• γ‎ vernal equinox

• Ω‎ longitude of the ascending node