Definition:Longitude (Celestial)

Definition
Let $P$ be a point on the celestial sphere.

Let $J$ be a great circle on the celestial sphere passing through $P$ and both of the north ecliptic pole and south ecliptic pole.

The celestial longitude $\lambda$ of $P$ is the (spherical) angle (measuring east) that $J$ makes with the vernal equinox.

It ranges from $0$ to $360 \degrees$.

Also known as
The term celestial longitude is also seen as ecliptic longitude.

Also see

 * Definition:Celestial Latitude