To describe the motions in the Milky Way, it is convenient to consider a cylindrical coordinate system centered on the Sun.
The galactic longitude is measured in the plane of the Milky Way, with the origin at the Sun, and l = 0 degrees pointing towards the Galactic Center and l = 90 degrees pointing in the direction in which the Sun is moving.
The galactic latitude is measured perpendicular to the plane, again as viewed from the Sun.
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