Aspects of this topic are discussed in the following places at Britannica.
Galactic longitude (denoted by the symbol l) is measured in degrees eastward of an imaginary line running across the plane of the Galaxy and connecting Earth (assumed to be on that plane) with a point near the galactic centre in the constellation Sagittarius. Before 1958, galactic longitude was measured from an arbitrarily chosen point, an intersection of the galactic and celestial...
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