什么是 Galactic coordinates？

The Galactic coordinates of a direction are its Galactic latitude $b$, the angle from the Galactic plane (positive towards the north Galactic pole), and its Galactic longitude $\ell$, measured in the plane from the direction of the Galactic Centre ($\ell = 0$) through $\ell = 90{}^{\circ}$, the direction towards which the Galaxy’s rotation carries the Sun. The plane was defined in 1958 as the principal plane of the Galaxy’s atomic hydrogen; the north Galactic pole lies at right ascension $192.25{}^{\circ}$ ($12^\mathrm{h}49^\mathrm{m}$) and declination $27.4{}^{\circ}$ (equinox 1950).