Galactic Coordinates

When describing the position of a star within the MW, it is useful to use Galactic Coordinates $(l,b)$, $0^o\le l\le 360^o$, and $-90^o\le b\le 90^o$. From the vantage point of the Sun, the direction to the galactic centre is taken to be $l=0$, $l=180^o$ is the ante-centre, and $b=0$ corresponds to the galactic plane. $b=\pm 90^o$ to the galactic poles. Knowing these direction in terms of right ascension $\alpha$ and declination $\delta$, one uses spherical trigonometry to convert between them. Since the plane of the MW is inclined with respect to the celestial equator, the planes $\delta=0$ and $b=0$ do not coincide.