Which coordinate system uses the Milky Way's plane as its equator?

The galactic coordinate system uses the Milky Way’s plane as its equator, or fundamental plane.

It is a celestial coordinate system designed to map objects in relation to our Galaxy rather than Earth’s horizon or the celestial equator. Its zero point in longitude is aligned approximately with the Galactic Center, while galactic latitude measures angular distance north or south of the Milky Way’s mid-plane.

The system is especially useful because the Milky Way forms a prominent band across the sky. Astronomers can describe whether a star, nebula, or survey target lies close to that band without relying on the changing orientation of Earth’s local horizon.

The coordinate system is heliocentric in practice: the Sun is treated as the coordinate origin, even though the Milky Way’s center defines the main direction. It is easily confused with equatorial coordinates, which use the celestial equator, or ecliptic coordinates, which follow the plane of Earth’s orbit around the Sun. Horizontal coordinates instead use an observer’s horizon.