Astronomers keep five different maps of the sky, each drawn around a different equator
There is no single address for a star. Astronomers choose among several coordinate systems, each built around a different great circle that splits the sky in half: your own horizon, Earth's equator, the plane of Earth's orbit, the disc of the Milky Way, or a band crowded with nearby galaxies.
All of them borrow the logic of latitude and longitude. Each picks a fundamental plane that plays the role of an equator, with poles 90 degrees above and below it, and a primary direction that serves as the zero of longitude. The name of each system comes from its choice of plane. When distance is unknown or unimportant, a direction on the imagined celestial sphere is enough.
The horizontal system is the most personal. It measures altitude above an observer's horizon and azimuth around it, so every star's coordinates drift constantly as Earth turns, once every 23 hours, 56 minutes and about 4 seconds relative to the stars. Awkward for catalogues, it is ideal for pointing a telescope from a particular spot; in one common convention azimuth is counted from the south point toward the west. Zenith distance is simply 90 degrees minus altitude.
For charts, the equatorial system wins. It projects Earth's equator onto the sky and fixes its zero at the March equinox, so star positions stay put, and modern star maps use it almost exclusively. Telescopes on equatorial mounts track stars through the night by following these coordinates. Because Earth's axis slowly shifts, maps specify a reference date, such as the older B1950 or the modern J2000.
The ecliptic system uses the plane of Earth's orbit. Centred on Earth, it was the main framework of ancient astronomy and defined the twelve zodiac signs; centred on the Solar System's centre of mass, it is used to compute planetary orbits. Galactic coordinates take the Milky Way's disc as the equator, with zero longitude pointing at the Galactic Center, while supergalactic coordinates follow a plane unusually rich in nearby galaxies.
Source: Astronomical coordinate systems
