Take the Earth’s lat/long grid and imagine projecting it out in an expanding bubble out toward the stars. Lock it in place so it stays fixed with the stars* rather than spinning with the Earth. This is the astronomer’s cartographic system.

For moving objects in the solar system, we describe the shape, orientation and timing of their orbits and then use some nasty math to turn that into celestial latitude and longitude.

* turns out “fixed with respect to the stars” is really complicated.