The measurement of the positions on the celestial sphere was among the earliest operations performed on heavenly bodies. On several aspects, the procedure is similar to that performed on the Earth by the geographer, however, there are peculiarities that have led to the definition of reference systems unique to astronomy. Some of these systems have originated from observations carried out over several millennia from the surface of the Earth; an astronomer in the extra-terrestrial space, or living on another planet, might have found ‘solutions more appropriate to his particular location.