For most of its history astronomy was not a separate discipline from astrology at all; the two names describe one undertaking, the systematic observation of the sky, practiced by the same people for calendrical, agricultural, navigational and divinatory purposes together. Babylonian scribes who tracked the risings of Venus to omen the fate of the king were doing the same positional observation that let them fix the length of the year; Ptolemy, whose Almagest set the mathematical model of the heavens that stood for over a thousand years, wrote the Tetrabiblos, a working astrological manual, as its companion volume rather than as a different field.
The split is a genuinely early modern event, not an ancient one. Copernicus, Kepler and Galileo were reworking the mathematical model of the heavens, not rejecting astrology as such, and Kepler himself cast horoscopes for a living while insisting on more rigorous physical foundations for planetary motion. What separated the disciplines over the following two centuries was less a single discovery than the gradual insistence, hardening through the seventeenth and eighteenth centuries, that a science of celestial mechanics should predict positions and explain causes without also claiming to predict human affairs. Astronomy kept the mathematics and the physics; astrology kept the older claim about what the positions mean for a life, and the atlas treats the two as related but distinct disciplines accordingly, astronomy's own entry here, astrology's several, one per tradition, elsewhere in this section.