Astrology and astronomy were a single field for most of recorded history, practised by the same people using the same instruments. Their separation was gradual and had identifiable causes.
They shared a purpose for centuries
Predicting celestial positions was demanding work, and the main institutional reason to fund it was that the positions were held to matter for human affairs.
Court astronomers in many societies were employed to produce both calculations and interpretations, and the two tasks were not distinguished by job title or training.
Much of the observational record that later astronomy depended on was compiled by people working within that framework.
A physical model changed the question
As mechanical accounts of planetary motion developed, explanation increasingly meant identifying a physical cause that could be stated generally.
Correspondences between celestial positions and events had no candidate mechanism of that kind, and the absence became more conspicuous as other phenomena acquired one.
The issue was not that the correspondences had been disproved. They had become the wrong shape of claim for what now counted as an explanation.
Institutional separation followed slowly
Universities and academies gradually stopped teaching interpretation alongside calculation, and the two functions were assigned to different people.
Publication venues divided in the same period, and practitioners came to be trained in one tradition without exposure to the other.
Individuals who worked across both, including some prominent early modern figures, are often read anachronistically as if the boundary already existed in their lifetimes.
The precession problem is often misstated
The gradual shift of the equinoxes means that the constellations behind the Sun on a given date differ from the sign names used in Western practice.
This is frequently presented as a discovery that undermines astrology, though the shift has been known and discussed within the tradition for a very long time.
Western practitioners generally treat the signs as divisions measured from the equinox rather than as the constellations themselves, which is a stated convention rather than an oversight.
Both traditions continued after the split
Astronomy became an observational and physical science, while astrological practice continued as an interpretive tradition with its own literature and training.
Public interest in the latter has risen and fallen repeatedly since, and periods of renewed popularity are usually connected to broader cultural conditions rather than to anything internal to the field.
Historians of science now study the shared period on its own terms, since reading it as a struggle between science and superstition obscures what its practitioners understood themselves to be doing.