Astrology and astronomy were one activity for most of recorded history, practised by the same people using the same observations. The separation happened gradually and for identifiable reasons.
Prediction required accurate positions
Casting a chart demands knowing where the planets were at a given moment, which requires observation, mathematical models and tables.
Much early positional astronomy was funded and motivated by that demand, and astronomers were routinely employed to produce almanacs and forecasts.
The observational tradition therefore owes a substantial debt to the practice that later separated from it, a point historians of science generally emphasise.
Physical explanation changed what counted as a cause
Once planetary motion was explained by a physical mechanism acting at a distance, the question arose of what mechanism could carry astrological influence.
No candidate emerged that fitted the developing physics, and the absence became more conspicuous as other phenomena received mechanical explanations.
Astrology's own defenders had offered various accounts, but these did not translate into the new framework, and the gap was not closed.
The precession problem was longstanding
The slow shift of the equinoxes means the tropical signs no longer align with the constellations they were named after, a discrepancy known since antiquity.
Practitioners addressed this by distinguishing signs from constellations or by adopting a fixed zodiac, and the debate continues within astrology today.
To critics outside the practice, the disagreement itself became evidence of an unresolved foundation rather than a technical detail.
Institutions divided along method
As universities and academies formalised around observation, mathematics and testable prediction, astrology lost its position within them.
Chairs, funding and publication moved to work that fitted the emerging methodological standards, and the practice continued outside those institutions.
The separation was therefore institutional as much as intellectual, and it happened at different rates in different countries.
Court patronage declined alongside academic standing. Where rulers had once retained astrologers for timing and counsel, those positions were withdrawn or converted into observational and calendrical roles.
Both continue with different aims
Astronomy proceeded as a physical science, while astrology persisted as a practice concerned with meaning, character and timing.
Contemporary astrologers frequently describe their work as symbolic and interpretive rather than causal, which is a different claim from the one tested and not supported in controlled studies.
Understanding the shared history explains why the two use overlapping vocabulary while answering questions that no longer resemble each other.