The astrology–astronomy split

What it is

The separation of the study of celestial mechanics from the practice of celestial interpretation — two activities that were, for most of their history, a single discipline pursued by the same people using the same instruments and the same mathematics.

The modern assumption is that astronomy is the science and astrology is the superstition it left behind. The historical picture is considerably stranger: the interpretive work paid for the mechanical work, and for centuries nobody regarded that as embarrassing.

Where it came from

The words themselves were used more or less interchangeably into the seventeenth century. Where a distinction was drawn, it often ran the other way from the modern one — astronomia for the mathematical description, astrologia for the science of celestial influence, with the second regarded as the point of the first.

The reason the two were joined is straightforward. If planetary positions matter, you need accurate planetary positions, and producing them is difficult mathematics requiring careful observation over long periods. Astrology supplied the demand that funded observational astronomy for something like two thousand years.

The people involved are not obscure figures:

• Ptolemy wrote the Almagest, the mathematical astronomy that stood for fourteen centuries, and the Tetrabiblos, the foundational text of Western astrology. He saw them as companion volumes: the first establishes where the planets are, the second what that means.
• Tycho Brahe built the finest pre-telescopic observatory in Europe and cast horoscopes, including one for the Danish king's son.
• Johannes Kepler was imperial mathematician — a post whose duties explicitly included astrological advice — and produced annual prognostications for income while deriving the laws of planetary motion. He is treated separately in his own article, because his position was more interesting than either camp usually reports.
• Galileo cast charts, including for his own daughters and for patrons.

Astrology was taught in universities, tied to medicine — a physician was expected to know astrology, because treatment timing depended on it — and was a standard part of educated learning.

How it is used

The split came in the late seventeenth and eighteenth centuries, and it was not a single event. Contributing pressures included the Copernican and Newtonian reorganisation of the heavens, which removed the framework of celestial spheres in which influence had made mechanical sense; the rise of experimental method and a demand for demonstrable mechanism; theological objection to determinism; and the professionalisation of the sciences, which had a strong interest in distinguishing themselves from practitioners who could be portrayed as fortune-tellers.

Astrology left the universities, largely disappeared from elite culture through the eighteenth and much of the nineteenth century, and returned as a popular and occult practice rather than an academic one — revived substantially through Theosophy in the late nineteenth century, which is where its modern psychological framing begins.

What we can and cannot say

We can say the joint history is not disputed and is frequently misrepresented by both sides. Sceptics who present astrology as always having been fringe are wrong about the record. Astrologers who present the split as science arrogantly discarding ancient wisdom are also wrong — the split happened partly because astrology could not produce the demonstrable results the new method demanded, and its practitioners knew it.

We can say the mechanism problem was real and was recognised at the time. Once the crystalline spheres went, and once Newton had described gravity as the force acting between celestial bodies, the question of how Mars was supposed to affect a birth became difficult in a way it had not been before. Astrology has never produced an answer to this that satisfies anyone outside it.

We can say the story usually told about Newton rebuking Halley — "I have studied the matter, sir; you have not" — is of uncertain provenance and probably apocryphal. It circulates widely as evidence that Newton defended astrology. It should not be relied on.

We cannot say astronomy owes astrology nothing. Centuries of position tables, instrument-making and mathematical technique were produced by people whose income depended on interpretive work. That is an uncomfortable inheritance for a discipline that now defines itself partly against astrology, and it is simply true.

Further reading

• Ann Geneva, Astrology and the Seventeenth Century Mind (1995) — on William Lilly's world and how astrology functioned before the split.
• Patrick Curry, Prophecy and Power: Astrology in Early Modern England (1989) — the best account of how and why it lost its standing.