Johannes Kepler

Called astrology the foolish daughter of astronomy and kept practising it — because he thought most of it was rubbish and a small part of it was real.

Who they were

Johannes Kepler (1571–1630) derived the three laws of planetary motion that displaced fourteen centuries of Ptolemaic astronomy, and made a substantial part of his living casting horoscopes.

He is the single most useful figure for understanding the relationship between the two disciplines, because he was neither a believer nor a debunker. He was a reformer who thought almost all contemporary astrology was baseless, and that a small residue of it was real and worth rescuing.

Where they came from

Born in Weil der Stadt in Swabia to a difficult and impoverished family, he trained for the Lutheran ministry at Tübingen, where he encountered Copernicanism through Michael Maestlin and was convinced by it early.

He taught mathematics in Graz, worked with Tycho Brahe in Prague, and on Tycho's death in 1601 succeeded him as imperial mathematician to Rudolf II — a post whose duties explicitly included astrological counsel. Access to Tycho's observations of Mars, the most accurate ever made, gave him the data from which the laws came.

His life was hard in ways that bear on his outlook. He lost his first wife and several children; his mother was tried as a witch and he spent years on her defence; he was displaced repeatedly by the Thirty Years' War, and was frequently unpaid.

What they taught

The three laws. Planets move in ellipses with the sun at one focus; a line from sun to planet sweeps equal areas in equal times; the square of the orbital period is proportional to the cube of the semi-major axis. These came from years of grinding calculation against Tycho's Mars data, and Kepler's willingness to abandon the circle — an assumption held since antiquity — when the numbers would not fit.

A reformed astrology. This is the part usually flattened into a slogan. Kepler's most quoted remark describes astrology as the foolish little daughter of the respectable mother astronomy, and observes that without the daughter's earnings the mother would starve. It is often read as pure contempt. It is not: he wrote it as someone who took the daughter seriously enough to try to educate her.

In De Fundamentis Astrologiae Certioribus (1601) — On the More Certain Foundations of Astrology — he set out what he thought could survive scrutiny and what could not. He rejected the zodiac signs as having no foundation. He rejected the house system. He rejected most traditional planetary significations and the elaborate apparatus of dignities.

What he kept was the aspects — the angular relationships between planets — because they are geometrical and harmonic, and because he believed the human soul possesses a faculty responsive to harmonic proportion in the same way the ear responds to musical intervals. He even proposed additional aspects based on further harmonic divisions. The connection to Harmonices Mundi (1619), where he sought musical proportion in planetary motion, is direct: the same conviction that the cosmos is ordered by harmony underlies both his astronomy and the fragment of astrology he defended.

He also documented weather observations against planetary aspects for years, treating it as an empirical question rather than a doctrinal one.

What we can and cannot say

We can say his own astrology did not work either, and he is a poor authority to cite in its favour. The harmonic-resonance mechanism has no support, and his weather correlations do not hold up.

We can say he is a poor authority for the opposite case too. He is regularly deployed by sceptics as a man who cast horoscopes cynically for money and by astrologers as a great scientist who believed. Neither is accurate. He thought the standard system was mostly worthless, said so in print at length, and held that a specific reformed core was genuine.

We can say his position is methodologically admirable regardless of being wrong. Presented with a tradition, he asked which parts had a basis and which did not, discarded the majority, tried to formulate the remainder so it could be checked, and then checked it. That is the right procedure applied to the wrong subject, by someone who did not have the tools to see how far the wrongness went.

We cannot say the horoscope work was merely cynical. He complained about it — the prognostications were a chore and he said so — while continuing to think there was something real underneath, and those two attitudes coexisted in him without difficulty for thirty years.

Who they were

Where they came from

What they taught

What we can and cannot say

Further reading