The debate has never lacked for qualitative arguments. On one side, Copernicus studied at institutions steeped in astrological tradition, trained under teachers who practised the art, and as a young student purchased books explicitly designed for casting horoscopes. Annotations made in his youth suggest he not only read it deeply but actively applied its methods. On the other side, none of his mature writings mentions astrology, no horoscopes in his hand survive, and his revolutionary model of the universe arguably undermined the cosmological foundations on which astrology rested. Both sets of arguments, as Borski and Kolkov carefully demonstrate, turn out to be logically inconclusive – too reliant on inductive leaps or reasoning from silence to settle the matter either way. Something more concrete was needed.

The researchers first turned to Copernicus’s birth chart, cast by Erasmus Reinhold. His recorded birth time of 4:48 pm – suspiciously precise for a merchant’s family with no precise time-keeping devices and no obvious reason to note the exact hour – long suggested astrological practices. A true believer in astrology, after all, would consider their own natal chart a matter of priority. Using purpose-built software to simulate 16th-century astrological methods, Borski and Kolkov tested the era’s most popular rectification algorithms. None of them produced 4:48 pm. A more likely explanation, they argue, is number symbolism: 4 hours and 48 minutes is exactly one-fifth of a 24-hour day, and the number five might have had a deep significance for Copernicus. For instance, in his own model of the universe, Earth occupies precisely the fifth orbital position, as Copernicus counted, from the sphere of fixed stars to the Sun.

The most compelling evidence, however, comes from the books which Copernicus owned. As a student, Copernicus purchased the so-called Alfonsine and Regiomontanus Tables, keeping both books for the rest of his life. The Alfonsine Tables were a universal tool for calculating planetary positions – useful for astronomy and astrology alike – while the Regiomontanus Tables were designed specifically for calculating the cusps of astrological houses, with little application outside the practice.

It appears that Copernicus corrected several misprints in the Alfonsine Tables by hand, and statistical analysis by Borski and Kolkov shows that these corrections align with ancient astronomical observations he used for his magnum opus with very high probability. Knowing that Copernicus did not hesitate to fix the misprints, Borski and Kolkov turned to the second book. The Regiomontanus Tables, that indispensable astrological instrument, contain easily spotted misprints too – including some on pages corresponding to the latitudes of cities where Copernicus lived and worked. Not a single one was corrected.

The conclusion is both clear and historically significant. Copernicus followed a standard astrological curriculum as a student, as any scholar of his age would, but the evidence strongly suggests that in his mature years, he set astrology aside entirely. Beyond settling a long-running biographical dispute, the paper by George Borski and Ivan Kolkov offers a broader philosophical lesson: quantitative analysis of primary sources can resolve historical controversies that qualitative narrative arguments cannot.