Astronomical tables hold a decidedly unglamorous place in the history of science. With hundreds of pages covered in nothing but column after column of numbers, they rarely interest more than the most hard-core of mathematicians. Very few people would consider them repositories for important scientific discoveries or stories of personal drama, but one volume in the Royal Society’s collections shows how wrong this perception can be.

The volume in question is the library’s copy of a set of tables by the 15th-century astronomer Johannes Regiomontanus. While the main feature of such tables is that they predict the daily positions of the sun, moon, and planets for many years into the future, early users quickly discovered that the daily entries for celestial positions also offered convenient spaces for recording personal events. You might call it the birth of the pocket calendar, even if the pocket in this case would have to be about the size of a saddlebag.

Willibald Pirckheimer, the German humanist and former owner of this Regiomontanus book, seems to have carried his tables with him for over 15 years, making notes about important life events along the way. Biographies of Pirckheimer usually mention that he spent years studying in Italy during his youth, but his notes in the Royal Society volume provide specific dates within these travels. For example on 21 November 1489, he records his arrival in Padua, where he began studying law, and years later on 4 July 1495, he notes his return to Nuremberg.


Pirckheimer records his arrival in Padua where he studied law (left) and then his return to Nuremberg (right). From the Royal Society’s copy of ‘Almanach magistri Johanis de Monte Regio’, 1488.


On October 13th of that same year, Pirckheimer recorded his marriage to Crescentia Rieter in the tables, and the birth dates for their first two children likewise appear in February 1497 and July 1498 next to their corresponding planetary positions. In 1499, however, the diary entries leave university and family life behind and take a darker turn on account of the brief but bloody Swabian War that Holy Roman Emperor Maximilian I launched against the Swiss Confederacy.

As an imperial city, Nuremberg was required to supply the emperor with a contingent of troops, and the 28-year-old Pirckheimer was named commander. Still carrying his tables with him, he recorded the movements of his forces between cities and battlefields along the Swiss border. Like many soldiers on distant battlefields, he missed the birth of a child while he was away, but if we look at the tables for August 1499 we find that after he learned the news, he drew a pointer below his war notes to indicate the date and hour of birth for a new daughter who shared her mother’s name.


Below his notes on troop movements between towns, Pirckheimer draws a line to August 23, 1499, noting the birth of his daughter Crescentia at the fourth hour of the night. From the Royal Society’s copy of ‘Almanach magistri Johanis de Monte Regio’, 1488.


Why was Pirckheimer even carrying around a huge set of astronomical tables anyway? Most likely, one of his main interests in the tables was astrological. He maintained a life-long interest in astrology, repeatedly asking mathematical colleagues to prepare horoscopes for himself and his family members.

His own mathematical abilities were not meagre, however. He knowledgeably discussed estimates of the longitude of the Americas in connection with a lunar eclipse Columbus had observed there, and his new Latin translation of Claudius Ptolemy’s Geography would become the standard edition of that work for much of the 16th century. Indeed, one of the most valuable Pirckheimer volumes now at the Royal Society is his personal copy of the Geography translation, heavily annotated in preparation for a never finished update of the work.

In addition, one of the discoveries that emerges from the Regiomontanus tables is that Pirckheimer seems to have taken a personal interest in astronomical observations, one of which has a rather surprising history. It was most likely made at a house that would later belong to Pirckheimer’s best friend, the artist and mathematician Albrecht Dürer. Today this house is one of Nuremberg’s most visited museums, but few visitors realize it was once an observatory for astronomy as well.


The Albrecht Dürer house in Nuremberg, with the observation ledge built by Bernhard Walther outside a top-floor window around the back. Photographs by the author.


Before Dürer purchased it, the house belonged to the astronomer Bernard Walther, who had inherited Regiomontanus’s manuscripts and continued the latter’s intensive program of observing and measuring the heavens. He even built a small ledge for his astronomical instruments, which you can still see beneath a top-floor window at the rear of the house. Pirckheimer praised Walther’s abilities in a March 1504 letter to his fellow humanist Conrad Celtis, but it is his diary notes in the Royal Society volume that show how closely their interests coincided.

When I visited the Royal Society library last summer from Cairo, I noticed occasional astronomical remarks starting to appear in Pirckheimer’s notes in the years after his return from the war. Some of these match with Walther’s more complete observation logs that survive from a printed edition forty years later, but it was Pirckheimer’s declaration that “today I saw Mercury” on 18 March 1504 that sounded suspiciously familiar and sent me digging into the text of Nicholas Copernicus’s landmark work On the Revolutions of the Heavenly Spheres (1543). Sure enough, that same date matched a Mercury observation by Walther that Copernicus had incorporated into his models of planetary motion, and Pirckheimer must have also been present when it was made.


Pirckheimer records seeing the planet Mercury on 18 March 1504, one week after recording a measurement of the sun’s position that was also part of Bernhard Walther’s observation programme. From the Royal Society’s copy of ‘Almanach magistri Johanis de Monte Regio’, 1488.


How Walther’s Mercury observations made their way across many miles to Copernicus, and indeed how they continued to be studied for more than a century by later generations of astronomers, is a story I described in a separate blog post for the Guardian in connection with the recent transit of Mercury. For Pirckheimer and Walther, though, that evening in March 1504 was no doubt just another convivial gathering, and they could hardly have imagined it would later be commemorated on folio 169 of one of the most famous books in the history of science. Neither of them lived to see Copernicus’s book, and the only surviving manuscript record from their contribution to it appears to be in Pirckheimer’s diary notes at the Royal Society.

After Pirckheimer’s death in 1530, his library stayed with members of his family for just over a century before being purchased by Thomas Howard, second Earl of Arundel, while on a diplomatic mission on the continent amidst the Thirty Years War, and later passing to the Royal Society as the Arundel Library. The end result is that the astronomical tables Pirckheimer once carried with him to war ended up surviving in London as effectively a war refugee from abroad. It is but one example of the tales each book in a great collection might tell if we only had the means to look closely enough.


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