A 420-year-old first edition of Galileo’s debut book — the first to come to auction in more than a century

In 1633, the Roman Inquisition charged Galileo Galilei with heresy and sentenced him to life imprisonment (later commuted to permanent house arrest at his home near Florence). He was only saved from a harsher fate by promising at his trial to ‘abjure with a sincere heart… every error, heresy, and sect contrary to the Holy Catholic Church’.

Galileo’s crime had been to publish Dialogue Concerning the Two Chief World Systems a year earlier, a book in which he seemed to defend heliocentrism. Now universally accepted, this astronomical model was highly radical — and controversial — at the time, positing that Earth and the other planets revolve around the Sun. Developed by Nicolaus Copernicus in the 1540s, it challenged the geocentric model — associated with the Greek philosopher Aristotle and long held by the church — that Earth was the centre of the universe.

Because of his trial, Galileo is remembered today as a hero of scientific inquiry, a symbol of the fight for intellectual freedom. He had published his first book, Dialogo de Cecco di Ronchitti da Bruzene in perpuosito de la stella nuova, three decades earlier, and one can find in it some fascinating hints of the career that lay ahead of him.

An extremely rare first edition of that work is being offered in the Valuable Books and Manuscripts sale at Christie’s in London on 9 July 2025, as part of Classic Week.

Eleven other copies are known to exist, all of them found in institutions. Only seven of those are complete — as the example coming to auction is. Newly rediscovered in a European private collection, it is the first copy to appear on the market in more than a century.

Galileo co-wrote Dialogo early in 1605 with Girolamo Spinelli, one of his students at the University of Padua, where he taught mathematics and astronomy. Their inspiration was the appearance the previous October of what is today known as Kepler’s Supernova: one of just five such phenomena to have been observed with the naked eye in the Milky Way in the past millennium. It outshone even Jupiter in the night sky and, for several weeks of its 18-month appearance, remained visible in the daytime too.

It is the focus of discussion in Dialogo, an imagined conversation between a pair of Paduan peasants named Matteo and Natale. The latter refers to it as a ‘very bright star’ which ‘shines at night like an owl’s eye’ and can still be seen ‘in the morning when [it’s time] to prune the grapevines’.

Among the public, Kepler’s Supernova was a source of wonder, fright and curiosity. Astronomers, too, were puzzled by it. Galileo himself was still a few years away from pioneering his famous telescope (in 1609), which he would use to observe the skies with unprecedented clarity.

A supernova is a brilliantly luminous stellar explosion marking the end of a star’s life. At the turn of the 17th century, however, no concept of it existed. For Cesare Cremonini, an Aristotelian professor of natural philosophy at the University of Padua, the bright new sight had to be a terrestrial phenomenon. This was consistent, he felt, with the then widely accepted belief that the stars in the sky are fixed and unchangeable.

He explained his position in a pamphlet called Discorso intorno alla nuova stella, which he published in January 1605 under the pseudonym of Antonio Lorenzini. The pamphlet failed to impress Galileo, who wrote his book with Spinelli in response.

Early on in Dialogo, the peasant Natale outlines Cremonini’s argument — that everything is astronomically perfect as it is and that, ‘with one star added, the heavens wouldn’t be able to move’. The rest of the book serves as a refutation of that argument.

Mathematical measurement is championed as a means of explaining the supernova. (Thanks to historical instruments such as the cross-staff and astrolabe, it was already possible to measure the altitude of celestial bodies.)

Matteo alludes to some astronomical measurements done in real life by Galileo and a few correspondents of his around Europe. The differences in the men’s observation points, and the differences in what they recorded above them, collectively suggested that the supernova was located beyond the Moon — and, therefore, was a heavenly phenomenon rather than a terrestrial one.

Galileo is renowned for having been a scientist who shared his ideas in accessible fashion, and Dialogo is no exception. The dialogue between the peasants is highly readable, with a number of light-hearted passages — such as that near the end where Matteo longs for a follow-up text from Cremonini, on grounds that Discorso intorno alla nuova stella was ‘blather’ which had ‘given [them] lots to laugh about’.

In trying to ascertain the location of the new star, the peasants also explore the notion of parallax — the apparent displacement of an object due to a change in the observer’s viewpoint. They do so by comparing their distance from four trees in the countryside around them: a willow, a poplar, a mulberry and a walnut tree. At one point, Natale even tries, with comic clumsiness, to climb the walnut tree, chipping a fingernail and grazing his knee in the process.

The book is written in a rustic Paduan dialect rather than in Latin (the official language of intellectual discourse), which adds to the sense of informality. Galileo hailed from the Tuscan city of Pisa, and it seems likely that much of Spinelli’s input — as a native of Padua — was linguistic.

The translation of the book’s full title is ‘The dialogue of Cecco di Ronchitti da Bruzene concerning the new star’, with Cecco di Ronchitti da Bruzene being the name under which the co-authors published this work. The fact that they, like Cremonini before them, adopted a pseudonym gives a sense of the fraught intellectual climate that then existed.

Talking of names, it’s worth adding that the term ‘Kepler’s Supernova’ was applied retrospectively, in acknowledgment of the fact that the German astronomer Johannes Kepler had observed the phenomenon more carefully than anyone, over the course of at least a year. His observations formed the basis of a book he published in 1606, De stella nova (‘On the New Star’).

Astronomers today know that what Kepler and Galileo believed to be the birth of a star was actually the violent death of one. Their two texts, however, still played a part in hastening the end of unquestioned belief in the immutability of the heavens — and thus ushering in a new era of astronomical understanding, where heliocentricism supplanted geocentricism.

In many ways Galileo’s first book anticipated his most famous work, the aforementioned Dialogue Concerning the Two Chief World Systems (1632). These were the only texts, for example, that he wrote in dialogue form (though the respective interlocutors are different). Both works also reveal Galileo’s fondness for using measurements to advance an intellectual position — in the latter book, these included ones made with a telescope.

Dialogo in perpuosito de la stella nuova, then, represents a major stepping stone in one of the most notable career paths in the history of science. It’s easy to see that it was written by the same man who later claimed ‘all truths are easy to understand once they’re discovered — the point is to discover them’.

Valuable Books and Manuscripts takes place on 9 July 2025, and will be on view, 5-8 July, as part of Christie’s Classic Week season in London