Copernicus triumphans, sive de parallaxi orbis annui tractatus epistolaris. ad celsissimum & serenissimum principem ac dominum, dominum Christianum Daniæ & Norvegiae &c. hæredem.
Copenhagen: At the author's expense by Joannes Grammius, [1727]. First edition of one of the earliest printed works to announce stellar parallax, declaring that the Copernican system had at last been proved by observation. Drawing on Rømer’s meridian records of bright stars such as Sirius and Vega—the same observational tradition that had earlier enabled Rømer to establish the finite velocity of light—Horrebow interpreted their apparent annual shifts as evidence of the Earth’s orbital motion. Issued as a decisive proof of heliocentrism, it was presented as the long-sought demonstration that Copernicus had demanded and that Tycho had denied. The absence of measurable stellar parallax had been the central empirical objection to Copernicus since the sixteenth century. In De revolutionibus Copernicus himself insisted that the motion of the Earth must be accompanied by an annual displacement of the stars; he explained the lack of observed parallax by positing that the fixed stars were at an immense, almost inconceivable distance. Tycho Brahe, in contrast, argued that if the Earth moved, the stars should shift visibly, and their failure to do so was proof that the Earth remained fixed. This was not a minor quibble but the main weapon against Copernicanism. As Tycho put it in the De mundi aetherei recentioribus phaenomenis (1588), the Copernican system required stars of unimaginable size at impossible distances—an absurdity in his view. Throughout the seventeenth century the search for parallax became a constant theme in observational astronomy. The invention of the telescope offered new possibilities, but the hoped-for demonstration remained elusive. Galileo had attempted to use double-star observations, but with inadequate instruments. Robert Hooke, in his Attempt to Prove the Motion of the Earth (1674), claimed to have measured the displacement of Gamma Draconis, but the result was spurious and quickly discredited. John Flamsteed, the first Astronomer Royal, made further attempts at Greenwich but also failed. Adrien Auzout and Jean Picard in Paris worked on the problem with longer telescopes and refined meridian instruments, without success. The persistence of these efforts shows how urgent the parallax question remained: heliocentrism could be argued on grounds of simplicity, physics, or elegance, but only a demonstration of parallax would supply the irrefutable geometrical proof. It was into this tradition that Horrebow’s Copernicus triumphans entered. A pupil and later successor of Ole Rømer, Horrebow inherited not only his master’s instruments but also his methods. Rømer had pioneered the use of transit instruments, perfecting meridian observations with telescopic sights. More famously, he had used the timings of Jupiter’s satellite eclipses to infer the finite velocity of light, announcing the result in 1676. By the early eighteenth century his methods were a model of precision observational practice. Horrebow, steeped in this tradition, turned to Rømer’s meridian records of bright stars in the hope of finding the parallax that Copernicus required and Tycho had denied. The treatise, printed in 1727 at Horrebow’s own expense, is short but confident. He points to the annual displacements of Sirius and Vega, interprets them as parallactic, and concludes that the Copernican system has at last been demonstrated. Dedicated to the Crown Prince, the work has a ceremonial as well as scientific tone: it was not just a technical claim but a public assertion that the long-standing controversy was resolved. As Houzeau and Lancaster record, it is among the earliest printed attempts to prove the Earth’s motion by stellar parallax (HL 12741). The displacements were real but misinterpreted. In 1729 James Bradley, observing with Samuel Molyneux, announced to the Royal Society his discovery of the aberration of light. Each star, he showed, describes a small ellipse in the sky over the course of a year, not because of parallax but because of the combination of the finite speed of light with the Earth’s orbital velocity. Aberration produces a displacement of about 20 seconds of arc, large enough to explain what Hooke, Flamsteed, and Horrebow thought they were seeing. Stellar parallax itself is much smaller, typically less than one second of arc, and was only successfully measured in 1838 by Friedrich Bessel for 61 Cygni, followed shortly by Henderson for Alpha Centauri and Struve for Vega. Thorndike, summarising the episode, noted: “Horrebow thought he had discovered parallax in the observations of Vega and Sirius made by Rømer … but he was deceived by systematic errors and by what we now know as the aberration of light” (History of Magic and Experimental Science, VIII, p. 158). The Dictionary of Scientific Biography makes the same point more bluntly: “Horrebow … claimed to have demonstrated the annual parallax of the fixed stars, thus proving the Copernican system” (DSB VI, p. 567). The importance of the treatise lies not in its correctness but in its place within the sequence of efforts to prove heliocentrism by observation. Copernicus had made parallax the test; Tycho had rejected the system for its absence; seventeenth-century astronomers from Hooke to Flamsteed had attempted to detect it; Horrebow claimed success; Bradley reinterpreted the observations as aberration; Bessel finally measured the true effect. In this chain Copernicus triumphans is a printed record of a critical transitional stage, capturing the state of astronomy just before the new physics of aberration reshaped the interpretation of stellar displacements. Horrebow’s own background adds to the significance of the work. Rømer’s discovery of the finite speed of light was a major moment in the history of physics, and his observational methods were respected across Europe. Horrebow worked directly in that tradition. It is telling that the same programme of careful meridian timing produced first the velocity of light, then the displacements that misled Horrebow, and finally, in Bradley’s hands, the true explanation of aberration. The link between these discoveries is not accidental: the geometry of observation and the physics of light were bound together. That Horrebow’s error arose from the very methods that had produced Rømer’s triumph only underscores the closeness of the questions. The work also illustrates the intellectual plurality of early eighteenth-century astronomy. In his teaching text, the Clavis astronomiæ (first published in 1725 and present here in its third edition of 1730), Horrebow continued to present a Cartesian cosmology of vortices and aetherial globules. Newtonian gravitation he regarded as an “occult quality,” rejecting it as unsatisfactory. Yet in his observational practice he claimed the Copernican demonstration. The juxtaposition of these two works in one volume is striking: a Cartesian physics for students, a Copernican proof for readers of his treatise. The Copernicus triumphans continued to be cited into the nineteenth century. Christian August Friedrich Peters, in 1848, analysed the data and showed that no real parallax could be extracted. But by then the work had long entered the literature as a notable early claim. Its persistence shows how eagerly the astronomical community received any suggestion that parallax had at last been observed, and how difficult it was to separate error from discovery before the new physics of light was understood. In retrospect the Copernicus triumphans stands as an important printed witness to the state of astronomy in the late 1720s. It reveals the eagerness with which observers sought a proof of Copernicus, the continuing authority of Tycho’s objection, the influence of Rømer’s observational legacy, and the immediate context for Bradley’s discovery. Appearing only two years before Bradley’s paper to the Royal Society, it documents the precise moment when the interpretation of stellar displacements shifted, and it shows how close astronomers were to the truth even as they misunderstood what they saw. The Clavis astronomiæ, bound here with the treatise, provides a useful counterpoint. First published in 1725, it served as Horrebow’s introductory course in physical astronomy and remained in use for decades. The third edition of 1730 includes a systematic presentation of Cartesian physics: vortices, the constitution of the heavens, the structure of planetary systems, and the figure of the Earth. Its presence beside the Copernicus triumphans highlights the coexistence of different explanatory frameworks at the same time: Cartesian mechanics in the classroom, Copernican proof in the observatory. Together the two works illustrate the complex intellectual situation of early eighteenth-century astronomy. Newton’s gravitation was by then widely discussed, but Cartesian mechanics still dominated in teaching, and Copernican heliocentrism still awaited decisive proof. Horrebow’s writings embody all three: rejection of Newton, adherence to Descartes, and proclamation of Copernicus’s triumph. They also reveal the close connection between observation and theory: the same meridian observations that had revealed the finite velocity of light to Rømer now misled Horrebow, before yielding to Bradley the true discovery of aberration. The Copernicus triumphans is therefore not just a curiosity but a record of a decisive stage in the development of modern astronomy. It shows how the demand for parallactic proof persisted, how observers attempted to meet it, and how their efforts contributed, even through error, to the eventual discovery of new phenomena. It belongs to the long sequence that links Copernicus’s demand for parallax to Bessel’s successful measurement, and it provides historians with a rare printed witness to the moment just before the explanation of aberration. Houzeau–Lancaster 12741; DSB VI, pp. 566–67 (Horrebow); Thorndike VIII; Nielsen, Horrebow, Peder Nielsen; Gingerich, Census of Copernicus (context); Watson, Catalogue Seventeen (for Argoli and the geocentric line); Bradley, “Account of a New Motion of the Fixed Stars,” Philosophical Transactions 35 (1729).
4to (199 x155 mm). [Copernicus:] pp. [ii], 58, with one folding engraved plate. [Clavis:] pp. [xii], 112, [12], woodcut initials, head- and tailpieces, one large folding engraved plate. Contemporary vellum, manuscript title on spine (a little soiled).
Item #6456
Price: $12,500.00
