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Boscovich, the Discovery of Uranus and His Inclination to Theoretical Astronomy

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Boscovich, the Discovery of Uranus and His Inclination to Theoretical Astronomy Mem. S.A.It. Suppl. Vol. 22, 26 Memorie della c SAIt 2013 Supplementi Boscovich, the discovery of Uranus and his inclination to theoretical astronomy L. Guzzardi Istituto Nazionale di Astrofisica – Osservatorio Astronomico di Brera, Via Brera 28, I-20121 Milan, Italy, e-mail: [email protected] Abstract. On March 13th 1781 Frederick William Herschel observed a bizarre celestial body moving in the sky. Retrospectively, that astral body was not at all new at that point. It was observed by a number of astronomers since the end of 17th century (and maybe earlier). But they failed to find out its motion and catalogued it as a fixed star – each time a different one. On the other hand, Herschel realized it was moving, and catalogued it as a comet. That news of a new finding in the sky rapidly spread throughout Europe, and after some months the ‘Herschel’s comet’ was correctly recognized as a new planet, which will be named Uranus. The present paper assumes the event of the discovery of Uranus and the assessment of its planetary nature as a system of complicated, interrelated processes which involved a number of actors in the 17th-century astronomical community. In this framework, the role of the Dalmatian-born jesuit scientist Ruggiero G. Boscovich is emphasized and the meaning of this discovery is discussed as an example of his interest in theoretical research more than in observational science. 1. Introduction Zeta Tauri”, as he wrote down in his journal: he saw a brilliant disk which have the “curi- On March 13th 1781 Frederick William ous” appearence of “either [a] Nebulous Star Herschel, born Friedrich Wilhelm, originally or perhaps a Comet”, becoming convinced four a German composer who made his own way nights later that it was indeed a new comet in England as a skilled astronomer, observed “for it has changed the place” (Miner 1990 p. a ‘new’ celestial body in the sky over Bath 17). In the following months a consensus was (Somerset), where he had established his in- reached amongst astronomers that such comet struments – originally as an amateur – in the was in fact a planet. After discussion, it pre- garden of his house. Since 1779 he was at- vailed for the new finding the name advanced tending a programme devoted to the survey by the German astronomer Johann Elert Bode: of all the stars in the sky, with special atten- Uranus. tion for double stars. In March 1781, after hav- Retrospectively, as early as March 1781 ing catalogued the stars of 4th magnitude, he that astral body was not at all new. By begun more ambitiously with those brighter the end of 1781, Bode succeeded in prov- than 8th. On the night of March 13th, some- ing Uranus identity with the corresponding thing drew his attention “in the quartile near ‘stars’ in Flamsteed’s catalogue and other cat- alogues (Bode 1781, pp. 218-219). An ob- Send offprint requests to: L. Guzzardi ject of 6th magnitude, it was observed by a Guzzardi: Boscovich, Uranus and theoretical astronomy 27 number of astronomers since the end of 17th (Dadic´ 1965, p. 211 and Kuhn 1977, pp. century (and maybe earlier): John Flamsteed 170-173). To begin with, from Boscovich and (1690, 1712, 1715), James Bradley (1748, Lalande’s works we know that as early as 1750, 1753), Tobias Mayer (1756), Pierre May 1781 Saron concluded that the distance Charles Le Monnier (1750, 1768, 1769, 1771). of the observed object from the Sun was very And yet, because of the big distance involved, large, comparable to the known distance at they failed to find out its motion and cata- the present day (i.e. approximately between 18 logued it nearly everytime as a different star and 20 UA). Then, Saron and Mechain´ ap- in the sky. However, Herschel could see its plied Boscovich’s general method for the de- motion, therefore realizing it was no star and termination of orbits and calculated an ap- suggesting it was a comet. But alternative proximately circular path, typical for a planet. claims ascribing a planetary nature to the ob- This is at least Boscovich’s opinion, empha- ject, came as Herschel notified the new finding sizing Saron’s and Mechain’s´ contributions to to the Astronomer Royal, Nevil Maskelyne. the application of his methods for the deter- This happened just some days after Herschel’s mination of cometary and planetary paths (see early observations. Through Maskelyne, the Boscovich’s own account of the early history news of a new object moving in the sky (per- of Uranus discovery in Boscovich 1785, pp. haps a comet or more spectacularly a planet) 474-476). was spread throughout the European scientific So, who discovered the planet Uranus? Part community. of the answer depends on what is considered Other doubts about the cometary hypoth- to be the peculiar mark to a particular discov- esis advanced by Herschel in his communi- ery. If the initial stimulus is the most promi- cation to the Royal Society on March 26th nent feature, then the discoverer of Uranus was (Herschel 1781) were raised by the French Herschel; but note that what made his dis- Astronomer of the Navy Charles Messier, who covery astonishing, namely that it is a planet, pointed out that the moving body they were ob- is due to ideas and calculations in which he serving actually appeared to have none of the played no role. If results are regarded as the es- usual characters of a comet (for instance, tail sential aspect, then the discoverer was Lexell, and coma). because his calculations were more correct As Maskelyne indicated in a letter to than others; but this hypothesis sounds even Herschel on April 23d, only accurate calcu- more arbitrary than the previous one because lations of the path could prove whether it his priority is questionable and the parameter was a “regular planet moving in an orbit of correctness is fuzzy, either too strict or too nearly circular round the sun” or “a Comet loose. moving in a very eccentric ellipsis” (Miner Indeed, should we require an exact correct- 1990, p. 17). The Swedish astronomer Anders ness, a perfect identity with calculations per- J. Lexell is commonly regarded as the first formed by us? Surely not, at least because our to have found (June 1781) an appropriate computation is also affected by (minor) errors quasi-circular orbital path, proving it was a due to assumptions, instruments and approxi- planet; however, at that time the “new” ce- mations. One could reply, for a calculation to lestial body was no longer such sensational be correct only small differences between cur- breaking news, for many other researchers rent and past results should be allowed; but (amongst others Ruggiero Boscovich, Angelo how small should a small difference be? And de Cesaris, Joseph-Jer´ omeˆ de Lalande, Pierre to what extent could small differences between F.A. Mechain,´ Barnaba Oriani, Jean Baptiste calculations be regarded as trifling? de Saron, etc.) were trying to find out its na- Furthermore, there is a fundamental ques- ture and determine its path. tion in the discovery of Uranus that seems As pointed out by some historians of sci- to be overlooked in considering only individ- ence, this facts makes particularly difficult to ual contributions: the non-independency, and establish with finality who in fact came first even the interdependence, of the researchers 28 Guzzardi: Boscovich, Uranus and theoretical astronomy involved in the discovery of Uranus. This was sequence of the continuous fight with his je- a complicated process, from the early observa- suit collegues at Brera Observatory in Milan, tions until the determination of its nature as a he was removed from his office at the ob- planet; and astronomers shared their observa- servatory. Nevertheless, his post as professor tional data, views, methods, and computations of optics and astronomy in Milan was pre- in private letters and communications mostly served. He accounted for his behaviour and before they published their results. This en- the scientific activities carried out in Brera in tire process may be termed “shared discovery”. a detailed memorandum addressed to Milan Though historians and philosophers of science plenipotentiary Carlo Firmian, but having as usually stress the cases of scientific discover- its last receiver Prince Kaunitz, the power- ies made by independent researchers, a more ful Staatskanzler of Habsburg Monarchy, in complicated process of sharing knowledge can charge for Austrian foreign affairs. The mem- be proven to underlie many scientific discover- orandum was intended by Boscovich as a de- ies in the development of modern science, from fence of his own conduct; but he also blamed the circulation of blood (traditionally ascribed Brera Director Louis de La Grange and other to William Harvey in XVII century) to the in- collegues for hostility and intrigue against him troduction of the neutrino (generally attributed (see Proverbio 1987). to Wolfgang Pauli’s conjecture in 1934; see The story came to an end only at the begin- Guzzardi 2012 for more details). ning of 1773, when Boscovich finally resigned An implication of this view is that a bet- his professorship. He initially thought of going ter understanding of such discoveries can be to Poland, maybe travelling at first to his na- reached by investigating the “network” of sci- tive Ragusa, where his mother was still alive. entists involved and their microcommunities But the suppression of the Jesuit order (1773), and research traditions. In what follows I fo- rapidly extending from the European countries cus on Boscovich’s network of professional to the Papal State, made the things worse, and and amateur colleagues in astronomy in or- Boscovich accepted his French friends’ sug- der to account for Boscovich’s own contribu- gestion to go to France, where he would be tion in the process of discovering the planet appointed as the Director of Naval Optics of Uranus.
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