Astronomical station for the eclipse of 1860 in Spain

[Desierto de Las Palmas, Castellón de la Plana, 1860]
Photography on sensitive paper
250 x 180 mm
Historical archives of INAF-Astronomical Observatory of Rome, Monte Porzio Catone (Rome)

“If for all the astronomers the 1860 eclipse had a powerful appeal, for me it was uniquely charming, because the main subject of my studies was the physical structure of the Sun since many years, and I was already searching for several preparations and studies to carry on in those precious moments”. With these words Father Secchi reported to the members of the Tiberine Academy of Rome the results of his expedition to Spain for the total solar eclipse of 18 july 1860, the first and only international scientific expedition in which he took part. For Secchi it was an important chance to deal with various questions about the structure of the Sun, especially on the nature of the protuberances. The illustrations and the daguerreotypes made during the 1842 and 1851 eclipses suggested that the protuberances were phenomena occurring on the Sun surface, but this interpretation was not fully agreed-on.
Whether the “red prominences” observed around the Moon during the minutes of the eclipse were just an optical illusion or a real physic entity, whether then their nature was solid or gaseous, whether the corona surrounding the Moon during the Sun eclipsing corresponded to the solar atmosphere or not, and if there were other optical or physical phenomena that could be related to the Moon during the eclipse, all those questions had paramount importance for Secchi. His approach, indeed, was aimed to a unitary vision of the universe, and the eclipse observation, the study of the physical characteristics of the terrestrial and heavenly bodies, and , in general, the investigation of the existing interactions in nature ultimately allowed to unveil the mechanicistic order of the creation and the continuos process of movement and transformation of the matter originated by the Divine action.
To clarify the nature of the protuberances, Secchi decided to take some pictures using Collodium photographic plates, a technique which use in astronomical field was yet pioneristic. Following the suggestions of Antonio Aguilar, director of the Royal Observatory of Madrid, Secchi chose as venue for the observation the Carmelite convent of the Las Palmas Desert, not far from Castellón de la Plana, in the totality zone. José Monserrat, professor of chemistry at Valencia University, was appointed to the photographical coverage of the event, achieved with the Cauchoix telescope, brang by Secchi from the Collegio Romano and placed near to the ancient convent. From the mountaintop of the mount San Miguel Secchi and Aguilar realized the visual observations with a Fraunhofer refractor telscope.
The sight of the eclipse was observed in its whole magnificence and the equipe coordinated by Secchi was able to realize some pictures of the totality phase, which analysis persuaded Secchi about the correct interpretation of the protuberances as solar phenomena. Moreover, the subsequent confrontation among his photographs and the ones obtained by the British astronomer Warren De La Rue took in Rivabellosa, five hundred kilometers away distant from Secchi’s observation site, highlighted an almost perfect coincidence of the positions of the protuberances on the photographic plates.
The work carried on by Secchi and De La Rue led to the conclusion that the protuberances were physical phenomena characteristic of the Sun, justifying the importance of photography in the observation of astronomical phenomena. 1860 eclipse was a true laboratory of international collaboration for the analysis of the phenomenon. The eclipse was observed by more than one hundred and fifty scientists from North America, Spain, and North Africa. The subsequent widespread of the photographic activity, together with spectroscopic techniques applied to stars, supported the strenghtening of an international community of solar physicians, in which Secchi had a keyrole.

Matteo Realdi, Vrije Universiteit Amsterdam

Bibliography
Chinnici I. (2018), “Ricordando Angelo Secchi: cronaca per la spedizione in Spagna del 1860”, Giornale di Astronomia, 44:2, pp. 23-26;
Hufbauer K. (1991), Exploring the Sun. Solar science since Galileo. Baltimore and London, The Johns Hopkins University Press;
Mazzotti M. (2010), “The Jesuit on the roof: observatory sciences, metaphysics, and nation-building”, in: Aubin D. & al. (a cura di), The Heavens on Earth. Observatories and Astronomy in Nineteenth-Century Science and Culture, Durham and London, Duke University Press, pp. 58-85;
Secchi A. (1860), Relazione delle osservazioni fatte in Spagna durante l’eclisse totale del 18 Luglio 1860, Roma, Tipografia delle Belle Arti.