Astronomers in the astronomical station of Augusta

[Tommaso Tagliarini]
[Augusta, 1870]
Photography
150 x 200 mm
Historical archives of INAF-Palermo Astronomical Observatory

“According to the deliberations of this illustrious Commission [established by the Italian Government to organize the scientific expedition for the 1870 eclipse], I should especially take care of the solar photographs during the totality phase, being the question about the real form of the corona the most interesting for science. The scientific program self-proposed by Father Secchi for the solar eclipse of the 22 december 1870 that would cover the south-eastern Sicily(Figure 1), was extremely clear and ambitious. The eclipse was an unique chance for the Italian scientists to put up a scientific expedition with wide scientific targets. It was at the preliminary meeting of september 1869 of the eclipse commission presided over by Giovanni Santini that Secchi was tasked with the photographic observations, point on which he had insisted, finally obtaining support. Ten years earlier, during the total eclipse of Sun of 1860 in Spain, Secchi obtained some among the first photographic images of the solar corona thanks to a meticulous preparation of the telescope and of the photographic technique, reaching important results.
The Commission financed the whole expense for the improvements required by Secchi to the Cauchoix telescope, in need of a solid mechanic for tracking, although the telescope of the Roman College belonged to another State. For the photographic component of the expedition, very expensive, some saving were required. Rather than bringing a photographer from Rome or Naples as Secchi would have preferred, Santini after suggestions of Cacciatore suggested a valid photographer from Palermo, Tommaso Tagliarini, who was very passionate about that collaboration, and which already proved himself valid during the eclipse in the southern Italy on 6 March 1867.
The photographic technique in vogue at that time, relying on humid Colloid,required a great experience in maneuver chemicals and a good velocity and ability of execution. Secchi planned the observations meticulously. The totality phase would last 110 seconds, and Secchi calculated that at least three photographs could be taken: two with 10 seconds of exposure for the protuberances, at the start and at the end of the totality phase, and one with 70 seconds of exposure for the solar corona in the middle. In this way 20 seconds remained for the changes in exposure and for a potential fourth exposure. Anyway the calculation of the exposure times was really complicated since it depended by a variety of factors, such as the “chemical force” of the humid Colloid prepared seconds before by the photographer. Moreover, the transparency of atmosphere and the strength of the “chemical rays”(blue-purple), able to impress the photographic plate, would play a key role for the success of the photographic sequence.
Prior to the day of the eclipse a meticulous set-up of the telescope and several exercises with the photographic equipments were made, to minimize the problems during the totality phase(Figure 2a-b). The weather during the starting point of the eclipse “showed sad presages” . When the phenomenon started weather was good, but just 15 minutes prior to the totality phase scattered clouds and high veils of cloud appeared in the observation field. Everything suddenly deteriorates in the exact instant of the totality phase, when”unfortunately[…] a cumulus with a veiled remnant” appeared covering the whole scene.
“ Discouragement seized the photographers” and the distraction caused by the sudden phenomenon provoked some untidy reaction. Secchi handled the situation with uncommon stiffness, changing the sequence of the exposures when, more or less one minute after the start, the cloud quickly moved away, unveiling the Sun. The final result left the photographers “disgusted” and it seemed that all the exposures were totally lost, but Secchi insisting noticed how “at the third pose a beautiful crown of protuberances had remained etched. This was an arc less than half-circle, but important and thus the figure was religiously preserved after being fixed.

Simone Zaggia, INAF-Osservatorio Astronomico di Padova

Bibliography
Rapporti sulle osservazioni dell’eclissi totale di sole del 22 dicembre 1870, Palermo, Stabilimento Tipografico LAO, 1872;
E. Olostro Cirella, M. Gargano, L'eclisse di Sole del 1870: una nuova prospettiva per l'astrofisica italiana, I. Chinnici, a cura di, Starlight, Napoli, Arte'm, 2016, pp. 56-78;
E. Proverbio, La nascita della fisica solare ed il contributo di Pietro Tacchini allo sviluppo delle ricerche di astronomia solare in Italia, Atti della “Fondazione Giorgio Ronchi”, 2001, n.6, p.1165;
Lettere Santini-Secchi, Archivio OAPD; I Tagliarini, due fotografi nella Sicilia del Grand Tour, Provincia di Palermo.