“Solar prominences observed at the Collegio Romano 1871”

[Angelo Secchi]
Drawing, 216 x 310 mm
Historical archives of INAF-Astronomical Observatory of Rome, Monte Porzio Catone (Rome)

In 1869 Angelo Secchi began regular spectroscopic observations of the chromosphere and the solar protuberances, in order to study their nature and their relationship with the stains and the facolae present on the solar disk.
The figure reproduced here shows some examples of the protuberance drawings performed during the observations, carried out predominantly in correspondence with the spectral line Hα, using the Merz Equatorial telescope in combination with the spectroscopic instruments .
From the observation of the protuberances, Secchi inferred that “in their infinite and capricious forms” they are “also susceptible to a certain classification”. As his colleague Pietro Tacchini, with whom he collaborated in Solar Observations, Secchi divided the regions observed in three classes “distinct: those with live jets, short and large, isolated and low and mostly divergent. Those with a filamentous structure and less lively, enormously higher, but less shiny, those with the structure with cumulus clouds. ” Secchi studied the details of the structure of the observed protuberances, estimated their size, hypothesized their nature. For instance, he wrote: “Figure 1 represents a very high prominence of about 2 ‘ 1/2″ or about 9 Earth diameters of height, mostly nebular, although there were hidden jets. It is really made up by 3 orders, the lower is formed by 2 masses, which extend horizontally after emerging vertically, the principal under the number 1 rises for three orders, and extends on the following on the right, which is much less energetic. This resembles to the clouds that we call strata, and with its distribution suggests different temperatures for different heights of the Sun. ” Moreover, from the evolution of the observed regions Secchi estimated the velocity of the plasma jets that form the protuberances and realized that for some regions “the projection of one on the other, in those that look vast, produces an unspeakable confusion” (Figure 1). He noted the elicity of the prominences (Figure 2), reporting that “their general trend often has the obvious form of spiral”. Secchi recognized the link between the spots, the facolae and the protuberances, reporting that “it is certainly a great fact that there are never spots without facolae. And now we know that the spots and the facolae are accompanied by difference in height in the photosphere and by luminous jets, “the protuberances.
The drawings reproduced here in draft accompanied a publication of Secchi on the acts of the new Lyncean Academy.

Ilaria Ermolli, INAF-Osservatorio Astronomico di Roma
Marco Ferrucci, INAF-Osservatorio Astronomico di Roma