Sunspot

Macchia solare osservata il giorno 5 maggio [1855] a 3 ore pom[eridiane] all’Equatoriale di Merz dell’Osservatorio del Collegio Romano
[Angelo Secchi]
Historical archives of INAF-Astronomical Observatory of Rome, Monte Porzio Catone (Rome)

There is no doubt that Secchi had a predilection for the study of the Sun: the numerous texts and manuscripts dedicated to solar physics prove it. In this study he applied photography and spectroscopy, publishing one of the main solar physics treaties of the nineteenth century, Le Soleil.
The image reproduced here depicts a watercolor of the sunspot that Secchi observed at the Merz Telescope of the Observatory of the Roman College on the afternoon of May 5, 1855, from 15.00 hours. This is a sketch was also printed: in fact, this image appears in the lower right corner of the panel of the Memories of the Observatory of 1856.
During his study, Secchi carried out numerous observations of the Sun that concerned both the photosphere, that is the region of the disk visible in white light, and the chromosphere, which he observed outside the total eclipses, thanks to sophisticated Spectroscopic techniques, in collaboration with Pietro Tacchini.
As far as the photosphere is concerned, keeping up with an observational tradition that dates back to Galileo Galilei and to the Jesuit astronomer Christoph Scheiner, he systematically followed the evolution of stains and clumps of stains. Secchi also studied the Photospheric granulation, due to the convective motions present in the outermost layers of the solar structure. The granules were interpreted by him as emission of incandescent matter (flames) projected towards the outside of the solar disk.
These kind of observations are still being conducted in solar observatories around the world through modern imaging techniques and by solar telescopes orbiting into space.

Aldo Altamore, Università degli Studi “Roma Tre”
Ileana Chinnici, INAF-Osservatorio Astronomico di Palermo

Bibliography
A. Secchi, Le Stelle, Fratelli Dumolard, Milano, 1877, p. 54 e ss;
A. Secchi, Le Soleil, Gauthier-Villars, Parigi,1875, tomo I, p. 56 e ss;
https://sohowww.nascom.nasa.gov/gallery/images/spotcollage.html )