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-Rome Astronomical Observatory, 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, at 15:00 hours. This sketch was also printed: in fact, this image appears in the lower right corner of the plate of the Memoirs of the Observatory of 1856.
During his study, Secchi carried out many solar observations 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 sunspot and clusters of sunspot. Secchi also studied the photospheric granulation, which is due to the convective motions present in the outermost layers of the solar structure. The granules were interpreted by him as emissions of incandescent matter (flames) out of the solar surface.
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 )