Le Soleil

Exposé des principales découvertes modernes sur la structure de cet astre, son influence dans l’univers et ses relations avec les autres corps célestes
Angelo Secchi
Paris, Gauthier-Villars, imprimeur-libraire 1875-1877
Vol. II, Tav. C
260 x 210 mm
Library of Arcetri Astrophysical Observatory, Florence

This work can rightfully be considered the main astronomical treatise on the Nineteenth-century sun, especially in the completely revised and enlarged edition, published in two volumes, again in French, in 1875 and 1877. Its success is testified also by the translations in German (Die Sonne, 1872), in Spanish (El Sol, 1879) and in Italian (the Sun, 1884). Since Secchi’s earliest work on meteorology, his interest had been stimulated to study the effects of solar radiation and to the extent of its intensity, for which it had devised and built a special instrument, the thermo- heliometer. Secchi was then actively interested in the study of sunspots, which at the time were considered vortices or eruptions in the solar photosphere. But the fame of Secchi in solar studies is mainly due to observations made in Spain during the solar eclipse of 1860, which allowed to definitively establish the solar nature of the protuberances, some of which are reproduced here in lithography. The solar chromosphere quickly became the subject of spectroscopic studies, especially from 1868 when Norman Lockyer and Jules Janssen independently discovered the method for observing the protuberances in full sun, without waiting for the rare and short opportunities offered by total eclipses.
All these new discoveries were collected and examined in detail in the treatise Le Soleil, in particular in the enlarged edition of 1875-1877, very well illustrated and also accompanied by an atlas of the solar spectrum. The first edition of Le Soleil was subdivided into three parts of a fairly unequal length: a very long first part, “Structure du Soleil”, a sort of broad compendium of the knowledge of the time about the Sun, followed by two shorter parts titled “activité extérieure du Soleil” (Dedicated to the effects of solar radiation on the planetary system), and “Les soleils ou les étoiles” (containing an explanation of classification of stars in spectral classes to which Secchi worked for a decade). Compared to the first edition, the one of 1875-1877 in two volumes, which has become necessary by the continuous succession of new discoveries, must be considered, according to the words of Secchi, “a completely rewritten and almost entirely new work .” In it the author finally believes that the time to give “scientific unity” and “logical coordination to the immense multitude of facts collected in recent years”, has finally come, testifying the now attained independence and maturity of the studies of solar physics.

Daniele Galli - INAF-Osservatorio Astrofisico di Arcetri