SECCHI, (PIETRO) ANGELO

(b. Reggio nell’ Emilia, Italy, 18 June 1818: d. Rome, Italy, 26 February 1878)

astronomy, astrophysics.

Secchi’s father was a cabinetmaker, and the family was of modest circumstances. Secchi himself attended the Jesuit school in Reggio nell’Emilia, then, toward the end of 1833, when he was fifteen, entered the Jesuit novitiate in Rome. Two years later he entered the Collegio Romano, where he distinguished himself in a course that included physics and mathematics. In 1841 he was appointed instructor in these subjects at the Jesuit college in Loreto and soon became known for the originatily of his lectures. Between 1844 and 1848 Secchi was obliged to return to his theological studies, but was able simultaneously to continue his astronomical work under Francesco de Vico, director of the observatory of the Collegio Romano and professor of astronomy at the Gregorian University in Rome.

When the Jesuits were expelled from Rome in 1848. Secchi went first to the flourishing Jesuit college at Stonyhurst, Lancashire, England. He then established himself at Georgetown University in Washington, D.C., where he acted as assistant to the director of the observatory, Father P. Curley and made further studies in both theoretical and practical astronomy. He met the hydrographer M. F. Maury, and became acquainted with Maury’s important meteorological works. In 1849 the ban against the Jesuits was lifted, and Secchi was able to return to Rome, where he took up an appointment as director of the observatory of the Collegio Romano.

Secchi recognized immediately that the observatory and its equipment were inadequate. He determined both to build a new observatory and to reshape the course of the research to be performed there, placing a new emphasis on astrophysics. His predecessor de Vico had already conducted important observations on comets, the rotation of Venus, and the satellites of Saturn, and Secchi, using the outmoded instruments of the old installation, began to examine the physical aspect of the ring of Saturn. In 1851 he began to study the sun, measuring the intensity of solar radiation with a thermoelectric pile, a technique previously employed by Joseph Henry to determine the temperature of a sunspot. Secchi first measured the radiation of the sun during the eclipse of 1851, then applied this method to the full solar disk in an attempt to establish the relationship between the temperature of the disk at its center and that at its edges. In the course of his work during the eclipse he made several daguerreotypes that must be reckoned among the earliest applications of photography in the study of the celestial bodies. His results indicated that the center of the solar disk emits almost twice as much radiation as does its borders.

The following year Secchi collated the two fragments into which Biela’s comet had split during its appearance in 1846. He was able to observe a weak star through one of the segments, and was thereby able to demonstrate the thinness of the matter of which the comet was constituted. In 1853 Secchi discovered a comet with a multiple nucleus. He was, during this period in which astronomers were first beginning to understand the true nature of comets, more interested in determining their physical composition than in observing their position, and his interest extended to falling stars, which he introduced as a subject for study by his students at the Collegio Romano. His own observations led him to the conclusion that falling stars were of cosmic origin and thus paved the way for the work of Schiaparelli. At about the same time he made his first investigations of nebulae, by which he sought to confirm the results obtained by William Herschel, James Clark Ross, William Huggins, and Hermann Vogel.

Secchi was further engaged in preparing the new observatory that was being constructed over the church of St. Ignatius, part of the Collegio Romano. He installed a Merz equatorial telescope with an aperture of 24.5 centimeters and a focal distance of 4.3 meters, and with it resumed his observations of Saturn. With the new instrument, Secchi was able to determine the physical characteristics of the planet, including its polar flattening and the eccentricity of its ring. Having made further observations of Mars, during its opposition of 1859, Secchi announced that he had seen two permanent “canals” between the two reddish equatorial continents of that planet: he thus introduced a term which was widely taken up by his successors. In the same year he made studies of the moon, during which he measured and and made a detailed drawing of the crater of Copernicus for the purpose of noting seasonal variations, should such occur. He attempted to apply photography to the study of that body, but determined that his refractor was not suitable for photographic use, since its visual objective was corrected for the human eye, rather than for the special qualities of photographic emulsions.

About 1860 Secchi began to make observations and drawings of Jupiter. He noted that disturbances similar to terrestrial storms occurred in its atmosphere and, with the Merz objective, studied its satellites in some detail. In particular, he observed the spots on the Jovian moons, their periods of rotation about their axes, and the characteristics of their reflected light. He made spectroscopic studies of both Jupiter and Saturn and discovered the presence of special absorption bands in their spectra, which led him to conclude that the atmospheres of these planets contained elements different from terrestrial ones. He also made note of the almost permanent transparency of the atmosphere of Mars and proved that Uranus and Neptune do not have discontinuous spectra, but rather demonstrate bands that have the same qualities (although of much greater intensity) as those that he had found in the spectra of Jupiter and Saturn.

Secchi soon turned his attention to the sun. From the beginning he was convinced of the applicability of his research to a wide variety of celestial and terrestrial phenomena, a belief that he later summed up (in the introduction to his Le soleil), when he wrote that

Whatever our researches and [whatever] the knowledge that we acquire from them, it will not be in our power to regulate the influence of the sun. Nevertheless, the action of this star is too intimately related to the phenomena of life, heat, and light to render useless the studies that may enable us to investigate its nature. And, on the other hand, who knows whether or not an intimate relationship may exist between certain solar phenomena and some terrestrial ones that it would be important for us to predict with some degree of certainty?

Secchi made use of a good helioscopic eyepiece and projections to observe, on the photosphere, a great number of small luminous granules. These granules were of a variety of sizes and shapes, although the commonest were oval: they stood out upon a darker (although not entirely black) ground. This granulation was broken, most notably at the edges of the disk, by luminous tongues, which Secchi named “facole,” and by the small black holes (which he called “pori”) that are the points from which sunspots originate. Secchi noted that the luminous granules represented the extremities of columns of the warmer gases that arise from the cooler and less luminous solar surfaces. Having observed that the formation of sunspots, which generally appeared after a period of surface agitation, was usually accompanied by the appearance of less brilliant luminous tongues (now called “flares”), Secchi determined to investigate them. He concluded that these flares were, in fact, complex groups of gases with several nuclei (or dark central shadows), surrounded by halfshadows.

Secchi also observed the chromospheric eruptions that cross the nuclei of sunspots and split them into segments. It was clear to him that such wide, rapid, and complex surface movements could not occur in a solid substance, and he therefore suggested that the entire photosphere must be composed of an elastic fluid, similar to a gas, through which the sunspots move in a manner similar to terrestrial cyclones. He noticed that these vortices are more frequent during a period in which sunspots are being formed, when the surface movements that create the spots create currents that converge toward the nuclei. This represented further evidence of the gaseous nature of the sunspots and of the photosphere. Secchi lastly applied the law of diminution of angular velocity to the movement of sunspots from the equator to the poles of the solar surface to ascertain that the sun, or at least the photosphere, moves in accordance or at least the photosphere, moves in accordance with the laws that govern a fluid mass.

Secchi wished to study the sun spectroscopically, and to that end commissioned Hofmann and Merz to construct spectroscopes incorporating a series of prisms, while G. B. Amici constructed instruments for direct viewing. Taking up Kirchhoff’s researches. Secchi demonstrated that the absorbing stratum of the sun, later identified as an inversion layer, must be very thin. With a large dispersion spectroscope, which he attached to the Merz equatorial. Secchi was able to observe the inversion of the hydrogen line on the chromosphere, which occurred an instant before the appearance of the dark D lines of Fraunhofer. Since the continuous spectra, with the exception of a few lines of sodium and magnesia, were also inverted, Secchi concluded that the stratum that in that instant partially inverts the dark lines of certain metals (such as the rose stratum of the chromosphere) also inverts the dark lines of hydrogen. This spectral analysis provided further confirmation of a solar atmosphere similar to the terrestrial one, although containing many lines of unknown origin, possibly those of elements that did not exist on earth. Secchi’s spectroscopic examination of sunspots led him to recognize that the lines they exhibit are those of the solar atmosphere, although more or less widened, intensified, or weakened.

Secchi also realized the importance of observing the chromosphere and the corona during total eclipses of the sun. His first opportunity to do so occurred in 1860, when the totality of a solar eclipse was visible in Spain. Secchi traveled to Desierto de las Palmas, near Castellön de la Plana, where he made observations with a Cauchoix refractor; he then compared his results with those De la Rue had made with a Kew photoheliograph at Rivabellosa. Secchi thus concluded that the prominences seen during the eclipse were real, rather than a play of light as some had suggested; that they were solar in origin; and that the corona was also real and thicker at the equator than at the poles, and thickest at forty-five degrees. After the 1868 eclipse Secchi used the technique, simultaneously developed by Janssen and Lockyer, of enlarging the aperture of the spectroscope directed toward the solar border to observe the prominences and chromosphere in full sunlight. He applied this method further in 1869, when he began, with Respighi, the series of observations of the “spectroscopic images of the solar border” that he published in Memorie della Societä degli spettroscopisti italiani in the following year.

Secchi also published a number of observations on solar prominences in his treatise Le soleil, issued in French in 1875–1877. This work was illustrated with magnificent color plates of the chromosphere and of the vaious types of prominences. Among those shown were the small flames (now called “spiculae”) that he observed in the region of sunspots, converging toward the center of the eruption. These, he noted, became higher, more slender, and extremely luminous at the solar poles, where considerable activity manifested itself even during periods of relative calm on the solar surface. He classified the prominences as “quiescent” and “eruptive”—terminology that is still current.

Secchi made an especially careful study of the forces to which solar prominences are subjected, measuring the velocity of the masses expelled by the sun and their movement in the solar atmosphere. He used the spectroscope to observe the shifts, caused by the Doppler effect, of the lines present in the spectra of the prominences and observed that the expelled matter is not only launched upward but also frequently animated by vortical movements that give a spiral appearance to the luminous spouts. He speculated that the variations in velocity now attributed to the presence of magnetic fields must be caused by an as yet unrecognized periodic force. He observed that secular variations, of very short duration, also occurred at the diameter of the sun, and pointed out that irregularities in the shape of the sun were most apparent during periods of (and in the regions of) maximum photospheric agitation.

While conducting his solar studies, Secchi was further concerned with the study of the physical constitution of comets. During the appearance of the comet of 1861 he observed that the head of the comet of 1861 he observed that the head of the comet emitted jets of discontinuous gases, which formed parabolic envelopes about it and, by a backward movement, produced the tail. In observing the comet in 1862, he was able to see that the jets of gases altered from night to night in correspondence to the comet’s distance from the sun. He established the presence of carbon in the spectrum of the comet of 1866 and noted, in addition to the lines of emission, the continuous spectrum that indicated the existence of direct or reflected light. He found carbon associated with hydrogen or oxygen in the bands exhibited by comets of later years.

In 1862 Secchi, in furtherance of the early work on falling stars that had been performed at the Collegio Romano, carried out simultaneous observations at Rome and at Civitavecchia in order to determine the altitude of falling stars, which he calculated as being between seventy-five and 250 kilometers, with a falling velocity of ninety kilometers per second. From these and later observations he also established the similarity of falling stars, asteroids, and aerolites. Using the spectroscope he demonstrated that all these phenomena contained such metals as iron, magnesium, and sodium; he was particularly active during the rain of fire of 27 November 1872, “in which,” he wrote, “the layers of distant light resemble the surge of snowflakes.”

Secchi’s spectroscopic research on luminous stars began in 1863, when he was visited in Rome by Janssen, who had a small spectroscope with him. Together they attached the spectroscope to a Merz equatorial and observed the stars, communicating their joint findings to the Paris Academy. Secchi and Jannsen found Fraunhofer lines in the stellar spectra they examined, and identified some of these with terrestrial elements: Secchi then began to work with more sophisticated equipment to define the differences in solar spectra already noted by Donati and Huggins. He determined to investigate the spectra of a large number of stars, and set out his plan in a communication to the Pontifica Accademia Tiberinia on 27 January 1868. He prefaced his report with the statement that: “In substance I wanted to see whether, just as the stars are countless, their composition is also proportionately varied. This was my query, and having been fortunate enough to perfect the observation instrument, the harvest was abundant, even more than I had hoped.” The instrument to which he referred was a spectroscope equipped with a luminous aperture with which he could analyze even the weakest stars: with it Secchi was able to recognize a specific spectral type in a number of principal stars.

Secchi then determined to examine other stars by means of simple differential methods. Toward the end of 1869 he decided to adopt the apparatus that Fraunhofer had used, which consisted of a flint prism, fifteen centimeters in diameter, with a refracting angle of twelve degrees. He mounted this prism directly in front of the twenty-three-centimeter objective of the Merz equatorial telescope; he thus obtained greater luminosity and a larger dispersion than had been possible with the direct-vision prism, inserted between the objective and the ocular piece, that had been possible with the direct-vision prism, inserted between the objective and the ocular piece, that he had used previously. He next experimented with a circular prism, constructed by Merz, which he attached to the Cauchoix equatorial, the objective having an aperture equal to that of the prism. With this new combination he discovered that “whereas the stars are very numerous, their spectra are reduced to a few well-defined and distinct forms, which for the sake of brevity we may call types.” He went on to note that he had examined at least 4,000 stars, and was able to divide them into five types, with the high-temperature white stars at one end of the scale This classification still bears Secchi’s name: it was soon adopted almost universally.

Secchi also classified nebulae according to his spectroscopic examination, into planetary, elliptical, and irregular forms. He examined examples of each type and found spectral lines of emission produced both by hydrogen and by elements then unknown on earth. The presence of these unknown elements led him to deduce that some nebulae are masses of pure gas—that is, that not all of them are resolvable into stars—a confirmation of theory put forth by Herschel and accepted by Secchi himself. Of the elliptical nebulae of the external galaxies he was particularly interested in Andromeda, of which he studied the continuous spectrum.

In his observations of Andromeda Secchi noticed two black canals transversing the nebula. It seemed to him that these must be zones of non-luminous matter, projected on the nebula proper and intercepting the light of the stars, a phenomenon that he discovered in other parts of the Milky Way. He noted that it was improbable that these canals should, in fact, represent apertures, particularly given the gaseous nature of nebulae: many years later observations confirmed the presence of dark masses dispersed in space, which are seen projected on the background of a sky made luminous by light-emitting cosmic matter.

Secchi had almost ceased to make observations by 1873, when the Jesuits were again expelled from the Collegio Romano. He was nonetheless allowed to remain at his observatory through the intervention of the government, and he spent his last years preparing his scholarly writings for their final editions. He also continued to publish notes in the Memorie della Società degli spettroscoopisti italiani, of which he had been, with Tacchini, a founder in 1870.

BIBLIOGRAPHY

I. Original Works Secchi’s works include Le soleil (Paris, 1875–1877); Le stella (Milan, 1877); and L’astronomia in Roma nel pontificato di Pio IX (Rome, 1877), A complete list of his writings is given by Bricarelli, cited below.

II. Secondary Literature. On Secchi and his works, see G. Abetti. Padre Angelo Secchi (Milan, 1928): and Storia dell’ astronomia (Florence, 1963), 159, 187–203, 205–206, 228–229, 255, 259, 266, 270, 278, 299, 301, 304, 364, 377, 389–391. 395, 401; and Bricarelli, “Della vita e delle opere del P. Angelo Secchi,” in Memorie dell’ Accademia pontificia dei Nuovi Lincei. 4 (1888).

Giorgio Abetti