Angelo Secchi - Astronomical Works

Astronomical Works

Father Secchi made contributions to many areas of astronomy.

• He revised Struve's catalog of double stars, compiling data for over 10,000 binaries.
• He discovered three comets, including Comet Secchi.
• He produced an exact map of the lunar crater Copernicus.
• He drew some of the first color illustrations of Mars and was the first to refer to "canali", the Italian word for channels, on the planetary surface.

Fr. Secchi was especially interested in the Sun, which he observed continually throughout his career.

• He observed and made drawings of solar eruptions and sunspots, and compiled records of sunspot activity.
• In 1860 and 1870, he organized expeditions to observe solar eclipses.
• He proved that the solar corona and coronal prominences observed during a solar eclipse were part of the Sun, and not artifacts of the eclipse.
• He discovered solar spicules.

However, his main area of interest was astronomical spectroscopy. He invented the heliospectrograph, star spectrograph, and telespectroscope. He showed that certain absorption lines in the spectrum of the Sun were caused by absorption in the Earth's atmosphere.

Starting in 1863, he began collecting the spectra of stars, accumulating some 4,000 stellar spectrograms. Through analysis of this data, he discovered that the stars come in a limited number of distinct types and subtypes, which could be distinguished by their different spectral patterns. From this concept, he developed the first system of stellar classification: the five Secchi classes. While his system was superseded by the Harvard system, he still stands as discoverer of the principle of stellar classification, which is a fundamental element of astrophysics. His recognition of molecular bands of carbon radicals in the spectra of some stars made him the discoverer of carbon stars, which made one of his spectral classes.