June 29: Pietro Angelo Secchi

Father Pietro Angelo Secchi

June 29, 1818 – February 26, 1878

Father Pietro Angelo Secchi SJ was an Italian astronomer. He was Director of the Observatory at the Pontifical Gregorian University (then called the Roman College) for 28 years. He was a pioneer in astronomical spectroscopy, and was one of the first scientists to state authoritatively that the Sun is a star.

He was born in Reggio Emilia, where he studied at the Jesuit gymnasium. At the age of 16, he entered the Jesuit Order in Rome. He continued his studies at the Roman College, and demonstrated great scientific ability. In 1839, he was appointed tutor of mathematics and physics at the College. In 1841, he became Professor of Physics at the Jesuit College in Loreto. In 1844 he began theological studies in Rome, and was ordained a priest on 12 September 1847. In 1848, due to the Roman Revolution, the Jesuits had to leave Rome. Fr. Secchi spent the next two years in Britain and the United States, where he taught for a time at Georgetown University in Washington, DC. He also took his doctoral examination in theology there.

During his stay in America, he met Commander Matthew Fontaine Maury, the first Director of the United States Naval Observatory in Washington. He studied with Maury and corresponded with him for many years.

He returned to Rome in 1850. On the recommendation of his late colleague Francesco de Vico, he became head of the Observatory of the College at age 32. Under his direction, the crumbling Observatory was relocated to a new facility on top of the Church of St. Ignatius (the chapel of the the College). Fr. Secchi served as Director until his death.

His position was challenged after 1870, when the remnant of the Papal States around Rome was taken over by the Kingdom of Italy. In 1873, the College was declared property of the Italian government. When the government moved to take over the Observatory as well, Fr. Secchi protested vigorously, and threatened to leave the Observatory for one of several positions offered to him by foreign observatories. He was offered important scientific positions and political dignities by the government, but refused to pledge allegiance to the Kingdom in place of the Pope. The royal government did not dare to interfere with him, and he continued as Director.

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 a detailed map of Mars.

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.

The lunar crater Secchi and the Martian crater Secchi are both named after him. The two STEREO (Solar TErrestrial RElations Observatory) spacecraft each carry an instrument package called SECCHI (Sun Earth Connection Coronal and Heliospheric Investigation).