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Apr 20 Angelo Secchi, Spectroscopy and the Papal Yacht

Today the Vaticans astronomer Fr Angelo Secchi came aboard the papal yacht on the Azure waters of the Mediterranean to demonstrate his new invention. Secchi disks are still used widely now as a simple measure of water pollution. We also look at his life and his immense contribution to the world of astronomy. His analysis of light , building on the discoveries of Isaac Newton has lead to him being called the 'Father of Spectroscopy' . One of the first to authoritatively call the Sun a star he invented the heliospectrograph, star spectrograph, and telespectroscope. Starting in 1863, he began collecting the spectra of stars, accumulating some 4,000 stellar spectrograms.


Pope Pius IX was curious about how the azure waters could be so clear. He watched as Father Secchi lowered a 43cm white disk into the water, sending it deeper and deeper until it could no longer be seen. The depth at which the disk is no longer visible was taken as a measure of the transparency of the water. This measure is still known as the Secchi depth and is reached when the reflectance equals the intensity of light backscattered from the water. The pope was delighted and Secchi disks are still in use today as a simple measure of water quality. The papal yacht, however, fared less well, following the unification of Italy and the loss of the Papal States. Pius found himself landlocked, declaring himself ‘to be a prisoner of the Vatican’, although in truth it was he that refused to leave. He ordered the yacht to be secretly moved to Toulon to prevent her being subsumed into the new Italian navy. This ship had been built to transport Pius on a planned, but never fulfilled, pilgrimage to the Holy Land and, since then, had been mainly involved in the suppression of piracy. The French sheltered the vessel in Toulon and eventually the pope, having lacked a seaport for eight years, ordered her to be. Previously Fr Secchi had supervised the construction of lighthouses for the ports of the Papal States. The Papal Navy, had been founded in the ninth century and had accrued battle honours that included Lepanto, the major defeat of the fleet of the Ottoman Empire in the Gulf of Patras.

Secchi Disks are still widely used in marine studies and have become the universal tool for measuring water clarity, at a time where awareness and concern about water pollution is growing. Since its invention, similar disks, with a black-and-yellow pattern, are used as fiducial markers on vehicles in crash tests, crash-test dummies, and other kinetic experiments. Fr Secchi had invented it as a part of his life long quest to understanding light and had been one of the first scientists to state authoritatively that the Sun is a star. He was the first astronomer to develop a spectral classification of star types. As part of his research, he had studied how light was dispersed and this, in turn, led him to study water quality. However, his main area of interest was astronomical spectroscopy, measuring energy that radiated from distant bodies, including visible light and radio. This had interested scientists since Isaac Newton first used a simple prism to observe the refractive properties of light. The spectrum of electromagnetic radiation revealed many of stars properties, such as their chemical composition, temperature, density, mass, distance, and luminosity.

Fr Secchi’s many scientific achievements were achieved in spite of the political instability in Italy. As a member of the International Jesuit order was able to make important international connections. In 1848, due to the Roman Revolution, the Jesuits had been ordered to leave Rome. Angelo Secchi spent the next two years in the United Kingdom at Stonyhurst College in the north of England where an English Jesuit, Alfred Weld, was in charge of the Stonyhurst Observatory. This inspired Sechhi to take up science. He moved on to the United States, where he taught for a time at Georgetown University in Washington, D.C. and met Commander Matthew Fontaine Maury, the first director of the United States Naval Observatory. Fr Secchi was elected to the American Philosophical Society in 1860. His international reputation would protect him when he returned to Italy and the remnant of the Papal States around Rome was taken over by the Kingdom of Italy. The Gregorian University, where Secchi was based was declared property of the Italian government. When the government moved to take over the observatory as well, 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 Italian 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. He died in 1878 at age 59, in Rome.


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