Astronomer Angelo Secchi demonstrates the Secchi disk, which measures water clarity, aboard Pope Pius IX's yacht, the L'Immaculata Concezion.

Angelo Secchi was an Italian Catholic priest and astronomer from the Italian region of Emilia. He was director of the observatory at the Pontifical Gregorian University 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.

The Secchi disk, as created in 1865 by Angelo Secchi, is a plain white, circular disk 30 cm (12 in) in diameter used to measure water transparency or turbidity in bodies of water. The disc is mounted on a pole or line and lowered slowly down in the water. The depth at which the disk is no longer visible is taken as a measure of the transparency of the water. This measure is known as the Secchi depth and is related to water turbidity. Since its invention, the disk has also been used in a modified, smaller 20 cm (8 in) diameter, black-and-white design to measure freshwater transparency.

Pope Pius IX was head of the Catholic Church from 1846 to 1878. His reign of nearly 32 years is the longest verified of any pope in history; if including unverified reigns, his reign was second to that of Peter the Apostle. He was notable for convoking the First Vatican Council in 1868 which defined the dogma of papal infallibility before taking a break in summer of 1870. The council never reconvened. At the same time, France started the French-Prussian War and removed the troops that protected the Papal States, which allowed the Capture of Rome by the Kingdom of Italy on 20 September 1870. Thereafter, he refused to leave Vatican City, declaring himself a "prisoner in the Vatican".