Johann Georg Hagen
Johann (John) Georg Hagen (March 6, 1847 in Bregenz, Austria – September 5, 1930 in Rome, Italy), was an Austrian Jesuit priest and astronomer. Naturalized American citizen he was called to Rome by Pope Pius IX in 1906 to be the first Jesuit director of the new Vatican Observatory.
Entering the Jesuit Order
Johann entered the Society of Jesus, commonly known as the Jesuits, in Gorheim, Germany in 1863. He attended a Jesuit College in Feldkirch, Austria and also studied mathematics and astronomy at the University of Bonn and the University of Münster. He volunteered for the ambulance service in the Franco-Prussian War, but was struck with typhoid fever.
Emigration to US
In June 1880, he left England for the United States. There he began teaching at Sacred Heart College in Prairie du Chien, Wisconsin. There he cultivated his interest in astronomy and built a small observatory for making astronomical observations. In Wisconsin, he became a naturalized citizen.
He was called to serve as the Director of the Georgetown University Observatory in 1888. There he continued his research and published numerous articles and texts. In mathematics, the Rothe–Hagen identity is named after him; it appears in his three-volume 1891 publication, Synopsis of Higher Mathematics.
- Atlas Stellarum Variabilium (Atlas of variable stars) (in Latin). Berlin: Felix L. Dames. 1890-1908. 
- with G. A. Fargis: The photochronograph, and its applications to star transits. Georgetown, D. C.: Georgetown College Observatory. 1891.
- Synopsis der höheren Mathematik. 4 vols. Berlin: F. L. Dames.Vol. 1: Arithmetische und algebraische Analyse. 1891.Vol. 2: Geometrie der algebraischen Gebilde. 1894.Vol. 3: Differential- und Integralrechnung. 1905.Vol. 4: Differentialgeometrie der Ebene und des Raumes. 1930.
- Index operum Leonardi Euleri. Berlin: F. L. Dames. 1896.
- "On the history of the extensions of the calculus". Bull. Amer. Math. Soc. 6 (9): 381–390. 1900. MR 1557730.
- La rotation de la terre, ses preuves mécaniques anciennes et nouvelles. Rome: Tipografia Vaticana. 1911.
- Wisconsin Journal of History, December 1941, page 180.
- Stein, J. (1931), "Johann Georg Hagen, S. J.", Popular Astronomy 39: 8, Bibcode:1931PA.....39....8S
- Parkhurst, J. A. (1907). "Review: Atlas Stellarum Variabilium, by Johann G. Hagen". Astrophysical Journal 25: 361–362. Bibcode:1907ApJ....25..361.. doi:10.1086/141461.
- Jacoby, Harold (1891). "Review: The photochronograph, and its applications to star transits, by J. G. Hagen and G. A. Fargis". Bull. Amer. Math. Soc. 1 (2): 44–46.
- Ziwet, Alexander (1897). "Review: Index operum Leonardi Euleri, by J. G. Hagen". Bull. Amer. Math. Soc. 3 (7): 256.
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