The Gregorian calendar (used today in most of the world) was issued in 1582. Pope Gregory XIII was annoyed by the Julian calendar (since 45 B.C. and off by 11 days). Almost simultaneously, he ordered the establishment of a Vatican astronomy laboratory to study the sun. Pope Leo XIII in 1891 formally refounded the Specola Vaticana (Vatican Observatory) and located it on a hillside behind the dome of St. Peter’s Basilica. The city lights interfered with seeing the stars, so Pope Pius XI provided a new location for the Observatory at the Papal Summer Residence at Castel Gandolfo in the Alban Hills some 25 kilometers southeast of Rome.
In 1981, for the first time in its history, the Observatory founded a second research center, the Vatican Observatory Research Group (VORG), in Tucson, Arizona in the United States, one of the world’s largest and most modern centers for observational astronomy. In 1993, the Observatory, in collaboration with Steward Observatory, completed the construction of the Vatican Advanced Technology Telescope (VATT, first optical-infrared telescope and pioneering new technology with international collaboration) on Mt. Graham, Arizona, probably the best astronomical site in the Continental United States. (For more information, go to www.vaticanobservatory.org)
The dedication plaque of the VATT reads: This new tower for studying the stars has been erected during the XV year of the reign of John Paul II on this peaceful site so fit for such studies, and it has been equipped with a new large mirror for detecting the faintest glimmers of light from distant objects. May whoever searches here night and day the far reaches of space use it joyfully with the help of God.