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About Time in Vatican

Inter Gravissimas: The Bull That Rewrote the Calendar

On February 24, 1582, Pope Gregory XIII issued a papal bull titled Inter gravissimas, announcing that the Julian calendar, in use since the time of Julius Caesar, would be corrected because its year was running about eleven minutes too long. Over many centuries that small error had pushed the spring equinox, and with it the calculation of Easter, roughly ten days away from where it had fallen at the time of the Council of Nicaea in 325. The bull ordered that the day following Thursday, October 4, 1582, would be counted as Friday, October 15, erasing ten calendar dates in a single stroke, and it also revised the leap year rule so that century years would only be leap years when divisible by four hundred.

The reform had been prepared for years by a papal commission, with the mathematical groundwork laid by the astronomer Aloysius Lilius and refined by the Jesuit mathematician Christopher Clavius. Because the pope had authority only over the Catholic Church, adoption outside Rome depended on individual governments choosing to comply, and the text of the bull was formally posted at the gate of Saint Peter's Basilica before being sent to Catholic monarchs across Europe. Spain, Portugal, most of Italy and Poland Lithuania adopted the new calendar within the same October, while Protestant and Orthodox regions held out for generations, some not switching until the twentieth century.

The Vatican Observatory: Astronomy Beneath Saint Peter's

Formally re-founded in 1891 by Pope Leo XIII, the Vatican Observatory, known in Italian as the Specola Vaticana, grew out of a much older tradition of papal interest in astronomy that stretched back to the sixteenth century calendar reform itself. Its first modern home stood on a hillside directly behind the dome of Saint Peter's Basilica, where astronomers took part in an ambitious international project to photographically map the entire sky, a program known as the Carte du Ciel.

By the early 1930s the growing brightness of Rome's electric lighting had made it impossible to study faint stars from the city center, so in 1935 Pope Pius XI relocated the observatory to the papal summer residence at Castel Gandolfo in the Alban Hills. There, Jesuit astronomers installed new telescopes supplied by the German firm Carl Zeiss and founded a laboratory for spectrochemical analysis. Light pollution eventually caught up with Castel Gandolfo as well, leading the Vatican Observatory to open a second research base in Tucson, Arizona, in 1981, where it later built the Vatican Advanced Technology Telescope atop Mount Graham in cooperation with an American university observatory.

The Tower of the Winds: Where the Old Calendar Was Proven Wrong

Rising above the Gallery of Maps that connects two wings of the Apostolic Palace, the Gregorian Tower, also called the Tower of the Winds, was built between 1578 and 1580 specifically to give astronomers a place to measure the sun's position in support of the coming calendar reform. Inside, the mathematician and cartographer Ignazio Danti installed a meridian line on the floor, a simple but precise instrument that let observers track exactly where a beam of sunlight fell through a small opening at solar noon on different days of the year.

When the line was first tested, the spring equinox sunlight fell on March 11 rather than March 21, physically demonstrating that the Julian calendar had drifted ten days out of alignment with the seasons, evidence that helped justify Pope Gregory XIII's decision to proceed with the reform. Christopher Clavius and his fellow Jesuit astronomer Christoph Grienberger are counted among the first scientists to make regular observations from the tower. In later centuries the same rooftop briefly housed the exiled Queen Christina of Sweden and, in the nineteenth century, became the first home of the modern Vatican Observatory before the roof was converted into an observation terrace.

June 7, 1929: The Day Vatican City Became a State

After nearly sixty years of dispute between the papacy and the Kingdom of Italy over the former Papal States, representatives of Pope Pius XI and the Italian government signed the Lateran Treaty at the Lateran Palace on February 11, 1929. The agreement recognized full papal sovereignty over a newly defined territory of just over 100 acres surrounding Saint Peter's Basilica, to be known as Vatican City State, in exchange for the Holy See formally recognizing the Kingdom of Italy and renouncing its claim to the wider territories lost decades earlier.

The treaty did not take legal effect immediately. Ratification documents were exchanged and the Lateran Pacts formally entered into force on June 7, 1929, the date on which Vatican City legally came into existence as the world's smallest sovereign state. Within days a Vatican postal service and a distinct currency were established, practical signs of the new state's independence, and the tiny territory has remained governed under the terms of that 1929 settlement, later confirmed in the Italian constitution, ever since.

Six in the Morning, Noon, and Six in the Evening: The Angelus Bell

Across the Catholic world, and within the Vatican itself, a short devotional prayer called the Angelus has traditionally been marked three times each day by the ringing of church bells at roughly six in the morning, noon, and six in the evening. Its roots reach back to a scattered medieval practice of reciting Hail Marys at the sound of an evening bell, a custom historians trace to Franciscan communities as early as the thirteenth century. Various popes gradually formalized and extended the devotion over the following two centuries, encouraging first an evening bell, then a midday ringing associated with prayers for peace, and eventually a morning bell as well, though sources differ on the precise years individual popes acted and the practice should be understood as a gradual accumulation of custom rather than a single clean invention.

Today the bells of Saint Peter's and other Roman churches still mark the midday Angelus, and every Sunday at noon crowds gather in Saint Peter's Square to watch the pope lead the prayer from a window overlooking the piazza, a tradition of public papal recitation that dates specifically to 1964. During the fifty days between Easter and Pentecost the Angelus is replaced by a different prayer, the Regina Caeli, keeping the same three times of day but marking the liturgical season with different words.

Christopher Clavius: The Mathematician Behind the Modern Calendar

Born in the German city of Bamberg in 1538, Christopher Clavius joined the Jesuit order as a young man and eventually became the leading professor of mathematics at the Collegio Romano in Rome, a position he held for roughly four decades. In 1579 Pope Gregory XIII assigned him to help finalize the technical basis for a reformed calendar, building on earlier calculations by the astronomer Aloysius Lilius, and Clavius became the reform's principal scientific defender once it was announced in 1582, publishing detailed explanations and rebuttals to critics over the following decades.

Beyond the calendar, Clavius wrote influential mathematics textbooks used in schools across Europe and among missionaries as far away as China and Japan for more than fifty years, and he helped establish rigorous mathematics as a core subject within Jesuit education at a time when the discipline was often dismissed by philosophers. Though he never fully accepted the heliocentric model of the solar system, he took Galileo Galilei's early telescopic observations seriously and lent them institutional credibility within the Collegio Romano shortly before his death in Rome in 1612.

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