A measure from above

The historical perception of post-Renaissance Italian astronomy has become so over-charged with the Roman Catholic Church's condemnation of Galileo in 1633 that it is commonly assumed that no significant science took place south of the Alps until the 19th century. But as John Heilbron's learned, elegant and finely phrased book reminds us, this was not the case. For as a religion whose principal celebration, Easter, can move across several weeks after the spring equinox, and whose claim to universality requires the resurrection of Jesus Christ to be celebrated by all Christians on the same Sunday in any given year, the church has always needed astronomers.

But the mathematical path to Easter Day led through a minefield of indivisible numbers. Yes, there may be 365 days and 13 28-day lunar months a year, but astronomical numbers are never round. Instead, they tail off into a string of awkward fractions. Of course, the fractions do not matter if you are only concerned with short runs of years. But short-termism was never a part of the Christian verities, and the church needed exact values for the lengths of the solar and lunar years if the resurrection was to be celebrated worldwide on the same moveable day, down to Armageddon. Things were getting acutely problematical by the 16th century, moreover, as the astronomical date of the spring equinox was almost ten days behind the old Roman calendar, making the true Paschal, or Easter, moon difficult to identify.

Heilbron looks at the history of these early calendrical matters as an essential preliminary to the great problem of calendar reform and management, which played such an important part in astronomy for more than two centuries after Egnatio Danti set up his first measuring instruments on the south wall of Santa Maria Novella, Florence, in 1574, prior to Pope Gregory's new calendar in 1582.

The key to reforming the calendar lay in measuring the precise lengths of shadows cast by the sun at the midwinter and midsummer solstices. Yet shadows are difficult things to delineate, and Danti revived a technique first used by Paolo de Pozzo Toscanelli in Santa Maria del Fiore, Florence, 100 years before: he admitted a thin shaft of sunlight into the cathedral's vast interior at noon each day, and noted where the "pinhole" projection of its disk fell on the marble pavement between December and June. Hence, the sun came into the church.

Danti's solar measuring instrument, now erected inside the Florentine church of Santa Maria Novella, came to be known as a meridiana , and when the death of his patron Cosimo de Medici made it expedient for the Dominican order to move Danti from Florence to Bologna, he laid down an even larger instrument in the cathedral of San Petronio. This massive sundial had a projection hole 65 Bolognese feet above the cathedral floor, and at midwinter cast the solar image 170 feet along a graduated brass line sunk into the carefully levelled marble pavement. With such massive projection distances, one could measure the tiniest changes in the sun's daily elevation above the horizon, from which it was possible to obtain a much more accurate value for the length of the solar year.

Considering the prominent position that astronomers held in the Renaissance Church - Danti became Bishop of Alatri - it was all the more incongruous that Galileo, as a good Catholic, should have been punished for his Copernican views in 1633. As Heilbron reminds us, however, condemnations for heresy came in varying orders of severity, and Galileo's was a relatively minor one. His was not a theological heresy, for the fixity of the earth was a philosophical and not a canonical topic; his offence stemmed from breaking the injunction of 1616, which forbade the teaching of the moving earth as a fact, rather than as an unproven hypothesis. That, plus the overbearing vainglory of Pope Urban VIII, who had once been Galileo's patron and friend, but who in 1633 incorrectly felt that the old Florentine astronomer was mocking him. In short, Galileo's condemnation for heresy was more about wounded pride, presumed insinuations and the disobedience of mere star-gazers in the eyes of worldly popes than it was about astronomy. Soon thereafter the church realised that it had acted rashly, though Galileo's name remained on the "Index of Prohibited Books" until 1822, and he was not fully pardoned until 1970.

It is ironic that the religious order that played the leading role in the Galileo affair, the Jesuits, should also have provided the Catholic world with some of its finest astronomers. Bologna in particular, squarely in the papal states, was its powerhouse. Though not a priest, Giovanni Domenico Cassini was Jesuit-educated, a friend of cardinals and popes and one of the city's scientific ornaments before being lured by a vast salary to serve Louis XIV of France in 1669. In 1655, however, the 30-year-old Cassini laid down a new meridiana in Bologna's San Petronio cathedral that was more precise than Danti's. Not only could it correct the calendar to a greater level of precision, it revealed new data about the heavens. Cassini and his Jesuit friends, Giambattista Riccoli and Francesco Grimaldi, were able to demonstrate fresh solar orbital characteristics when the sun shone into San Petronio.

By the 1660s, however, evidence for Copernicus's and Galileo's moving earth was increasing, and Jesuit astronomers were becoming adept at dealing with its physical and mathematical implications while keeping on the right side of the law: by calling it a hypothesis. And as the meridiana had proved itself to be such a delicate instrument of astronomical research, others came to be built in Italy, France and elsewhere. Eighteenth-century clerical geophysicists such as Roger Boscovitch started using these great ecclesiastical sun lines to quantify local variations in the earth's spherical form, along with other Newtonian criteria. And Leonardo Ximenes's 1755 Florentine instrument provided the foundation for several original investigations in experimental physics. Meridianae even became political instruments, as in the case of the 1786 brass line laid down in the Duomo cathedral in Milan by the occupying Austrians, who used it to enforce European mean time on Lombardy. It was in Belgium, however, that the last great meridianae were laid down after 1836 as King Leopold sought to bring British efficiency into his country and coordinate his train timetables.

Though Heilbron supplies all the necessary geometry to demonstrate how the meridianae were constructed and used within the great architectural masterpieces into which they were incorporated, his book is arranged and illustrated in such a way that non-mathematical persons can enjoy it. In addition to dealing with astronomical problems, it teems with the inspired, exotic and enthusiastic individuals who wrestled with them. Perhaps most of all, it deals with their spectacular optical creations, the meridianae , which with "their unique blend of art and history, science and sanctity, can still give visitors a glimpse of the sublime".

Allan Chapman is a member of the faculty of modern history and of Wadham College, University of Oxford.