John III Sobieski was known for his scientific interests and his support for scholars. The fact that his name appeared right next to the intricate sundial mechanism in the church of Santa Maria degli Angeli e dei Martiri would certainly have appealed to him.

The sundial as a sign of the Church's triumph

The making of the meridiana, or sundial, was commissioned by Pope Clement XI (Giovanni Francesco Albani), which is why it is also known as the Linea Clementina. It was constructed by Francesco Bianchini, a botanist, mathematician, physicist and astronomer, expert in theology, archaeology and ancient languages. He was assisted in the work by Giacomo Maraldi. The main practical aim of the project was, in addition to indicating the correct time, which made it possible to regulate the still inaccurate mechanical clocks, to determine the date of Easter and to verify the correctness of the Gregorian calendar reform. Both of these matters were of fundamental importance to the Roman Catholic Church. The problems of determining the date of the movable feast of Easter stemmed from the days of Christ's death and resurrection being given according to the Jewish calendar, based on the phases of the moon. The Christian calendar, on the other hand, is solar. The issue was resolved at the First Council of Nicea (325). It was agreed that Easter would fall on the first Sunday after the full moon following the vernal equinox. The Julian calendar in force at the time (introduced by Julius Caesar) was reformed in the 16th century by Pope Gregory XIII. However, the changes introduced were still criticised and met with resistance, especially in some non-Catholic Churches. The construction of the meridiana was therefore an extremely important undertaking, and much depended on its accuracy. Its precision would be further proof that faith and science are not in conflict with each other.

The church of Santa Maria dei Angeli e dei Martiri was chosen for several reasons. Firstly, in a huge basilica, drawing a line nearly 45 m long was not a problem. In turn, the size of the clock translated into its accuracy. Secondly, in the semi-darkness of the temple interior, it was easier to see the wandering of a small patch of light across the floor. Even the windows were covered from the outside for observation purposes, as evidenced by the hooks on the facades of the building.

The historical context was no less important. The temple was built on the ruins of the ancient thermal baths of Diocletian, one of the fiercest persecutors of Christians. The construction of a sundial to mark church festivals therefore had symbolic significance as an expression of the Church's triumph. Finally, it was in this church that Giovanni Francesco Albani celebrated his first Mass on 6 October 1700, having been ordained only a few years earlier. On 23 November of the same year, he was elected to the throne of Peter and took the name Clement XI. The inauguration of the meridiana took place on 6 October 1702, exactly two years after Albani celebrated his first Mass.

Mechanism of operation

A hole was punched in the southern wall of the basilica, through which the sun's rays entered, forming an oval-shaped patch of light on the floor. The clocks were constructed in such a way that the diameter of the hole in relation to the height at which it was placed above the ground remained in a ratio of about 1:1000. In the case of Santa Maria degli Angeli, the hole is 20.3 m above the floor and was originally only 2 cm in diameter. It was later enlarged twice.

A 44.89m-long bronze line, framed by light-coloured marble, was drawn along the north-south axis on the floor. The working principle, which is presented here in the necessary simplification, was that a spot of light crossed this line every day at solar noon. This point was furthest from the hole (i.e. conventionally at the end of the line) at the winter solstice, as this is when the sun is lowest above the horizon, and closest - exactly 6.75 m from the base of the hole - at the summer solstice. As the seasons changed, so did the diameter of this spot of light: from 22 cm at the summer solstice to 110 cm at the winter solstice. At the time of the spring equinox, which formed the basis for determining Easter, the spot of light was around the centre of the line. The range of possible dates for Easter, i.e. the days between 22 March and 25 April, are marked by the inscriptions 'Terminus Paschae'.

On either side of the line are numerical markings and explanatory notes, as well as marble compositions depicting the twelve signs of the zodiac, designed by Francesco Tedeschi. One side of the line shows the summer and autumn constellations, the other the winter and spring constellations. Closest to the opening we have the symbol of Cancer and furthest to Capricorn: the Sun's entry into the former sign marks the summer solstice and into the latter the winter solstice. Observations carried out at Santa Maria degli Angeli in the first half of the 18th century detected a slight deviation in the indications. This was brought to the attention of Luigi Vanvitelli, who led the reconstruction of the temple in 1750. After corrections, the meridian continued to regulate the clocks of the Romans for another century, until 1846.

Sun and Moon 19 years after the Battle of Vienna

On either side of the line are also bronze plaques bearing the name of Clement XI. The privilege of placing two more was granted by the Pope to Maria Kazimiera Sobieska, widow of King John III Sobieski, who had lived in Rome since 1699. Her relations with the recently elected successor of St Peter went very well in the early years of his pontificate. The Queen even credited herself - rather without any real basis - with influencing his election during the conclave.

The plaques were placed even before the ceremonial inauguration of the meridian, on 12 September 1702, exactly nineteen years after the Polish king's victory at the Battle of Vienna. One plaque is a direct reference to this clash, while the other is dedicated to Maria Kazimiera. The text on it refers to these nineteen years after her husband's military triumph. This period is associated with the so-called Metonic cycle, named after the 5th century BC Athenian astronomer. According to Meton, every nineteen years the phases of the moon fall on the same days of the year, so that in the year of the inauguration of the meridians, movable church festivals such as Easter fell identically to the year of John III Sobieski's victorious battle at the walls of Vienna. In this sophisticated way, the past and the present were combined at the Linea Clementina, Maria Kazimiera's presence in the Eternal City with her spouse's services to Rome and, finally, the defence of Christianity and the Church on the battlefield with their triumph achieved through science.

Two plaques dedicated to King John III Sobieski and Maria Kazimiera Sobieska

Church of Santa Maria degli Angeli e dei Martiri in Rome

1702

bronze

Inscription on the plaque dedicated to John III Sobieski:

"Diem XII Septembri / Christiano Populo / Faustam Felicem / Ioannis III Polon. Reg. / Viennensi Victoria / de Turcis. Reportata / Anno MDCLXXXIII / Innocentio XI / Sedente".

"On 12 September, happy and auspicious for the Christian people, occurred the victory of John III King of Poland over the Turks at Vienna in the year 1683, during the pontificate of Innocent XI".

The inscription on the plaque dedicated to Maria Kazimiera Sobieska:

"Maria Casimira / Poloniae Regina Uxor / Periodo Decemnovennali / Absoluta Restitutisque / Solis ac Lunae Motibus / Romae Signavit / Anno MDCCII / Clementis Undecimi / Secundo".

"Maria Kazimiera, wife queen of Poland, after the lapse of nineteen years and the renewal of the revolutions of the Sun and the Moon, placed (this plaque) in Rome in the year 1702, the second (of the pontificate of) Clement XI".