On the floor of the basilica of "Santa Maria degli Angeli" a great meridian has shown the time and the evolution of human history for nearly three centuries. However, for many years this scientific and artistic monument has been quite forgotten and neglected even by the most interested visitors; only some curious hasty tourist once in a while stops to wonder and look at the marvellous representations of the signs of the zodiac, trying in vain to understand the meaning of all those numbers in a line as well as the purpose of this work of art that mysteriously crosses the floor of the whole basilica.

The development of these great meridians is motivated by three fundamental objectives:

the necessity to give astronomers suitable observatories for their research;

the opportunity to give to the populations of highly - civilised cities the signal of noon in order to adjust the widespread mechanical clocks;

the requirement of the Roman Catholic Church to control the movement of celestial bodies so as to obtain a correct calendar determining the date of Easter. In fact in 325 a.C. the Council of Nicea established that "all the churches would have celebrated Easter on the Sunday following the full moon after the spring equinox".

The great meridian of Santa Maria degli Angeli in Rome can certainly be included in this category of instruments exclusively built for religious purposes. In the 18^th century Pope Clement XI ordered Bianchini to build the beautiful meridian of Santa Maria degli Angeli in Rome in 1702. Bianchini chose the structure given by Michelangelo in 1566 and took as a model the meridian of San Petronio in Bologna designed by Cassini. The meridian was then inaugurated on the 6^th of October 1702. But because of the rehandling carried out by Vanvitelli n 1749, the meridian lost some of its importance as an astronomic observatory; a recent memorial tablet indicates that the meridian was used has a solar clock until 1846 when it was replaced by the Gianiculum cannon.

The system includes a double meridian constituted by two gnomons, a boreal one and an austral one. The austral gnomon originates from a hole positioned on the southern wall trough which a sunbeam penetrates when it is on the superior meridian. The gnomonic hole is situated at the height of 20.30 metres; the size of the ellipse varies from 22 to 110 cm according to the meridian height of the sun; the accuracy of the measure at the moment of its transit depends on the clearness and the speed at which the dot moves that is of about a couple of seconds. The meridian line is materialised by a brass strip at centre of which there is a thin line determining the real meridian; this meridian is 44 meters long, 38 of which are usable. From the boreal gnomon it was possible to observe the North star, but the hole was eliminated with Vanvitelli’s restructuring. Naturally, the higher the gnomonic hole was situated , the more accurate were the observations and nearly all the great meridians were set up in churches since they were at that time the only buildings that could house such devices. Alongside the meridian there are some squares representing the constellations: they correspond to the points on a tangent line where the solar dot passes when the sun enters the respective sign of the Zodiac. On the left of the brass strip there is the indication in degrees of the zenithal distance of the star projected on that point. On the right there is the indication of its trigonometric tangent line.

The point on the meridian corresponding to the equinox (that is the "0"declination of the sun, centre of the zodiac signs Aries and Libra)represents a fundamental point of the instrument therefore justifying the purpose and the existence of the whole device. The centre of the solar dot passes exactly through this point at the moment of the equinox, whenever it occurs exactly at noon local time on the day. The designer rightly wanted to point out the meaning of the equinoctial point drawing upon it the complete image(shadow and penumbra)of the ellipse produced by the sunbeam. Since the transit of the sun on the day of the equinox at exactly noon local time maybe merely accidental , Bianchini inserted a graduation into the ellipse drawing which, at the moment of the sun transit on the meridian that is slightly staggered if compared to the ellipse drawing on the floor, accurately indicates how many hours before the equinox has occurred or how many hours later on it will take place. Of course, the same operation can be performed by measuring the "tangent line" of the sun at noon; from this it is possible to obtain the declination at that moment and therefore deduce the "0"declination time, that is of the equinox. In order to stress even more the importance of the equinoctial point and to make its observation easier, on the left of the meridian line corresponding to the equinoctial point there is a series of stars set into the floor indicating the course of the solar dot while it approaches the equinoctial meridian transit. In other words, these little stars mark the equator line on the floor and their connecting line is therefore a straight line. The meridian was an important means for the observation of the stars which was performed with the help of telescopes up to 20 metres-long. Another astronomic element included in this system was the length of the day, from sunrise to sunset. Bianchini wanted to underline the points representing important dates: one example is the points marking the maximum date limit of Easter.

Great damage has been caused to the meridian: for instance most of the inscriptions have disappeared and the brass strip materialising the meridian and showing the notches of the tangent line has been badly restructured. The great meridian of Santa Maria degli Angeli therefore needs to be restored: the whole strip should be correctly re-assembled and most of the stars of the constellation that have disappeared need to be reset thus giving the great meridian of the roman Basilica of Santa Maria degli Angeli a new life.