This room, built in 1727, was planned for meridian observations. Great arches grounded on the lower floors intersect along diagonals, firmly supporting the "meridian wall". This wall crosses the room diagonally, from north to south. Fixed instruments were anchored to it. The original roof was flat and was provided with shutters to allow observations, in the same place where the present roof has been raised. The meridian room was close to the astronomer's working and resting rooms.

These rooms were respectively the room to the right of the passage and the room upstairs, where the astronomer's living quarters were located.

The instruments made in Rome by Domenico Lusverg were the first to be put in the meridian room. These instruments were originally intended for the "Specola Marsiliana", which operated from 1702 to 1709. They are the unique testimonials of the superb 17th century French craftsmanship, due to the destruction of the older similar instruments in Paris and Greenwich.

Lusverg's instruments were replaced in 1742 by Sisson's, which were purchased in England thanks to the considerable sum remitted to the city of Bologna by Pope Clement XII. The instrument for meridian observations and that for measuring transit time were separated in this equipment for the first time in Italy. A mural instrument was still used to measure the altitude of the heavenly bodies at their transit over the meridian, while a transit telescope was used to measure transit time.

The latter allows to maintain a correct orientation along the meridian, with the varying altitude of the observed star, due to its structure and kinematics, thereby solving the problem of the poor planarity of the limbs of mural instruments.

The Bolognese architect Ercole Lelli replaced the old wood floor with the present one, in 1742, on the occasion of the installing of the English instruments in functioning condition. He drew the fine apparent and mean solar time brass meridian line, and decorated the walls by painting columns on them, which recall the bearing columns of the transit telescope, and which have by now almost disappeared.

Nowadays in the room are the Lusverg mural semicircle and some movable quadrants by Lusverg and by Sante Menini of Bologna, as well as the Sisson mural and movable quadrants and a couple of 18th century clocks.

The first attempts to use spheres to represent the sky using lines and reference points date back to the Chaldeans and the ancient Egyptians.

The Greeks were influenced by them when making celestial globes, starting maybe in the 6th or 5th century B.C. The idea of tranferring lands, on which man lived and around which he sailed, on spheres is certainly subsequent.

From the beginning these globes had a twofold use: both as an aid for sailing and as a didactic and explanatory aid for the positions and motions of celestial bodies.

Globography had a widespread growth in Europe after the 15th century. In those times the sphericity of the Earth was widely recognized after Magellan's journey and new travels and discoveries redesigned the world map completely.

The first celestial and terrestrial globes, all covered with printed sectors of paper, appeared in the mid-16th century.

It is natural therefore that both celestial and terrestrial globes, and armillary spheres would be used by astronomers as working, studying and teaching instruments.

Some of these globes have been brought back in the central room on the 6th floor of the tower.

They were given to the observatory, on various occasions, by General Marsili, Cardinal Antonio Davia and Pope Benedict XIV.

The oldest exhibited globes date back to the early 17th century. They were made by the Dutchman Willem Janszoon Blaeuw, one of the most clever sphere craftsmen. He was a student of Tycho Brahe and clever both at astronomy and at printing. The sample of celestial and terrestrial globes, made in couples as was the custom at the time, is valuable for the clarity of the drawings and for the pictures covering the surfaces.

A couple of table globes by the English craftsman Senex and a couple by the Dutch Gerhard and Leonard Valk, both from the first half of the 18th century, are furthermore exhibited. The Horizon of the latter is supported by gilded putti.

Two armillary spheres complete the set, both showing the Copernican system.

The bigger one is made of gilded cardboard and the smaller, a table one, of brass. Both are from the first half of the 18th century.

On the walls two 16th century parchment nautical maps can be seen, magnificently illustrated and painted, and two valuable early 17th century large Chinese maps. Both come from Beijing Observatory, which was founded by the Jesuits.

The first one, a geographic one, was made by Father Matteo Ricci and is one of the few remaining in the world. The second one, a celestial one, was made by the Mathematician Schall von Bell.

The upper part of the observatory tower was called "torretta". Its sides are oriented towards the cardinal points and rotated with respect to the main tower structure.

The iron attachment used for managing long telescopes was built into the parapets, which were massive and high in order to protect from the wind. Some instruments could be used inside the observation room, protected by eight full-length windows. A four feet wide circular aperture in the middle of the vault allowed the astronomers to observe the zenith.

Telescope tubes and their accessories were lodged in the high store-rooms at the turret corners.

From the date of its completion, in 1725, until now, the observation room did not suffer remarkable changes. Nowadays the early 18th century wood telescope tubes are exhibited there. The original ones made for the 10.5-foot and the 22-foot lenses made in the late 17th century by Giuseppe Campani (the well-known manufacturer of lenses and telescopes in Rome) and those made by Ercole Lelli, in order to use the numerous long focal Campani lenses, stand out among the others. Some of these lenses are in the show-cases. Valuable telescopes of the late 18th century can be seen, some of which by the English craftsman Dollond, including both lens and mirror telescopes. Furthermore, a small equatorial machine by Adams, a movable quadrant by the Frenchman Megnié, and an early 19th century mirror telescope by Amici of Modena are exhibited. The large meridian circle, made in the half of the19th century by Ertel, has been replaced in an oval room built on one of the triangular terraces of the tower.

Some instruments used by Bolognese astronomers for celestial, terrestrial and meteorological observations, are exhibited in the show-cases, together with geometrical and mathematical tools. A 13th century Arabian astrolabe attributed to Ibn Baso the Elder, a 16th century astrolabe by Gualterus Arsenius, from Gemma Frisius' school, a small Gregorian type telescope (one of the first in Italy), a few small sundials, and a Venus globe, are worthy of note.

You can see the site inside the spiral staircase leading to the turret, where in 1790 Giovan Battista Guglielmini conducted an experiment to measure the deviation from a vertical line of falling bodies. The Bolognese astronomer provided one of the first proofs of the Earth's rotation. The first European experiments on the electric nature of lightning have been conducted from the top of the tower, in mid-18th century, by the astronomer Petronio Matteucci and the physician Giuseppe Veratti.