In the great hall on the second floor of the Paris Observatory - built between 1667 and 1671 from plans by architect Claude Perrault - can be found one of the wonders of the Observatory : the great meridian line. Designed and sketched by Jean-Dominique Cassini (1625-1712), shortly after his arrival at the Paris Observatory in 1669, it was intended as the first line of a monumental sundial. However, it was his son, Jacques Cassini (1677-1756) who, in 1729, some fifty years later, finished the construction; his aim was to make a high-precision astronomical instrument so as to solve one of the greatest controversies of the new century: namely, is there inexorable variation in the axis of rotation of the Earth?
The care and thoroughness with which this brass line over 31 meters long was built, made it a huge astronomical instrument for taking high precision measurements. It was no longer simply a sundial as conceived by Cassini's father ; rather it was more like a mural quadrant whose radius, represented by the imposing height of the gnomon (i.e. the orifice through which the sunlight was allowed to enter into the room) located about 10 meters above the ground, thereby surpassed in size the larger existing divided instruments. The almost exclusive function of the Meridian was to determine as accurately as possible the inclination of the axis of rotation of the Earth on its orbit, or what astronomers refer to as "the obliquity of the ecliptic". It was also meant to settle a controversy that was becoming increasingly important: does the obliquity of the ecliptic decrease with time gradually and inexorably?
The actual principle of this line is simple. It is based on measuring the meridian altitude of the Sun, which varies during the year, from one solstice to another. Through the gnomon (a Greek word meaning "indicator"), a simple orifice with a diameter equal to the thousandth part of its height - slightly less than one centimeter in the case of the Cassini Meridian - the sunlight entered the hall, without hindrance, and went along the floor forming an elliptical image, which is more or less large, more or less elongated, as the Sun is high (in summer) or low (in winter) on the horizon as it passes through the local meridian. This meridian is a great imaginary circle that connects the north and south poles of the Earth. Thus, it is said that at this precise moment the sun marks the exact location of true noon. The height difference between these extreme positions, corresponding to the summer and winter solstice points, is equal to twice the obliquity of the ecliptic. In other words, if the axis of the Earth was not tilted towards the ecliptic - this being the plane in which the Earth travels around the Sun - the Sun would appear every day at noon at the same altitude. This, moreover, would be equal to the latitude of the place of observation, so that the meridian line would be reduced to a simple point and would therefore be of no interest at all.