During its construction, the plans for the large room on the second floor of the Paris Observatory were modified by Jean-Dominique Cassini in order that a large meridian line could be established. However, it was his son who finished the construction during the period 1729-1732.
It was designed to be an extremely accurate astronomical instrument, whose function was to measure the meridian altitude of the Sun – to an accuracy of 10 arc seconds – in order to detect a possible change in the obliquity of the ecliptic (a slow change in the direction of the terrestrial rotation axis).
It is made of 32 brass strips, each almost 1 m long, embedded in white marble slabs ; on this line is projected the elliptical image of the Sun from a gnomon consisting of a hole drilled at a height of 9,9377 m in a copper plate ; the principle is that of the pinhole camera (the "camera obscura" in the language of the time).
The marble path has a total width of one royal foot (32,48 cm), which ensures that the image will remain within its limits from one solstice to the other. Twelve additional marble slabs, engraved with zodiacal signs, complete the paving. The brass strip is graduated along its western and eastern edges, to give, respectively, the apparent altitude of the solar centre and the tangent of its zenith distance.
This was the first of a series of very large meridian instruments which flourished up to the end of the XVIIIth century (notably at Saint-Sulpice, 1743 ; Santa-Maria del Fiore in Florence, 1756). It is the only instrument of its type built within a non-religious establishment. For 25 years - up to 1755 - Jacques Cassini used it regularly, and obtained thereby almost 450 observations. Today, a simple lens has replaced the gnomon.