The meridian line consists of 32 brass rulers attached to one another. Each of these rulers has a length of just under a meter - 993.77 mm precisely. This length corresponds to that of a pendulum that completes one swing every second at the latitude of Paris. This measurement of the length of the pendulum swing per second had been conducted in Paris by the astronomer Jean Picard (1620-1682). Jean-Dominique Cassini wanted to immortalize it in the meridian line by giving the height of the gnomon a value equal to ten times the length of Picard's pendulum.
The line is divided into two sets of graduation. One facing west, graduated in degrees, indicates the angle of the Sun's altitude above the horizon when it is perfectly bisected by the line, which happens at true noon. The other is oriented toward east, giving the tangent of the zenithal angle, multiplied by a factor of 1000, which is the angle between the vertical and the direction of the Sun.
Bands of white marble, along the line of brass on both sides, collect the Sun's image so as to make it brighter. The total width of these bands is 32.4 centimeters and is calculated so that it is fully occupied by the image of the Sun at the winter solstice, in its maximum transverse dimension.
The line is also marked by twelve white marble slabs on which are engraved the signs of the zodiac. Their position along the line is determined according to the times when the Sun enters into each of these signs. Although the name of the creator of these engravings has not come down to us, he would appear nonetheless to have been greatly inspired by images reproduced in the work of Johannes Bayer (1572-1625), the Uranometria, which was published in 1603.
The meridian line came to be known by its full metonymic name of "The Meridian" when its gnomon was taken into account. Formerly, it was a simple circular hole drilled in a metal plate that was attached horizontally to the inside wall of the hall. Nowadays, this is a lens of 8.5 cm in diameter. Nobody knows exactly when the substitution occured, presumably in the end of the eighteenth century.. This lens is intended solely to collect more sunlight than a small hole could, in order to increase the brightness of the image on the ground. However, outside the focus area of the lens, the image appears blurry and unfocused, with hazy contours. It is not until early May and August that it is possible to obtain a sharp image of the Sun on the floor of the meridian room. Sunspots can even be detected during periods of high solar activity.