1761 Desnos / Mornas Tour Diagram of Astronomical Observation

This is a beautifully hand-colored example of Desnos and Mornas' c. 1761 sheet on astronomy and planetary science from their Atlas Méthodique et Elémentaire. A fascinating work of Enlightenment-era natural philosophy, it reflects the process of the development of modern astronomy.

A Closer Look

The sheet contains a central section with an illustration of a sextant, below which appears a diagram of the Gnomon of St. Sulpice (discussed below) and its use in determining the solstices and equinoxes. Columns of text at left and right discuss the Sun and the planets, their relation to the naming of the days of the week, and the development of astronomical observation of the planets from the Ancients through Galileo, Huygens, and Cassini. Most of the discussion deals with the Sun and its nature, sunspots, the elliptical revolution of planets around the Sun, and the solstices. Taken as a whole, the sheet reflects an increasingly modern understanding of astronomy, including that the stars seen in the sky are each the center of their own solar system ('tourbillon').

The Gnomon of St. Sulpice

The current church of Saint-Sulpice in the 6th arrondissement of Paris was built in the mid-17th century and elaborated several times thereafter, to the point that it is the second largest church in Paris after Notre-Dame. Upon one of these renovations in 1727, the church's priest Jean-Baptiste Languet de Gergy (1674 - 1750) asked for a gnomon to be installed in a south-facing window of the church's transept. At solar noon each day, a beam of sunlight passes through the gnomon and lands on a brass meridian line inlaid into the church floor, which runs north-south and is marked with calibrated scales. At the far (north) end of the meridian line is an obelisk, the top portion of which is lit with the winter solstice. Gergy wanted to use the gnomon to more accurately set clocks in Paris and to determine the proper date for Easter. It was also used by later scientists to measure variations of the ecliptic (revolution of the Earth around the Sun), variations in the tilt of the Earth's axis, and the date of the perihelion (when the Earth is closest to the Sun in its annual elliptical orbit).

Publication History and Census

This sheet was issued in the 1761 edition of the Atlas General Methodique et Elementaire, pour l’Etude de la Geographie et de l’Histoire Moderne by Louis-Charles Desnos and Claude Buy du Mornas. This sheet is not independently cataloged in any institutional collections, while the entire atlas is well-represented.