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Rare late state of this "Northern Celestial Planisphere projected on the plane of the equator, originally created by Chrysologue de Gy, a Capuchin father from the convent on Rue Saint-Honoré, a student of Pierre Charles Le Monnier, a celebrated astronomer from the Academy of Sciences.
Father Chrysologue improved existing northern and southern celestial planispheres by adding the nine hundred stars from Nicolas-Louis de Lacaille's 'Caelum Australe'. Encouraged by his teacher Lemonnier, Chrysologue de Gy first published these two planispheres in 1778, approved by the academy and under its privilege. His two large planispheres depicted the two celestial hemispheres in a convex stereographic polar projection, with the projection of the terrestrial poles as the center and the projection of the equator as the outer circumference.
At the time of creation, the images represented one of the best syntheses of the representation of the celestial map at the end of the 18th century: 4466 stars, 79 nebulae, accurate updates of observations from Flamsteed, Hevelius, Halley, Le Monnier, Lacaille and 900 of his stars, Messier, and Mayer. The stars, identified by letters according to the Bayer method, are divided into 14 classes. Images of the constellations are shown and named in French.
The equator circle, forming the perimeter of the hemispheres, shows a graduated scale divided into units of 15 arc seconds of right ascension. On the outer edge of the tables is a triple graduated scale, the first in 24 hours divided into 60 minutes correlated with the movement of the Sun, the other two in 360° identifying terrestrial longitude with the meridian of reference.
In the lower corners of the map of the northern hemisphere, there are two detailed maps of the Pleiades and Hyades by Pierre Charles Le Monnier.
