Planisphère mobile.

Au moyen du quel on peut apprendre l’Astronomie seul et sans le secours des Mathématiques. Imité de l’Anglais par Melle. Ginot Desroys. Prix 4 f. Paris : chez Bachelier, 1847.

Planisphere, 355 mm in diameter, comprising four die-cut discs (or volvelles) of engraved paper over card, pointer, central screw, and holes for suspension at the two “poles” of the largest disc; edges painted green; detailed instructions for the seventh edition are printed on the verso; lightly stained, but a very good example.

This rare scientific instrument is a portable, adjustable star chart that displays exactly which stars and constellations are visible in the sky at any given date and time, and which can be used without any prerequisite mathematical knowledge. It was designed by Ginot Desroys (or Des-Rois), a female scientific instrument maker and author of a number of works on astronomy and geography, as well as the educational card game Jeu des petits voyageurs aux cinq parties du monde. The layout of the constellations (which are labelled in both French and Latin) is based on that made by the astronomer Alexis Jarrin.

The four discs comprise the Etoile Solaire (North Star), and an elliptical compass window and pointer which moves across the third disc to reveal the constellations of the night sky at any given date; this in turn can be aligned with the last and largest disc, which is marked with the hours of the day or night.