Petrus Peregrinus de Maricourt

Petrus Peregrinus de Maricourt (Latin), Pierre Pelerin de Maricourt (French), or Peter Peregrinus of Maricourt (fl. 1269), was a French mathematician, physicist, and writer who conducted experiments on magnetism and wrote the first extant treatise describing the properties of magnets. His work is particularly noted for containing the earliest detailed discussion of freely pivoting compass needles, a fundamental component of the dry compass soon to appear in medieval navigation. He also wrote a treatise on the construction and use of a universal astrolabe. Peregrinus's text on the magnet is entitled in many of the manuscripts of it Epistola Petri Peregrini de Maricourt ad Sygerum de Foucaucourt, militem, de magnete ("Letter of Peter Peregrinus of Maricourt to Sygerus of Foucaucourt, Soldier, on the Magnet") but it is more commonly known by its short title, Epistola de magnete ("Letter on the Magnet"). The letter is addressed to an otherwise unknown Picard countryman named Sygerus (Sigerus, Ysaerus) of Foucaucourt, possibly a friend and neighbor of the author; Foucaucourt borders on the home area of Peregrinus around Maricourt, in the present-day department of the Somme, near Péronne. In only one of the 39 surviving manuscript copies the letter also bears the closing legend Actum in castris in obsidione Luceriæ anno domini 1269º 8º die augusti ("Done in camp during the siege of Lucera, August 8, 1269"), which might indicate that Peregrinus was in the army of Charles, duke of Anjou and king of Sicily, who in 1269 laid siege to the city of Lucera. However, given that only one manuscript attests this, the evidence is weak. There is no indication of why Peter received the sobriquet Peregrinus (or "pilgrim"), but it suggests that he may have been either a pilgrim at one point or a crusader; and the attack on Lucera of 1269 had been sanctioned as a crusade by the Pope. So Petrus Peregrinus may have served in that army. "You must realize, dearest friend," Peregrinus writes, "that while the investigator in this subject must understand nature and not be ignorant of the celestial motions, he must also be very diligent in the use of his own hands, so that through the operation of this stone he may show wonderful effects."