Pierre Pelerin de Maricourt (French), Petrus Peregrinus de Maricourt (Latin) or Peter Peregrinus of Maricourt; (fl. 1269) was a 13th-century French scholar 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’ text on the magnet is titled in many of the manuscripts as Epistola Petri Peregrini de Maricourt ad Sygerum de Foucaucourt, militem, de magnete ("Letter on the Magnet of Peter Peregrinus of Maricourt to Sygerus of Foucaucourt, Soldier"), but it is more commonly known by its short title, Epistola de magnete. Pierre's letter is thus addressed to an otherwise unknown Picard countryman named Sygerus (Sigerus, Ysaerus) of Foucaucourt, possibly a friend and neighbor of the author; Foucaucourt borders on Pierre's home area de Maricourt, in the department of the Somme, near Péronne.
The letter also bears the closing legend (but in only one of the 39 manuscript copies) 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 Pierre de Maricourt was in the army of Charles, duke of Anjou and king of Sicily, who, in 1269, laid siege to the city of Lucera. But given that only one manuscript attests this, the argument is weak. There is no indication of why he received the sobriquet Peregrinus ("pilgrim") but it suggests that he may have been either a pilgrim at one point or a crusader; and the 1269 attack on Lucera 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."