Peregrinus, Petrus (Peter the Pilgrim), c.1220-?, medieval scholar and soldier. The tutor of Roger Bacon, he wrote the first important study of magnetism, Epistola de Magnete, in which he described simple magnetic attraction and repulsion. He improved the compass by placing the lodestone on a pivot and surrounding it with a graduated directional scale. His ideas were further elaborated in the 16th cent. by William Gilbert.

Petrus comes from the Greek meaning "rock", and is the common English prefix "petro-" used to describe rock-based substances, like petros-oleum or "rock oil." As the source of Peter, it is a common name for people from antiquity through the medieval era, and may refer to one of the following:

As a name:

• Petrus, Saint Peter, the first pope
• Priest Petrus, 5th century Illyrian priest
• Petrus Abailardus, Latin name for Peter Abelard
• Petrus Alfonsus, Spanish Jewish writer
• Petrus Aureolus
• Petrus Baldus de Ubaldis
• Petrus Bernardinus, Florentine sectarian
• Petrus Canisius, 16th century German Catholic theologian and polemicist
• Petrus Capuanus or Peter of Capua, an Italian scholastic
• Petrus Christus, (ca. 1410/1420 - ca. 1475/76), Dutch painter
• Petrus de Abano or Pietro d'Abano (1257 – 1315) Italian philosopher and doctor
• Petrus de Amalfia, 14th century Italian music theorist
• Petrus de Cruce, late 13th century composer and author, one of the most significant and influential composers of the late ars antiqua
• Petrus de Natalibus, Venetian hagiographer
• Petrus de Picardia, 13th century French music theorist
• Petrus de Sancto Dionysio, 14th century French music theorist
• Petrus Diaconus, several people
• Petrus frater dictus Palma ociosa, 14th century French music theorist, one of the first to illustrate current musical innovations
• Petrus Jacobus Joubert
• Petrus Le Viser, late 13th century musician, important in the history of tempo and rhythm
• Petrus optimus notator, 13th century musical scribe in Paris, important for notating Notre Dame polyphony
• Petrus Peregrinus = Peter de Maricourt, 13th century French writer on magnetism and astrolabes
• Petrus Ramus
• Petrus Trothun Aurelianis, 13th century singer of the Notre Dame School

Other:

• Château Pétrus, a Pomerol Bordeaux wine producer