Abstract
The paper is concerned with a medieval astronomer called Petrus Philomena de Dacia, who was particularly active during the period 1291–1303. The very few primary sources of the story of his life are mentioned, together with a list of about a dozen treatises ascribed to him; they are contained in about two hundred manuscripts. The unity of this literary estate is discussed. Finally, the author tries to present a reconstruction of Peter's most original contribution to medieval astronomy, namely his Semissa, or Equatorium, which constituted an important step forward in the history of astronomical “computing-machines”, being the first instrument capable of computing the ecliptic longitudes of all the planets by means of a single device. A reconstruction of this instrument is exhibited.
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