Natural Knowledge in the Latin Middle Ages 103But in 1292 in Paris, William of St. Cloud measured the precession of the
equinoxes, leading him to reject the variable rate of the Arabic theory of
trepidation. Once the Alfonsine Tables entered Paris, Johannes de Muris
both modifi ed them and tested their accuracy (he built a giant quadrant
with a radius of more than four meters). He and nine colleagues observed
the solar eclipse in 1337 with “several good astrolabes.”^95 The Provençal
rabbi and savant Levi ben Gerson (d. 1344) initiated in Europe interior
meridian- line solar observations, determined the solstice, invented the
“Jacob’s staff” to facilitate longitudinal measurements, and used his own
observations to criticize Ptolemy’s lunar theory, to which he created an al-
ternative.^96 Johannes Regiomontanus (d. 1476) also criticized the failure of
existing tables to predict planetary positions accurately and of Ptolemaic
theory to predict planetary sizes correctly.^97
The literature on experiment in the Middle Ages is growing. A widely
diffused late- thirteenth- century alchemical treatise, pseudo- Geber’s Summa
perfectionis, involved procedures that were fully experimental, since they
were used as tools of analysis, discovery, and identifi cation. Signifi cantly,
its approach to theory was empirical rather than metaphysical, drawing
on corpuscular themes in Aristotle’s work.^98 Theodoric of Freiberg used
artifi cial apparatus (probably a spherical urine glass) to model the path
of sunlight in the raindrop, and geometry to explain the primary and
secondary rainbows. Peter of Maricourt devised a host of experiments to
produce the fi rst systematic study of the magnet.^99 Anatomy, once tied
to craft and under the control of barber- surgeons, also became a subject
in late- medieval medical faculties, as the practice of dissection acquired
pedagogical utility.^100
Whereas “lifers” such as Siger of Brabant (d. ca. 1280), John Buridan,
and John of Gmunden spent entire careers teaching in the arts faculty,
most masters of arts taught but a few years, others for a decade or more.^101
The few who entered medicine or theology had the greatest opportunity
to extend their study of the “natural books” or the mathematical sciences
at the university. Like today’s graduate assistants, they survived for years
by teaching in the arts faculty, while studying in the higher faculty.^102
Many kept abreast of natural philosophy, remained engrossed by its ques-
tions, and practiced its methodological autonomy from theology. When
Aquinas commented once again on Aristotle’s Physics shortly before his
death, it was presumably because the subject still fascinated him, not be-
cause his theology needed a handmaiden in extremis.
A few years later, driven by conservative theologians and enacted
by the bishop of Paris, the Condemnation of 1277 sought to assert the
dominance of theology over philosophy by challenging some limits on