Page 224 - Art In The Age Of Exploration (Great Section on Chinese Art Ming Dynasty)

which bounds the projection. Although the rete
resembles a modern planisphere made of trans-
parent plastics, it is a mirror image because it
shows the celestial sphere, not as seen from the
earth, but as seen "externally"; for instance, as
seen on a celestial globe. The rete can be rotated
over a plate, designed for use in a specific latitude,
on which are engraved the horizon for that lati-
tude; circles of altitude at equal intervals between
the horizon and the zenith (almucantars); lines
showing unequal, planetary hours: hours deter-
mined by dividing the interval between sunset
and sunrise into twelve equal divisions and the
interval between sunrise and sunset into a further
twelve divisions, six o'clock being at midnight and
at midday, and the hour-length varying through-
out the year; and sometimes lines delineating the
twelve astrological houses. Usually a number of
plates are provided for different latitudes, that for
the place of immediate use being placed on top of
the pile of plates which are, with the rete on top,
placed in a recess, mater, in the body of the astro-
labe. Together with a sighting-rule, alidade, the
whole is held in place by a pin and wedge, horse,
through the center of the astrolabe, which repre-
sents the celestial pole. Rotation of the rete over
the plate —which is prevented from turning by a
lug — simulates the apparent rotation of the stars
about the pole, creating a form of analogue-
computer.
Time-telling by night provides a simple exam-
ple of one use of an astrolabe: The instrument is
freely suspended by the ring and the altitude of a
star represented on the rete is measured, using in it is a second recess for plates. These include a
the alidade in conjunction with the scale of plate for latitude 48° and another of the twelve 122
degrees. The rete is then turned until the pointer astrological houses. A small magnetic compass is Attributed to Hans Dorn
for the observed star lies on the circle of altitude inset in the front of the suspension-piece; this is
corresponding to the observed altitude, east or an unusually early example of such practice. Viennese, 1430/1440-1509
west of the meridian as appropriate. This done, The need for a different plate for every latitude MARTIN BYLICA'S TORQUETUM
the rete shows the positions of the stars in rela- led to the design of universal astrolabes requiring
tion to the horizon of the place of observation. A only a single plate; these instruments, however, 1487
straight line taken with a rule from the center of were not always as convenient for the solution of Buda?
the astrolabe through the position of the sun in certain problems. Such a plate is found at the back brass 7
the ecliptic, as marked on the ecliptic circle of the of this astrolabe: a stereographic projection of the height 71 (27 /s); base 43.3 x 56 (17 x 22)
rete (that is, the declination on the day of obser- celestial sphere from the vernal point onto the references: Rosinska 1974; Schallaburg 1982, 339-
vation, ascertainable from the zodiac calendar colura of the solstices, known in medieval Europe 340, no. 285; Poulle 1983, 32-35; Turner 1987,
17-18
scale usually engraved on the back of European as saphdea Azarchelis, because it was devised by
astrolabes) will indicate the time in equal hours the astronomer, Ibn az-Zarqalluh, in eleventh- Jagiellonian University, Cracow
on the scale of hours on the limb (circumferential century Toledo. His Arabic treatise was translated
border) or by an analogous procedure in unequal into Hebrew first, then into Latin, ensuring its The torquetum "may rather be considered an
hours on the hour-lines on the plate below diffusion in the Christian West. Like the insertion example of conspicuous intellectual consumption
the rete. of the small magnetic compass, a universal pro- than a much used instrument" (Turner 1987, 18).
Forty-eight stars are indicated on the rete of jection was innovative at this time because there This perhaps explains, apart from the piece's obvi-
Martin Bylica's astrolabe. The bases of the point- are very few medieval examples outside the man- ous fragility, why only two known medieval
ers of the brightest stars are more elaborately dec- uscript tradition (for example, orthographic pro- examples have survived — although several more
orated than the others (compare to the stars on jection on the astrolabe on the globe, cat. 120). A are known from the sixteenth century and the
the globe, cat. 120). This astrolabe is unusual in large, similar astrolabe, also attributed to Dorn, is instrument is described in medieval manuscripts
that the limb of the front bears the zodiac calendar equally innovative and dated three years earlier to and early printed books. One of the two surviving
scale correlating the solar declination with the 1483 (Museo di Storia della Scienza, Florence). torqueta was bequeathed to the Jagiellonian Uni-
date, as well as the customary scales of degrees F.R.M. versity by Martin Bylica (as were cats. 120,121).
and of equal hours (0-12, twice). The zodiac cal- The other belonged to Nicholas of Cues at the
endar scale was not engraved on the back because beginning of the fifteenth century and is now at
Bernkastel-Cues.

EUROPE AND THE MEDITERRANEAN WORLD 223