Angelo Poliziano’s 1484 Description Of A Planetary Clock

In studying the writings of scholars of ages past, one often begins to suspect that they were aware of more things than we generally give them credit for.  We begin to understand that the progress of knowledge is not always “upwards” in a steadily sloping straight line; there are periods of setbacks, stagnation, and decay.  And very often we perceive that men of great ability can be trapped in environments that are hostile to the development of their talents.

Consider a letter written by the Renaissance humanist Angelo Poliziano in 1484 to his friend Francesco della Casa, a Florentine ambassador and advisor employed by the Medici.  In this letter Poliziano describes a mechanism that was (presumably) able to describe the orbits of the various planets known at that time.  Poliziano has not seen the machine in some time; he apologizes that he has to write from memory.  The machine he describes was constructed by one Lorenzo della Volpaia, an artisan who came from a family specializing in clocks and scientific instruments.  Here he discusses the mechanism:

There is a small square pillar, roughly four-and-a-half feet high, which, tapering to a point, is topped in the manner of a pyramid.  Above it, in place of a capital, is a flat bronze disc, marked out in gold and colors, and on one side of which each path of the planets is laid out.  It is less than a foot-and-a-half in diameter, and it is moved within by gears, while a stationary band, marked with twenty-four hourly intervals, embraces the outer edge.  Inside this on the surface of the rotating disc, the twelve zodiacal signs are divided according to the proper degrees.  On the interior, eight miniature discs, roughly equal in size, are visible.  Of these, two occupy the central point, so that a lower one, slightly larger, represents the sun, and an upper one, the moon.  A radius extending from the sun to the band is able simultaneously to indicate, on the latter, hours, while on the zodiac, months, days, and the number of degrees, and what they call the true and medial movement of the sun.

A pointer likewise extends from the moon as an indicator of the lunar hours, which in fact are designed below on the circumference of the larger miniature disc; passing through the center of the lunar epicycle, and touching the zodiac, it announces the medial point in the movement of the planet.  Another, likewise rising from the same place, and intersecting the center of the lunar body (i.e., the rim of the epicycle), makes its true location plain.  The result is that slowness and speed, motion and each orbit, along with the conjunctions and the times of the full moon, are visible.  Surrounding these are six miniature discs, one of which, known as the “tail and head of the Dragon,” tells us about solar eclipses, as well as lunar ones.  The rest are assigned to the planets; from each one of these protrude twin needles, indicators of their movements, just as I described for the moon.

But these also exhibit retrograde motion, which does not happen at all in the case of the moon, inasmuch as its epicycle goes in the opposite direction.  Likewise is the system of conjunctions, reversals, and transverses made clear for each.  There is, in addition, another band, the size of that of the zodiac, intersecting the aforementioned six miniature discs of the planets on top, from which the gradient movement of the constellations and the intervals of the day (i.e, the time of sunrise) may be seen.  The discs by which the individual planets are conveyed move back and forth, toward east in the daytime, and toward west at night.  By contrast, the very large disc bends the planets toward the east at night, toward the west in the daytime, in twenty-four-hour intervals.  [Trans. by Shane Butler]

Anticipating that his friend might doubt the existence of such a marvelous machine, Poliziano quotes Ovid, saying, tarda solet magnis rebus inesse fides, or “in great matters, it often happens that belief comes slowly.”  We, as readers, can make several observations about this letter.  The first is the impressive power of memory and description possessed by its writer.  Poliziano is speaking from memory, and he neither designed the machine nor, as far as we know, operated it.  The second observation we make is on the advanced state of mechanical knowledge in Italy in 1484.  We hear of rudimentary versions of such machines in classical antiquity (e.g., the so-called “Antikythera Mechanism”), but this knowledge was not available to the engineers of that era.

The planetary clock described by Poliziano has not survived.  One wonders what other machines might have existed, then or earlier in history, that have now been irretrievably lost to the inroads of time.



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