Topics

• The time span of the period called Middle Ages is defined differently by different authors. For our purposes, we will take a it to extend approximately from the fall of the Roman Empire (476) to the fall of Constantinople (1493). A very good general resource for this period is Fordham University's Internet Medieval Sourcebook. You should also check Paul Gans' The Medieval Technology Timeline at NYU, and James McNelis' The Medieval Science Page. In this lecture we will continue the hardware theme of the previous lecture.
• As we can see in The Medieval Technology Timeline, and with important exceptions we will discuss below, most of the innovations in this period are in the area of agriculture and industry, and are not directly related to computing or information technology. Concerning the latter, however, we must mention paper. "Paper was invented by the Chinese prior to the first century AD," but "entered Europe through Islamic Spain and Sicily in the tenth century […] and it was not until the 13th century that its use became at all widespread and production begun." At about this time (1285) spectacles appeared in Europe. Once again it would seem that the science behind such invention was the work of Moslem mathematicians and astronomers.
  The compass was a Chinese invention, probably dating from the first century AD, and was definitely in use on Chinese ships by the ninth century. It reached Europe around the 12th century.
  I will also mention another Chinese invention that reached Europe in the 11th century, the horizontal loom, because later attempts to automate and program this tool played an important part in the development of the computer.
• For our purposes, the most remarkable inventions of the Middle Ages were perhaps the armillary spheres, the astrolabe and the quadrant. Of course we should also mention clocks. As we pointed out in the previous lecture, the medieval art and science of clocks continued the seeingly unbroken tradition of the Antikythera Mechanism.

   

  

  Early Armillary Spheres (16th Century)
  Museum of the History of Science, Oxford

   

  "It appears that from the times of Eratosthenes, in the 3rd century BC, a representation of the celestial sphere was widely used by natural philosophers of the Alexandrian school, consisting of graduated metallic rings representing the equator, the ecliptic and certain meridians and parallels. This "skeleton" of metal rings (armillae) was supported by a fixed ring representing the horizon of the observer and could be adapted to the latitude and longitude of the place. At the centre of this armillary sphere the Earth was represented together with, in the more complicated models, the Sun, Moon and planets. By correctly placing bodies inside the sphere and the rings, it was possible to solve problems of spherical astronomy and compute the coordinates of the stars on the celestial sphere. Obviously, the lack of precision dictated by the dimensions of the instrument favoured its use for more didactic and explicative purposes." [ from Museo della Specola at the University of Bologna, Italy ]

• The armillary sphere represented a model of the celestial sphere, and its use therefore was (or attempted to be) somewhat intuitive. The astrolable on the contrary embodied a more ambitious goal: "projecting a hemisphere onto the plane." This is impossible without introducing distortions, and much of the work of the Arab and Muslim astronomers and mathematicians centered around techniques intended to formalize the relationship between the motions observed in the heaven and the motions of the components of the astrolabe. In particular this work lead to the production of tables of trigonometric functions.

• "The oldest known astrolabes were created a few centuries BC, possibly by Hipparchus. They were [not] improved and more features were [not] added to them until the Middle Ages, by which time they had become very complex instruments. The Arabian astronomers made extensive use of the astrolabe. One of the best descriptions of the astrolabe and its use was written in 1392 by Geoffrey Chaucer, in England." [ from Astrolabe History ]

   

  

  Astrolabe from Baghdad (1131)
  Adler Planetarium & Astronomy Museum, Chicago

   

  "The astrolabe, whose mathematical theory is based on the stereographic projection of the sphere, was invented in late antiquity, but its extensive development in Islam made it the pocket watch of the medievals. In its original form, it required a different plate of horizon coordinates for each latitude, but in the 11th century the Spanish Muslim astronomer az-Zarqallu invented a single plate that worked for all latitudes. Slightly earlier, astronomers in the East had experimented with plane projections of the sphere, and al-Biruni invented such a projection that could be used to produce a map of a hemisphere. The culminating masterpiece was the astrolabe of the Syrian Ibn ash-Shatir (1305-75), a mathematical tool that could be used to solve all the standard problems of spherical astronomy in five different ways."
  [ from Arabic Mathematics ]

• A remarkable discovery was made by Sharaf al-Din al-Tusi (1201) who invented the linear astrolabe. Such instrument embodies the so-called Tusi couple, which allows circular motions to be transformed into linear motion. See George Saliba's article Greek Astronomy and the Medieval Arabic Tradition in American Scientist, where you can also play with a few Java applets illustrating the operation of the Tusi couple.

   

  

  The Tusi Couple
  (from G Saliba, Greek Astronomy and the Medieval Arabic Tradition,
  Am.Sci. v 90, n 6, 2002)

   

  It is interesting to note that, as we shall see later, some of the early, semi-mechanical, modern analog computers attempted to solve somwhat similar problems.

• "Quadrants were used to measure altitudes of celestial objects and were especially useful for mapping the sky. They consisted of a sighting device mounted on a quarter circle, or quadrant, which was engraved with degree markings. Quadrants ranged in size from small hand-held or table-mounted versions to large mural quadrants mounted on walls." [ from Smithsonian National Air & Space Museum ]

   

  

  Quadrant Astrolabe, ca 1325
  Smithsonian Air & Space Museum

   

• "In the first half of the 14th century, large mechanical clocks began to appear in the towers of several large Italian cities. We have no evidence or record of the working models preceding these public clocks, which were weight-driven and regulated by a verge-and-foliot escapement. Variations of the verge-and-foliot mechanism reigned for more than 300 years, but all had the same basic problem: the period of oscillation of the escapement depended heavily on the amount of driving force and the amount of friction in the drive. Like water flow, the rate was difficult to regulate. But it was only in 1656, Christiaan Huygens, a Dutch scientist, made the first pendulum clock, regulated by a mechanism with a "natural" period of oscillation. (Galileo Galilei is credited with inventing the pendulum-clock concept.)" [from NIST's A Walk Through Time ]

  Read Nigel Bunce and Jim Hunt's nice description of Two Medieval Clocks.

  "All clocks must have two basic components:

  • a regular, constant or repetitive process or action to mark off equal increments of time. Early examples of such processes included the movement of the sun across the sky, candles marked in increments, oil lamps with marked reservoirs, sand glasses (hourglasses), and in the Orient, knotted cords and small stone or metal mazes filled with incense that would burn at a certain pace…
  • a means of keeping track of the increments of time and displaying the result…"

  
  [from A Walk Through Time, Early Clocks ]

   

  

  The Salisbury Cathedral Clock (1386)
  The Science Museum, London

   

  This type of clock used the foliot balance and verge escapement system. Not until the 17th century was such system replaced by the pendulum and recoil escapement.

 
Readings, Resources and Questions

• A brief but good description of the operation of an astrolable can be found at the Museo della Specola of the University of Bologna.
• Another instrument that became rather sophisticated in the Middle Ages is the sundial. Do a little research about the state of development reached by this device in the period we are considering here.
• One of the items included throughout The Medieval Technology Timeline is the size of urban population. Why?
• Clocks are essential in almost any attempt to automate computation. Discuss why.