“His Almagest, and bookes great and small
His astrolabe, belonging to his art,”
-from The Millers Tale in Chaucer’s Canterbury Tales
So I finally have an astrolabe. My daughters found a shop at the Renaissance Fair in Charlotte that sold astrolabes that were hand made replicas of an ancient device and gave to me for Christmas. I have a certain fascination with the middle ages as well as ancient science, and the astrolabe, along with other early computing devices, always astound me. In my opinion the astrolabe is not only the most useful of these devises but also the most wonderful to look at. It could be used to tell the time, sunset/sunrise, celestial events, sidereal time (star time), navigation, architecture and surveying.
Astrolabes have their origin with the Greeks. The etymology derives from the Greek Astron (star) and Lambanein (to take) which formed the Greek word Astrolabos. Through the centuries the astrolabe found it’s admirers in the Roman Empire, Islam and into Medieval Europe. Islamic science brought the astrolabe to its full development during the first millennium. In the 11th century, Gerbert of Aurillac (Pope Sylvester II) was noted to have built an astrolabe. Gerbert was known for several other “game changers” in Europe such as bringing the number zero (Persian/Arabic numerals), the abacus, the armillary sphere and constructing a hydraulic organ.
Use of the Astrolabe in Europe became popular in the following centuries and we can see it’s legacy in many Illuminated Manuscripts, several clocks such as the famous Orloj, an astronomical clock in Prague, and even in the works of Geoffrey Chaucer.
The following is a quote from GC which I have provided in the Old English as well as a modern translation for those of you who remember reading the Canterbury Tales in the parallel text format.
Tractatus de Conclusionibus Astrolabii Bred and mylk for childeren astrolabe
Lyte Lowys my sone, I aperceyve wel by certeyne evydences thyn abilite to lerne sciences touching nombres and proporciouns; and as wel considre I thy besy praier in special to lerne the tretys of the Astrelabie. Than for as moche as a philosofre saith, “he wrappith him in his frend, that condescendith to the rightfulle praiers of his frend,” therfore have I yeven the a suffisant Astrolabie as for oure orizonte, compowned after the latitude of Oxenforde; upon which, by mediacioun of this litel tretys, I purpose to teche the a certein nombre of conclusions aperteynyng to the same instrument.
A Treatise on the Astrolabe
Little Lewis my son, I perceive well by certain evidences thine ability to learn sciences touching numbers and proportions; and as well consider I thy constant prayer in special to learn the treatise of the Astrolabe. Than for as much as a philosopher saith, “He wrappth him in his friend, that condescendth to the rightful prayers of his friend”, therefore have I given thee a suffisant Astrolabe as for our horizons, compounded after the latitude of Oxford; upon which, by means of this little treatise, I purpose to teach thee a certain number of conclusions pertaining to the same instrument.
Geoffrey Chaucer (c. 1343 – 25 October 1400)
The Astrolabe is made up of several parts which are aligned according to data that is provided and produces the result of additional information. For instance you could use the position of a star to tell the hour or date. Or you could use the date to determine the sunrise/sunset of any particular day. It is an example of an early computer like the Antikythera Mechanism or an Abacus, except it could do so much more. In the picture above is the Rete. It has several star positions which you could use to align on the Mater below it.
The Mater is made up of a Plate and a Limb which is the outer ring surrounding the plate. Several types of plates were made which could be inserted into the Mater for locations at different latitudes . The drawing to the left is a mater with a planispheric plate. Just like a planisphere, it used the concept of stereoscopic projection to map the sky.
The back of the Astrolabe is also designed with scales and calendar and a rule (Alidale). This would be used to determine the angle or altitude of a celestial point which would then be used to process information on the Mater.
Astrolabes do come in many styles and a lot of thought went into their design to make not only a useful tool but also a work of art. This only follows with the general feel of the Middle Ages, with its gothic cathedrals, illuminated manuscripts and maps of the world. Art seemed to accompany all aspects of life from architecture to science which is very different from our modern utilitarian world.