The Astrolabe

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Guest Post by HSCI 3013.002 students Bryce Bonnet, Lynn Bui and Cera Vu

What is an Astrolabe?

With a history that dates back more than two thousand years, the astrolabe has been utilized consistently by astronomers since antiquity. There are several types of astrolabe with varying levels of complexity, including the Mariner’s Astrolabe, the Quadrant Astrolabe, and the Planispheric Astrolabe.

Brief History of the Astrolabe

Astronomy and astrology were popular historical pastimes for all social classes. Without smart phones, social media, or computers to pass the time, looking up to the heavens proved to be common social activity. The astrolabe was initially designed to satisfy this demand; in the right hands, any star visible to the naked eye could be examined and analyzed.

A scientific breakthrough from the Islamic scientific world to the Europeans, the astrolabe was seen in the hands of scientists, elites, and monarchs alike from the Byzantine Empire to Muslim Spain. First used in ancient Greece before being extensively developed during the Islamic Golden Age by Arabian astronomers, the astrolabe became the key astronomical instrument of the western middle ages.

No one is sure who invented the astrolabe. A likely candidate is the Greek astronomer Hipparchus, who worked on the Isle of Rhodes around 150 BCE, but it evolved in complexity and usefulness over many centuries. We do know that the mathematical theory which serves as the foundation for the stereographic projection used in the planispheric astrolabe was provided in the second century CE, by Ptolemy, in his “Planisphaerium“(celestial plane).

Timeline

Early History

  • Hipparchus first described the stereographic projection used for an astrolabe around 150 BCE
  • Around 150 CE Ptolemy described an instrument very similar to an astrolabe
  • By the 9th century, the Islamic world was producing exceptionally designed astrolabes and sophisticated texts over their functions.

Medieval History

  • Hermann Contractus of Reichenau’s De utilitatibus astrolabii was one of the first texts on the astrolabe composed in Latin
  • Between the 11th and 13th centuries, most astrolabes in Europe were imported from Muslim Spain
  • In the 14th century, Geoffrey Chaucer wrote Treatise on the astrolabe that became the first ‘technical manual’ of its kind to be written in English (instead of Latin, Greek or Arabic); based off of Masha’allah’s Compositio et operatio astrolabii (The construction and use of the astrolabe).

Early Modern History

  • George Hartmann established a workshop in Nuremberg and quickly became one of the most important makers in early 16th century
  • By the end of the 16th century, astrolabes were made for emperors, monarchs, and princes, becoming a symbol of status and power
  • In the 17th century, production of astrolabes rapidly declined as it was replaced by other instruments, such as the sextant.

Properties of the Astrolabe

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There are many components of an astrolabe. This instrument is suspended by a cord that is connected to a protruding part, called a throne. The throne’s aesthetic variety reflects the time and location of its maker. Connected to the throne is a large circular body, called the mater. This body has a raised outer rim, called the limb, which commonly indicates the hours of the day and a degree scale. The front of the mater has a hollowed depression (womb) that is used to hold the plate, which is engraved with several circles and lines that specify a particular location. Sitting on top of the plate is a skeletal disc that symbolizes the ecliptic ring as well as several prominent starts. There are two pointers, one in the front (rule), and one in the back (alidade). The rule facilitates the reading of the astrolabe. The alidade is a measuring tool that measures the angle and altitude of buildings, stars, and the sun.

How to use the Astrolabe

https://www.youtube.com/watch?v=gu-yC-g4fcU

References:

AstroCrafts: How to Make An Astrolabe

Using An Astrolabe to Tell the Time

The Way to the Stars: Build Your Own Astrolabe

J.D. North, “The Astrolabe” Scientific American (1973)

Darin Hayton, ePamphlet Guide to the Astrolabe

Armillary Sphere

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A guest post by HSCI 3013.002 students Danya Majeed, James Reeves and Crystal Neill.

History:

Imagine you are an astronomer in medieval times.  You have studied Aristotelian physics.  You wholeheartedly agree with Ptolemy’s description of the universe and have thoroughly digested the precepts laid out in his book, Almagest, detailing the position of the earth and the celestial bodies in the cosmos.  But you, as an astronomer and aspiring astrologer, would like to use this knowledge for practical benefit.  You want to devise a practical model of the theories laid forth by Ptolemy and therefore have decided to build a model of the universe based on his precise mathematical calculations.

This is how the Armillary Sphere was born.

Ptolemy himself provides instructions on how to build this model of the cosmos in Chapter 1, Book 5 of his opus magnum, Almagest.  Who, among astronomers, was known to have actually built and used this instrument?

We know of three observatories in the Islamic world which used the armillary sphere.  In the 13th century, the Maragha Observatory (modern-day Iran) used an armillary sphere.  In the 15th century, the Samarkand Observatory (modern-day Uzbekistand) used an armillary sphere.  In the 16th century, the Istanbul Observatory (modern-day Turkey) used an armillary sphere.

In the European world, the construction of the armillary sphere was taken from models found in Andalus (modern-day Spain).  In the 16th century, well-known European astronomer Tycho Brahe constructed and used several armillary spheres.  He built them of steel, brass, and wood held together by screws, in order to avoid warping materials which would render the instrument inaccurate.  His armillary spheres were quite large, over 1.5 meters in diameter, and very accurate in their readings.  Our good friend Copernicus also constructed armillary spheres in this time period, and gave rise to ideas which would lead to a different take on the instrument altogether.

In the 17th century, Copernicus’ heliocentric model of the cosmos had gained enough traction for armillary spheres to be constructed with the sun in the center instead of the Earth.  These spheres also became more complex and astronomically accurate with the addition of rings for newly discovered planets, including Uranus and Neptune.

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Francesco de Mura, “Allegory of Arts (painted between circa 1747 and circa 1750). Wikimedia Commons.

In the 18th century, the armillary sphere became a symbol of the well-read scholar.  It can be found in paintings, manuscripts, and sculptures, representing the highly regarded and sought after science of astronomy as well as hinting at the polished and intellectual inclinations of the subject of the artwork.  Books such as ‘The Young Gentleman and Lady’s Astronomy’ were published, describing the function of the armillary sphere as part of a leisurely educational curriculum for the upper class.  The paintings “Allegory of the Arts” by Franscesco de Mura and A Young Princess by Jan Gossaert feature an armillary sphere, with the latter painting simply depicting a young girl holding the device in her hands.

The armillary sphere remains an emblem of scientific thought and an intellectual  milestones in the history of science.  In many ways, it represents the ingenuity and remarkably unmatched intelligence and perseverance of scientists throughout the centuries.  Today, the Oxford Museum of Natural History holds an impressive statute of 13th century scientist Roger Bacon holding an armillary sphere in his hands, our own University of Oklahoma has two armillary spheres in its special History of Science collections, and Dr. Crowther even has a picture of one on the desk in her office.

How It Works: 

This video describes the different parts: https://youtu.be/gBbZItACgyw

This video describes how to use it: https://youtu.be/pGNt_PKsNyY

Building:

This Armillary Sphere was made of laser cut particle board.  We debated whether we should build the model out of wood, metal, or 3D print it, but we settled on cutting it from wood.  Once we settled on a material and method of building, we then went through several different images and models to decide how to build it.  The design we went with worked best with a solid stand that would allow the meridian to rotate between the horizon.  Next, we used Adobe Illustrator to create the rings and print the cardinal directions and degrees, which was then cut or etched into the wood by the laser cutter.  The pieces were fit together, and two nails were used to allow the globe and its parts to rotate.  The Earth is a Styrofoam ball suspended by a wooden dowel.  Once the pieces were cut, it was a matter of putting them together and making sure that the zodiac was in the correct orientation.

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Further reading:

The Armillary Sphere