The Astrolabe

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

What is an Astrolabe?

The astrolabe is a complex inclinometer with several functions. With a history that dates back more than two thousand years, the astrolabe has been utilized consistently by astronomers and navigators 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.

The functions of the astrolabe (depending on which variation is being used) are:

  • Identify stars or planets
  • Calculate the position of celestial objects 
  • Determine local latitude given the local time and vice versa
  • Measure the time of the year
  • Compute what part of the sky is visible
  • Predict celestial events such as eclipses
  • Triangulate current location
  • Be used at night or day

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