Compositio et operatio astrolabii

The Arabic text of this work has been lost, and it survives only in a Latin translation possibly made by the twelfth-century scholar Joannes Hispalensis. It was one of the sources that Chaucer used for his own Treatise on the Astrolabe.

As the title suggests, the work falls into two parts. The first describes how to construct an astrolabe, and is provided with several diagrams and illustrations. The second part instructs the reader in the practical use of the instrument, and is divided into more than 40 'propositions', most of which were copied by Chaucer.

No copies of the manuscript in Arabic exist. St John's College MS F.25 is one of several copies of the work in Latin. It contains several illustrations showing how to construct an astrolabe. The manuscript dates from the thirteenth or fourteenth centuries and was donated to the College in 1635 by Thomas Wriothesley.

Click on any of these images to see a bigger version.

This picture shows the top of the mother of the astrolabe. The loop on the top used to hang the astrolabe from the thumb is labelled "armilla", the Latin for hoop or ring. The division of the rim into degrees is shown with divisions of 5 degrees working to the right from the top.

This is a diagram of the rete of the astrolabe. The internal ring bears the names of the signs of the Zodiac. This is the ecliptic, the path taken across the sky by the sun. Also spaced across the rete are the names of various stars.

This diagram shows how to construct the lines of altitude on the plate of the astrolabe. The plate is constructed specifically for a given latitude on the earth's surface. The latitude of Cambridge is 52° North, Edinburgh is 56° North, and Exeter is 51° North. The altitude lines enable the astrolabe user to see how high in the sky a given object will be.

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