Figure II. — Gearrd Astrolabe by Muhammad b. Abi Bakr of Isfahan, A. D. 1 221-1222. 

 {Photo courtesy of Science Museum, London.) 



explains how a special train of gearing may be used 

 to show the revolutions of the sun and moon at their 

 relative rates and to demonstrate the changing phase 

 of the moon, features of fundamental importance in the 

 Islamic (lunar) calendrical system. This device nec- 

 essarily uses gear wheels with a "non-round" number 

 of teeth {e.g., 7, 19, 59) as dictated by the astronomical 

 constants involved (see fig. 10). The teeth are shaped 

 like equilateral triangles and square shanks are used, 

 exactly as with the Antikythera machine. Horse- 

 headed wedges are used for fixing; a tradition bor- 

 rowed from the horse-shaped Faras used to fasten the 

 traditional astrolabe. Of special interest for us is 

 the lunar phase diagram, which is just the same in 



form and structure as the lunar volvellc that occurs 

 later in horology and is still so comm.only found 

 today, especially as a decoration for the dial of 

 grandfather clocks. 



Biruni's calendrical machine is llic earliest compli- 

 cated geared device on record and it is therefore all 

 the more significant that it carries a feature found in 

 later clocks. From the manuscript description alone 

 one could not tell whether it was designed for auto- 

 matic action or merely to be turned by hand. For- 

 tunately this point is made clear by the most happy 

 survival of an intact specimen of this very device, 

 without doubt the oldest geared machine in existence 

 in a complete state. 



98 



BULLETIN 218: CONTRIBUTIONS FROM THE MUSEUM OI' HISTORY AND TECHNOLOGY 



