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Inventory no. 48213 - Former Display Label

A Geared Computing Device, 1221-2

This Persian astrolabe with a geared calendar movement, made in Isfahân in 1221-2 A.D. by Muhammad b. Abî Bakr, is the oldest geared machine in existence in a complete state. It illustrates an important stage in the development of the various complex astronomical machines from which the mechanical clock derives.

The back of the astrolabe, which carries the train of gears and the calendar, is shown detached, on the right of the remainder of the astrolabe. The train of gears can be seen in the mirror.

The design of this part of the astrolabe is based on a text by one of the greatest Islamic scientists, al-Bîrûnî (973-1048 A.D.), who explained how a special train of gearing might be used to show the revolutions of the sun and moon at their relative rates and to demonstrate the changing phase of the moon. These phenomena were of fundamental importance in the lunar calendar used in Islam. In this instrument, the gearing has been cleverly simplified so that only one wheel has an odd number of teeth (13), compared with several in al-Bîrûnî's design. The wheels have teeth shaped like equilateral triangles, recalling the teeth on the wheels in the Hellenistic astronomical computing machine found in a wreck of c.80 A.D. at Antikythera and now in the National Museum, Athens.

When assembled, the geared calendar is operated by turning the {ain}ankabût of the astrolabe. The upper and smaller circular opening on the back reveals a lunar phase diagram, which may be compared with the lunar volvelles found on many European horological instruments and, even today, on the dials of grandfather clocks. The abjad numeral visible in the small rectangular opening gives the age of the moon (and therefore the date in a lunar calendar). Below, within a zodiacal calendar scale, are two concentric rings, the outer ring inset with a small gold disc representing the sun, the inner ring formerly having a similar inset representing the moon. The rotations of these rings show the relative positions of the sun and moon (opposition, conjunction, etc.) and the position of the sun in the zodiac.

The gear train count is as follows: 48 - 13 + 8 - 64 + 64 - 64 + 10 - 60. The pinion of 8 teeth has been incorrectly replaced by a pinion of 10. The gear of 48 teeth should make 13 (lunar) rotations while the double gear of 64 + 64 makes 6 revolutions of double months (of 29 - 30 days) and the gear of 60 makes a single turn in the hegiral year of 354 days.

The astrolabe is of brass with silver ornament, including pictorial representations of the signs of the zodiac, between figures of warriors, damascened on the rim. There are two plates, for latitudes 30°, 32°; 36° and 40°. There is no alidade, two sights on the {ain}ankabût serving instead.

The instrument is signed, across the back plate, 'Made by Muhammad b. Abî Bakr b. Muhammad ar-Râshidî al-Ibarî [or al-Abirî] al-Isfahanî'. It is dated, '618 [A.H. = 1221-2 A.D.]', at the end of a long inscription around the edge of the back. On one of the wheels is engraved, 'Its owner is the poor man Hasan Shâh'.

See Derek J. de Solla Price, 'On the Origin of Clockwork, Perpetual Motion Devices, and the Compass', Contributions from the Museum of History and Technology (United States National Museum, Bulletin 218), Washington, 1959, pp. 97 ff.

[IC 5]
Lewis Evans Collection

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Related Objects:

Inventory No. 48213, "Astrolabe with Geared Calendar, by Muhammad b. Abi Bakr, Isfahan, 1221/2" [1924-0/part], Muhammad b. AbiBakr