Oldest in the world
This is the world's oldest geared mechanism which has survived in a complete state.
There is an astrolabe on the front and a calendar plate on the back. The hollow space between the two is occupied by the gear train visible in the mirror.
The action of the gearwheels changes the display on the back of the instrument. The top circular window shows the phases of the moon. That visible appearance is matched by the age of the moon (the number in the small square window representing the moon's day in its monthly cycle).
The lower calendar disc shows the relative positions of the sun and moon. There is an outer zodiac circle surrounding two concentric rotating rings. One is inset with gold to represent the sun and its apparent motion; the other indicates the changing position of the moon. Together they help to show the causes and timings of eclipses.
Other narratives:
- Astronomical mechanisms
- Geared astrolabe inscription
- Inventory no. 48213 - Former Display Label
- Relationship between astrolabe and calendar
- Special Exhibition Label: 'Al-Mizan: Sciences and Arts in the Islamic World' (26/10/2010 - 20/03/2011)
- Exhibition Label - 'The Moon'; National Maritime Museum, 19 July 2019 - 5 January 2020