Astrolabes served as mechanical maps of the Universe. Sophisticated, hand-held instruments, they were used for centuries to teach people about the sky. The back side had a moveable sighting arm and a scale of degrees for measuring altitude. The front side was engraved with a flattened map of the heavens, which was used with other moveable parts to solve practical astronomical problems.
Islamic Astrolabe, A. This astrolabe has several interchangeable plates, each engraved with the celestial coordinates for a different latitude. The pointers on the top plate indicate the positions of 22 bright stars. The top plate can rotate to show where those stars will appear at different times or dates, much like a modern paper or plastic star finder.
The instrument could also be used to predict when the Sun or certain bright stars would rise or set on any date. A sextant can measure an angle on any plane, and works by a principle of double reflection. It is also far more accurate and can be used for a range of purposes including navigation finding latitude, longitude, local time. An astrolabe can only measure angles in a vertical plane and was principally used for latitude-finding, although you can also use it for purposes such as finding the height of something.
From the collection: The Greenwich Valentia Astrolabe. This example was found in under a rock on Valentia Island, close to the point off southern Ireland where three ships of the Spanish Armada were wrecked in The throne is low and moulded. The mater has been made from a single casting, cut out in a wheel shape with a greater weight left in the lower half to help the instrument hang vertically.
The face of the mater is engraved with circles and decorative lines but there are no numbers on the scale, suggesting that the instrument was never completed - perhaps it was one of a number of pieces of unfinished equipment hurried aboard a Spanish vessel in An alidade is fitted to the face of the instrument and held in place with a pin through its centre, about which it can rotate. This mariner's astrolabe will be going into the new Tudor and Stuart seafarers gallery.
Astronomical instrument with engraved markings showing the projection of the celestial coordinates. The markings are directly derived from those of the astrolabe, but are reduced and repositioned to fit inside the quarter-circle. In essence, this is an astrolabe folded twice on itself.
A distinctive version is the Davis quadrant. It consists of two complanar and facing octants, which measured the Sun's altitude from its reflection on a small mirror placed on the horizon plane. Horary quadrants are engraved on one of their sides with hour lines computed for a particular latitude. Some horary quadrants had a sliding cursor that could be positioned for a specific latitude, enabling the instrument to be used for finding the time at different latitudes.
Its special characteristic is its "two-limbed" design, i. This feature allowed two separate readings for morning hours and afternoon hours.
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