This astrolabe is a very rare example of the 'Mathematical Jewel' published by the English mathematician John Blagrave in 1585. Blagrave's design uses as a mater the universal projection of Gemma Frisius's 'astrolabum catholicum'. However, it combines this with a distinctive rete divided into two halves: a stereographic grid and the more familiar rete form with star pointers and an ecliptic semicircle.
The back of the instrument is merely outlined and bears the signature of the author, Charles Whitwell, and the date. The type of engraving allows the use of the instrument in navigation, for finding stars and reference points when latitude co-ordinates are unknown.
The astrolabe belonged to the group of objects brought by Sir Robert Dudley from England, which he later bequeathed to the Grand Duke of Tuscany.