The importance of this astrolabe lies in its Royal provenance and the astrological information it contains on the reverse. It was once believed to have been made for Queen Elizabeth on account of the inscribed royal arms flanked by 'ER' on the back of the instrument. However, recent research concludes that it was actually constructed for her half-brother, Edward VI, and indeed, the style of engraving on this instrument is remarkably similar to Gemini's signed and dated astrolabe made for Edward VI, currently in the Observatoire Royale de Belgique in Brussels (IC 450). Gemini also made a signed astrolabe for Queen Elizabeth in 1559, now at the Museum of the History of Science, Oxford (Inv. No. 36-6,42223).

Only the throne, limb and back of this instrument survive - the rete, plates, alidade and rule have been discarded. The throne, riveted to the limb, is of an elaborate and delicate design. The inside of the mater was initially unengraved, but since, Henry Sutton engraved a circular slide rule within it, in 1655. It is signed, 'H: Sutton fecit 1655'. The back of the astrolabe is decorated with a series of concentric rings, detailing specific astrological information about the planets, zodiac signs, lunar mansions, decans and terms. Most of this information has been copied directly from an astrological disc, published by Gerard Mercator in 1551. It is likely that this information reached Gemini through John Dee, Elizabeth's 'philosopher', who spent a good deal of time with Mercator while they both lived in Paris between 1548 and 1550.