The Canterbury Quadrant which London dealers Trevor Philip and Sons have sold to the British Museum for £411,250.
It will go on display in the BM's new medieval gallery.
The piece was bought by the Jermyn Street firm for £138,000 at Bonhams' Knightsbridge rooms on March 21 last year.
Unearthed during excavations prior to building work in Canterbury in 2005, the brass astrolabe served a multitude of functions, such as calculating the time and length of the day, the date of the next full moon and the height and depth of structures. It also served as a pocket calculator, being uniquely tiny with a 2 1/4in (7cm) radius.
Dated to c.1388, the year after Chaucer started writing The Canterbury Tales, the Canterbury Quadrant is one of only eight such instruments recorded.
