The square brass plate has four holes to enable the dial to be fixed to a permanent pedestal. Presumably it was originally erected in a garden belonging to its first owner, whose name is engraved beside the south point in the hour circle: '+Sur+Henry+darcy+'. Darcy had been knighted by the Earl of Leicester in 1566.
The hours run from 4 to 12 to 8 and are subdivided to 1/2. Inside the hour circle there is a decorated compass rose marked for the 8 principal directions.
The angle of the gnomon is approximately 52?, close to the latitude of London.
The maker's signature is given as '+ H + C?le + + 1579 +'.
In the course of its history, the instrument has been not only weathered but also marked by subsequent owners or passers-by: various initials and names have been scratched on its surface.
See R. T. Gunther, "The Great Astrolabe and other Scientific Instruments of Humphrey Cole", Archaeologia, 76 (1927), pp. 273-317. at p. 300.