Shadow square

An astrolabe like the one pictured would commonly include a Shadow Box on the back.

The shadow square, also known as an altitude scale,[1] was an instrument used to determine the linear height of an object, in conjunction with the alidade, for angular observations. An early example was described in an Arabic treatise likely dating to 9th or 10th-century Baghdad.[2] Shadow squares are often found on the backs of astrolabes.

  1. ^ "Shadow Square". Retrieved 2009-07-18.
  2. ^ King, David A. (2002). "A Vetustissimus Arabic Text on the Quadrans Vetus". Journal for the History of Astronomy. 33 (112): 237–255. Bibcode:2002JHA....33..237K. doi:10.1177/002182860203300302. S2CID 125329755.