The shadow square, also known as an altitude scale, was an instrument used to determine the linear height of an object, in conjunction with the alidade, for angular observations. It is an early example of an analog computer.[not in citation given] It was invented by Muhammad ibn Mūsā al-Khwārizmī in 9th-century Baghdad.[not in citation given] Shadow squares are often found on the backs of astrolabes.
The main use of a shadow square is to measure the linear height of an object using its shadow. It does so by simulating the ratio between an object and its shadow. 
- "Shadow Square". Retrieved 2009-07-18.
- "Astrolabe detail within shadow square". National Maritime Museum. Retrieved 2009-07-18.
- King, David A. (2002). "A Vetustissimus Arabic Text on the Quadrans Vetus". Journal for the History of Astronomy 33: 237–255. Bibcode:2002JHA....33..237K.
- "Shadow Square". Museo Galilieo. Retrieved March 12th 2014.