Al-Sijzi, the Iranian astronomer, believed that the Earth rotated around its axis and he constructed his astrolabes in accordance with this belief. Al-Biruni attempted to describe the four types of astrolabes invented by Al-Sijzi. In most astrolabes (i.e., planispheric astrolabes), the coordinate system depends on the stationary observer and the positions of the stars, which are spinning around the Earth, and are measured relative to the various coordinates. In the astrolabes made by Al-Sijzi, the stars are considered to have a fixed position in space, and the horizon of the observer rotates relative to the fixed points. This idea was originally proposed for constructing scaphoid astrolabes; however, other kinds of astrolabes (i.e., Mestry astrolabes, Marychy astrolabes, and cruciate astrolabes) were also based on this idea. In this paper, based on information in Biruni’s Al-Estiab, the structure of these four types of astrolabes is compared to flat instruments, and their operating principles are described based on the theory of the Earth's rotation.

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