Several Islamic astronomers questioned the Earth's apparent immobility, and "centrality within the universe". Some accepted that the "Earth rotates around its axis", such as Abu Sa'id al-Sijzi (died circa 1020). who invented an astrolabe based on a belief held by some of his contemporaries "that the motion we see is due to the Earth's movement and not to that of the sky". That others besides al-Sijzi held this view is further confirmed by a reference from an Arabic work in the 13th century which states: "According to the geometers engineers ("muhandisīn"), the earth is in constant circular motion, and what appears to be the motion of the heavens is actually due to the motion of the earth and not the stars".