| Abu Sa'id al-Sijzi | |
| Caption: | A page from Al Sijzi's geometrical treatise |
| Birth Date: | 945 CE |
| Birth Place: | Sijistan, Saffarid dynasty (modern-day Sistan)[1] |
| Death Date: | 1020 CE |
| Main Interests: | Mathematics, Astronomy, Astrology |
Abu Sa'id Ahmed ibn Mohammed ibn Abd al-Jalil al-Sijzi (c. 945 - c. 1020, also known as al-Sinjari and al-Sijazi; Persian: ابوسعید سجزی; Al-Sijzi is short for "Al-Sijistani") was an Iranian[2] Muslim astronomer, mathematician, and astrologer. He is notable for his correspondence with al-Biruni and for proposing that the Earth rotates around its axis in the 10th century.[3] [4]
He dedicated work to 'Adud al-Daula, who was probably his patron, and to the prince of Balkh. He also worked in Shiraz making astronomical observations from 969 to 970.
Al-Sijzi studied intersections of conic sections and circles. He replaced the old kinematical trisection of an angle by a purely geometric solution (intersection of a circle and an equilateral hyperbola.)
Al-Biruni tells us that Al-Sijzi invented an astrolabe, called "al-zūraqī", whose design was based on the idea that the Earth rotates:[5]
Al-Biruni also referred to Al-Sijzi as a prominent astronomer who defended the theory that the earth rotates in al-Qānūn al-Masʿūdī.[6]
The fact that some people did believe that the earth is moving on its own axis is further confirmed by a reference from the 13th century which states:
"According to the geometers [or 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."