Abu Sa'id Ahmed ibn Mohammed ibn Abd Jalil Sijzi (short for Sijistani) (Persian: ابوسعید سجزی) was a Persian astronomer and mathematician from Sistan, a region lying in the south-west of Afghanistan and south-east of Iran.
Sijzi is thought to have been born around 945 CE, and lived through about 1020. His main scientific focus was astronomy. He had a deep knowledge of literature which he used to his advantage. He dedicated work to 'Adud al-Daula and the prince of Balkh. He also worked in Shiraz making astronomical observations from 969 to 970. He also did a lot of geometry work.
Biruni tells us that Sijzi invented an astrolabe, called the "Zuraqi", whose design was based on the idea that the Earth moves:
"I have seen the astrolabe called Zuraqi invented by Abu Sa'id Sijzi. I liked it very much and praised him a great deal, as it is based on the idea entertained by some to the effect that the motion we see is due to the Earth's movement and not to that of the sky. By my life, it is a problem difficult of solution and refutation. [...] For it is the same whether you take it that the Earth is in motion or the sky. For, in both cases, it does not affect the Astronomical Science. It is just for the physicist to see if it is possible to refute it."
Sijzi was a mathematician who made a special study of the 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.)
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