Abu Sa'id al-Sijzi
A page from Al Sijzi'sgeometrical treatise
Born	945 CE Sijistan,Saffarid dynasty (modern-daySistan)[1]
Died	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 asal-Sinjari andal-Sijazi;Persian:ابوسعید سجزی;Al-Sijzi is short for "Al-Sijistani") was anIranian^[3]Muslim astronomer,mathematician, andastrologer. He is notable for his correspondence withal-Biruni and for proposing that theEarth rotates around its axis in the 10th century.^[4][5]

He dedicated work to'Adud al-Daula, who was probably his patron, and to the prince ofBalkh. He also worked inShiraz making astronomical observations from 969 to 970.

Al-Sijzi studied intersections of conic sections and circles. He replaced the old kinematicaltrisection of an angle by a purely geometric solution (intersection of a circle and an equilateral hyperbola.)

Al-Biruni tells us that Al-Sijzi invented anastrolabe, called "al-zūraqī", whose design was based on the idea that the Earth rotates:^[6]

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.

Al-Biruni also referred to Al-Sijzi as a prominent astronomer who defended the theory that the earth rotates inal-Qānūn al-Masʿūdī.^[4][7]

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."^[5]

• O'Connor, John J.;Robertson, Edmund F.,"Abu Said Ahmad ibn Muhammad Al-Sijzi",MacTutor History of Mathematics Archive,University of St Andrews
• Suter, Heinrich: Die Mathematiker und Astronomen der Araber und ihre Werke (80–81, 224, 1900).

• Hogendijk, Jan P. (1996).Al-Sijzi's Treatise on Geometrical Problem Solving(PDF). Tehran: Fatemi Publishing Co.ISBN 964-318-114-6. Archived fromthe original(PDF) on 2004-05-03.

• Brummelen, Glen van (2007)."Sijzī: Abū Saʿīd Aḥmad ibn Muḥammad ibn ʿAbd al‐Jalīl al‐Sijzī". In Thomas Hockey; et al. (eds.).The Biographical Encyclopedia of Astronomers. New York: Springer. p. 1059.ISBN 978-0-387-31022-0. (PDF version)
• Dold-Samplonius, Yvonne (2008) [1970-80]."Al-Sijzī Abū Sa'īd Aḥmad Ibn Muḥammad Ibn 'Abd Al-Jalīl".Complete Dictionary of Scientific Biography.Encyclopedia.com.

• Medieval Iranian astrologers
• 10th-century Iranian mathematicians
• 11th-century Iranian mathematicians
• 940s births
• 1020 deaths
• 10th-century Iranian astronomers
• Scholars under the Buyid dynasty
• 10th-century inventors
• 11th-century Iranian astronomers
• Astronomers of the medieval Islamic world

• Articles with short description
• Short description matches Wikidata
• Articles with hCards
• Articles containing Persian-language text
• Year of birth uncertain