Year 940 (CMXL) was a leap year starting on Wednesday (link will display the full calendar) of the Julian calendar.
Events
By place
Europe
The tribe of the Polans begins the construction of the following fortified settlements (Giecz, Bnin, Ląd, Gniezno, Poznań, Grzybowo and Ostrów Lednicki) in Greater Poland. The Piast Dynasty under Duke Siemomysł gains control over other groups of Polans along the Upper Vistula, and establishes their rule around Giecz (approximate date).
Japan
March 25 – Taira no Masakado, the self-proclaimed "New Emperor" (新皇), is subdued by local rivals who revolt against his rule. His forces are defeated by his cousin, Taira no Sadamori, in Shimōsa Province. Masakado's head is brought back to Emperor Suzaku in Tokyo.
By topic
Literature
Saadia Gaon, a Jewish rabbi and philosopher, compiles his Siddur (Jewish prayer book) in Arabic and synagogal poetry in modern-day Iraq (approximate date).
Religion
Narita-san ("New victory temple"), a Shingon Buddhist temple, is founded in Chiba (Japan).
Births
June 10 – Abu al-Wafa' Buzjani, Persian mathematician and astronomer (d. 998)
Abdollah ibn Bukhtishu, Syrian physician and scientist (d. 1058)
Abu-Mahmud Khojandi, Persian astronomer and mathematician (d. 1000)
Abū Sahl al-Qūhī, Persian mathematician and physicist (approximate date)
Adelaide-Blanche of Anjou, French queen and regent (d. 1026)
Al-Baqillani, Muslim theologian and jurist (approximate date)
Baldwin III ("the Young"), Frankish nobleman (approximate date)
Chavundaraya, Indian general, architect and poet (d. 989)