837

By place

Byzantine Empire

• Byzantine–Arab War: Emperor Theophilos leads a Byzantine expeditionary force into Mesopotamia. He sacks the cities Arsamosata and Sozopetra—which some sources claim as the birthplace of Abbasid caliph Al-Mu'tasim—and forces Melitene to pay tribute.
• The Slavs in the vicinity of Thessaloniki revolt against the Byzantine Empire. Theophilos undertakes an evacuation of some Byzantine captives, who are settled in trans-Danubian Bulgaria.

  Europe

• Presian I, ruler of the Bulgarian Empire, sends his prime-minister Isbul against the Smolyani. The Bulgarian army campaigns along the Aegean coasts, and conquers most of Thrace and Macedonia, including the fortress city of Philippi.
• The city of Naples is attacked by Saracens from Egypt demanding an annual payment.

  Britain

• King Drest IX dies after a 3-year reign. He is succeeded by his cousin Eóganan, as ruler of the Picts.

  By topic

Astronomy

• April 10 - Comet Halley passes approximately 5 million km from Earth, its closest ever approach.