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Year 1042 (MXLII) was a common year starting on Friday (link will display the full calendar) of the Julian calendar.
- April 18 or April 19 – Emperor Michael V of the Byzantine Empire attempts to remain sole emperor, by sending his adoptive mother and co-ruler Zoe of Byzantium to a monastery.
- April 19 – Michael V of the Byzantine Empire is deposed by popular revolt. Zoe, Empress of the Byzantine Empire with co-rulers since 1028, becomes reigning empress with her sister Theodora.
- June 8 – Edward the Confessor becomes King of England.
- June 11 – Empress Zoe marries for the third time, and elevates her husband to the throne as Constantine IX.
- George Maniaces, Byzantine general in Sicily, revolts against Constantine IX.
- Harald III of Norway, at this time leader of the Varangian Guard in the Byzantine Empire, returns to Norway, possibly because of his involvement in Maniaces' revolt.
- Magnus I of Norway becomes King of Denmark.
- The Normans in Italy move their capital to Melfi.
- Abad II succeeds his father to the throne of Sevilla.
- Incursions by the Western Xia, and the threat of further Liao Dynasty military actions against the Song Dynasty, force an increase in tribute payments to the Liao.
- The Almoravids, led by Abdullah Ibn Yasin, invade Morocco.
- Byzantine-Arab Wars: The Byzantines conquer Edessa, returning it to Christian hands, after 400 years of Islamic rule.
- Vladimir of Novgorod wages a campaign against the Tavastians (yem).