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Year 491 (CDXCI) was a common year starting on Tuesday (link will display the full calendar) of the Julian calendar. At the time, it was known as the Year of the Consulship of Olybrius without colleague (or, less frequently, year 1244 Ab urbe condita). The denomination 491 for this year has been used since the early medieval period, when the Anno Domini calendar era became the prevalent method in Europe for naming years.
|Ab urbe condita||1244|
|Balinese saka calendar||412–413|
|Chinese calendar||庚午年 (Metal Horse)|
3187 or 3127
— to —
辛未年 (Metal Goat)
3188 or 3128
|- Vikram Samvat||547–548|
|- Shaka Samvat||412–413|
|- Kali Yuga||3591–3592|
|Iranian calendar||131 BP – 130 BP|
|Islamic calendar||135 BH – 134 BH|
|Minguo calendar||1421 before ROC|
|Seleucid era||802/803 AG|
|Thai solar calendar||1033–1034|
617 or 236 or −536
— to —
618 or 237 or −535
- April 9 – Emperor Zeno, age 66, dies of dysentery (or of epilepsy) after a 17-year reign. He has no sons to succeed him and Anastasius, palace official (silentiarius) and favoured friend of empress Ariadne, is elevated to the throne.
- May 20 – Anastasius I marries Ariadne shortly after his accession. His reign is disturbed by religious distractions and a civil war started by Longinus, brother of late emperor Zeno.
- Anti-Isaurian riots break out in the Hippodrome at Constantinople. Longinus and several other Isaurians, including general Longinus of Cardala, are exiled to Thebaid (Egypt).
- Aelle of Sussex besieges and conquers the fortified town Anderitum in southern Britain. He massacres the population, apparently sub-Roman Brythons (according to the Anglo-Saxon Chronicle).
- July 9 – Odoacer makes a night assault with his Heruli guardsmen, engaging Theodoric the Great in Ad Pinetam. Both sides suffer heavy losses, but in the end Theodoric repulses the attack, forcing Odoacer back into Ravenna.