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|Millennium:||1st millennium BC|
|491 BC by topic|
|Gregorian calendar||491 BC|
|Ab urbe condita||263|
|Ancient Egypt era||XXVII dynasty, 35|
|- Pharaoh||Darius I of Persia, 31|
|Ancient Greek era||72nd Olympiad, year 2|
|Balinese saka calendar||N/A|
|Chinese calendar||己酉年 (Earth Rooster)|
2206 or 2146
— to —
庚戌年 (Metal Dog)
2207 or 2147
|Coptic calendar||−774 – −773|
|Ethiopian calendar||−498 – −497|
|- Vikram Samvat||−434 – −433|
|- Shaka Samvat||N/A|
|- Kali Yuga||2610–2611|
|Iranian calendar||1112 BP – 1111 BP|
|Islamic calendar||1146 BH – 1145 BH|
|Minguo calendar||2402 before ROC|
|Thai solar calendar||52–53|
−364 or −745 or −1517
— to —
−363 or −744 or −1516
Year 491 BC was a year of the pre-Julian Roman calendar. At the time, it was known as the Year of the Consulship of Augurinus and Atratinus (or, less frequently, year 263 Ab urbe condita). The denomination 491 BC 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.
- Darius I sends envoys to all Greek cities, demanding "earth and water for vassalage" which Athens and Sparta refuse.
- The Greek city of Aegina, fearing the loss of trade, submits to Persia. The Spartan king, Cleomenes I tries to punish Aegina for its submission to the Persians, but the other Spartan king, Demaratus, thwarts him.
- Cleomenes I engineers the deposing of Spartan co-ruler Demaratus (and his replacement by Cleomenes’ cousin Leotychidas) by bribing the oracle at Delphi to announce that this action was divine will. The two Spartan kings successfully capture the Persian collaborators in Aegina.
- Hippocrates, tyrant of Gela, loses his life in a battle against the Siculi, the native Sicilian people. He is succeeded as Tyrant of Gela by Gelo, who had been his commander of cavalry.
- During a famine in Rome, Gaius Marcius Coriolanus advises that the people should not receive grain unless they would consent to the abolition of the office of tribune. For this, the tribunes have him condemned to exile. Coriolanus then takes refuge with the leader of the Volsci and during the following years they together lead the Volscian army against Rome, turning back only in response to entreaties from his mother and his wife.
- The construction of a relief begins in the Apadana, a ceremonial complex at Persepolis. The relief pictures Darius I and Xerxes I receiving tribute and is now displayed in the Oriental Institute of the University of Chicago.