Jupiter is the King of the Gods. The eagle is his messenger. His weapon is the Thunderbolt (thunder and lightning). All other gods fear him, and Jupiter himself is said to fear only his wife Juno. Along with Neptune and Pluto, he is a son of Saturn, whom they imprisoned before dividing the world between themselves: Jupiter took the air, Neptune the sea, and Pluto ruled the earth (the Underworld).
Jupiter means Father Jove (Father in Latin is "pater"). The largest of Roma's temples is dedicated to Jupiter Optimus Maximus ('Jupiter Best and Greatest'), and the Romans believe he guards their cities and looks after their civilization from harm.
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