Final answer:
The Romans called Poseidon Neptune because Neptune was the Roman god of the sea, similar to Poseidon in Greek mythology.
Step-by-step explanation:
The Romans called Poseidon Neptune because in Roman mythology, Neptune was the god of the sea, just like Poseidon was in Greek mythology. When the Romans adopted Greek mythology, they often changed the names of the gods to fit their own language and culture. So, Poseidon became Neptune in Roman mythology. The naming of planets and moons after gods and heroes from Greek and Roman mythology is a tradition that has been followed by astronomers throughout history.
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