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In Greek mythology, Hesperos (Greek Ἓσπερος (The Evening Star), sometimes Latinized as Hesperus) was a son of Eos, by either Atlas, Astraios or Cephalus. His Roman equivalent was Vesper.

Variant names

Hesperus is the personification of the evening star, the planet Venus. His name is sometimes conflated with the names Eosphorus (Εώσφορος, "bringer of dawn"; Latin Aurora) or Phosphorus (Φώσφορος, "bringer of light", translated as Lux Ferre ( Lucifer ) in Latin) since they are all personifications of the same planet Venus.

When named thus by the ancient Greeks, it was thought that Phosphorus and Hesperus (Venus in the evening) were two different celestial objects. It was the Greek philosopher and mathematician Pythagoras who first realized that Phosphorus and Hesperus were the same object.

Phosphorus was the father of Ceyx, Daedalion and Leukone. He is also said to be the father of the Hesperides.

"Hesperus is Phosphorus"

"Hesperus is Phosphorus" is a famous sentence in the philosophy of language (see, e.g., proper name). Gottlob Frege used the terms "Hesperus" and "Phosphorus" to illustrate his distinction between sense and reference. Saul Kripke used the sentence to demonstrate that the knowledge of something necessary (in this case the identity of Hesperus and Phosphorous) could be discoverable rather than known a priori.