In Greek mythology, Hesperus (//; Ancient Greek: Ἕσπερος, romanized: Hésperos) is the Evening Star, the planet Venus in the evening. He is one of the Astra Planeta. A son of the dawn goddess Eos (Roman Aurora), he is the half-brother of her other son, Phosphorus (also called Eosphorus; the "Morning Star"). Hesperus' Roman equivalent is Vesper (cf. "evening", "supper", "evening star", "west"). By one account, Hesperus' father was Cephalus, a mortal, while Phosphorus was the star god Astraios. Other sources, however, state that Hesperus was the brother of Atlas, and thus the son of Iapetus.
Hesperus is the personification of the "evening star", the planet Venus in the evening. His name is sometimes conflated with the names for his brother, the personification of the planet as the "morning star" Eosphorus (Greek Ἐωσφόρος, "bearer of dawn") or Phosphorus (Ancient Greek: Φωσφόρος, "bearer of light", often translated as "Lucifer" in Latin), since they are all personifications of the same planet Venus. "Heosphoros" in the Greek Septuagint and "Lucifer" in Jerome's Latin Vulgate were used to translate the Hebrew "Helel" (Venus as the brilliant, bright or shining one), "son of Shahar (Dawn)" in the Hebrew version of Isaiah 14:12.
Maurus Servius Honoratus, in his commentaries on Virgil's Eclogues, mentions that Hesperus inhabited Mount Oeta in Thessaly and that there he had loved the young Hymenaeus, son of Dionysus and Ariadne. Servius makes no distinction between the Evening Star and the Morning Star, calling them both Hesperus and the lucifer of Ida.
In the philosophy of language, "Hesperus is Phosphorus" is a famous sentence in relation to the semantics of proper names. Gottlob Frege used the terms "the evening star" (der Abendstern) and "the morning star" (der Morgenstern) to illustrate his distinction between sense and reference, and subsequent philosophers changed the example to "Hesperus is Phosphorus" so that it utilized proper names. Saul Kripke used the sentence to posit that the knowledge of something necessary (in this case the identity of Hesperus and Phosphorus) could be empirical rather than knowable a priori.