Hesperus

In Greek mythology, Hesperus is the Evening Star, the planet Venus in the evening. A son of the dawn goddess Eos, he is the half-brother of her other son, Phosphorus. Hesperus' Roman equivalent is Vesper. By one account, Hesperus' father was Cephalus, a mortal, while Phosphorus' was the star god Astraeus. Other sources, however, state that Hesperus was the brother of Atlas, and thus the son of Iapetus.

Variant names

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 or Phosphorus, 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", "son of Shahar " in the Hebrew version of Isaiah 14:12.
Eosphorus/Hesperus was said to be the father of Ceyx and Daedalion. In some sources, he is also said to be the father of the Hesperides.
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.

"Hesperus is Phosphorus"

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" and "the morning star" 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 could be empirical rather than knowable a priori.