From Academic Kids
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 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.
"Hesperus is Phosphorus"
The terms "Hesperus" and "Phosphorus" and the sentence "Hesperus is Phosphorus." are of interest to language philosophers. The terms were used by Gottlob Frege to illustrate his idea of sense and reference. Saul Kripke used the sentence to demonstrate that the knowledge of something necessary (in this case an identity) is not always an a priori matter, but could (and in some cases, necessarily) be something empirically discoverable.
Modern Wiccan devotées and demonologists make Jerome's Lucifer or "Phosporus" the father of "Aradia" and Ceyx by Diana. Though they refer to a "Roman tradition", no such classical myth existed, and "Aradia" and "Phosporus" first appear in the writings of Charles Leland.