Phosphorus, from the Greek Φώσφορος means Venus when it is seen in the morning (the morning star). Its name comes from the Greek name for the morning star (from the Greek φως "light" and φόρος, "bearer", phosphorus means "bringer of light").
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.
The terms "Hesperus" and "Phosphorus" and the sentence "Hesperus is Phosphorus." are of interest to language philosophers. The terms were used by Frege to illustrate his idea of sense and reference. 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.