The line is from Gottlob Frege, I gather, though I just now came across it while reading David Chalmers. It’s new to me, though my brief career in academic philosophy centered on Wittgenstein, an admirer of Herr Frege.
Hesperus and Phosphorus are the latinized forms of the Greek personifications of the Evening and Morning Stars, respectively. (Frege’s point was roughly that “Hesperus is Phosphorus” might convey information—the Morning and Evening Stars are in fact the same planet—or might be tautological—Venus is Venus.)
Hesperus is the Roman Vesper, evening (and so vespers). Phosphorus is “bearer of light” (a form of the chemical element phosphorus glows as it reacts with oxygen). The Roman Phosphorus is Lucifer. (And -fer, “bearer” or “bringer”, also shows up in aquifer, conifer, Christopher (St Christopher carried a disguised Christ across a river), and so on.)
And here’s Isaiah 14:12:
When the Lord has given you rest from your pain and turmoil and the hard service with which you were made to serve, you will take up this taunt against the king of Babylon: How the oppressor has ceased! How his insolence has ceased! … How you are fallen from heaven, O Day Star, son of Dawn! How you are cut down to the ground, you who laid the nations low! You said in your heart, “I will ascend to heaven; I will raise my throne above the stars of God; I will sit on the mount of assembly on the heights of Zaphon; I will ascend to the tops of the clouds, I will make myself like the Most High.” But you are brought down to Sheol, to the depths of the Pit. Those who see you will stare at you, and ponder over you: “Is this the man who made the earth tremble, who shook kingdoms, who made the world like a desert and overthrew its cities, who would not let his prisoners go home?”
Isaiah is talking about a Babylonian king, but the passage was later applied to Satan. I like the image of Lucifer, the Day Star, appearing as the brightest object in the pre-dawn sky, only to be eclipsed by the overwhelming brilliance of the Sun God.