Lucifer = Planet Venus, not Satan

It is not only Jerusalem and Judah that are warned in the Book of Isaiah concerning the wrath of God. The surrounding heathen nations are also warned of doom, and first in line is Babylon.

It is easy to suspect that chapters 13 and 14, in which the doom of Babylon is foretold with savage imagery, is not really Isaianic. In Isiah's time, it was Assyria that was the conquering nation and Babylon lay under its thumb in more devastating fashion than Judah did. The paean of hatred and scorn should, it would be expected, be turned against Assyria and the new capital that Sennacherib had built at Nineveh.

On the other hand, a century after Isaiah's time, it was Babylon under Nebuchadnezzar that was the oppressor. It is reasonably likely, then, that this passage is of later origin and was possibly composed during the Exile at a time when Babylon seemed doomed to fall before the conquering armies of Cyrus the Persian.

Picturing Babylon as already fallen, the writer recites a taunting poem of sarcastic contempt for the mighty Babylonian monarch now brought low. In part, it goes:

Isaiah 14:12. How art thou fallen from heaven, O Lucifer, son of the morning! ...
Isaiah 14:13. For thou hast said in thine heart, I will ascend into heaven ...
Isaiah 14:14. ... I will be like the most High.
Isaiah 14:15. Yet though shalt be brought down to hell ...

The Hebrew word here translated as "Lucifer" is helel. Literally, it means "The Shining One," and is thought to refer to the planetary body we call Venus.

Venus is the brightest of the planets in our sky and, next to the sun and the moon, the brightest object in the heavens. Because of the position of its orbit between the earth's orbit and the sun, it is always seen (from earth) to be fairly close to the sun. When it is in that part of it's orbit that puts it to the east of the sun, it shines out most clearly after sunset, and sets never more than three hours afterward. It is then visible only in the evening and is called the evening star.

On the other side of its orbit, when Venus is to the west of the sun, the planet rises first and for a short period of time (never more than three hours), it shines in the eastern sky as dawn gradually breaks. It is then the morning star.

It is only natural that cultures unlearned in astronomy and not particularly observant of the heavens would consider the evening star and the morning star to be two separate bodies. In Isaiah's time, even the clever Greeks were of this opinion. It was not until two centuries after Isaiah's time that the Greek philosopher Pythagoras discovered the two to be the single body that the Greeks then came to call Aphrodite and the Romans (and ourselves) Venus. It is very likely that Pythagoras discovered this in the course of his travels in the East (tradition says he visited Babylonia and it was the Babylonians who were the great astronomers of ancient times).

Venus, in its morning star aspect, could be called the "daystar" for its rising heralds the coming of day. It is also the "son of the morning" for it is only as morning approaches that it is possible to see it. Thus, the Revised Standard Version translates verse 14:12 as "How art thou fallen from heaven, O day-star, son of the morning."

The Greeks, in the period when they thought Venus to be two bodies, called the evening star "Hesperos" and the morning star "Phosphoros." Hesperos means "west" and it is always in the west that the evening star appears. Phosphoros means "light-bringer" and it is therefore the essential equivalent of "daystar." By the Romans, the Greek terms were translated directly into Latin. The evening star became "Vesper" ("west") and the morning star became "Lucifer" ("lightbringer").

The Hebrew helel is therefore translated as Phosphoros in Greek versions of the Bible; and as Lucifer in Latin versions.

The use of the term "Lucifer" in connection with the overweening pride of the Babylonian king is an ironic thrust at the habit of applying fulsome metaphors for royalty. Flattering courtiers would think nothing of naming their king the Morning Star, as though to imply that the sight of him was as welcome as that of the morning star heralding the dawn after a long, cold winter's night. This habit of flattery is confined neither to the East nor to ancient times. Louis XIV of France, two and a half centuries ago, was well known as the Sun King.

The writer of the verses concerning Lucifer ironically described his fall from absolute power to captivity and death as the fall of the morning star from the heavens to Hell.

With time, however, these verses came to gain a more esoteric meaning. By New Testament times, the Jews had developed, in full detail, the legend that Satan had been the leader of the "fallen angels." These were angels who rebelled against God by refusing to bow down before Adam when that first man was created, using as their argument that they were made of light and man only of clay. Satan, the leader of the rebels, thought, in his pride, to supplant God. The rebelling angels were, however, hurled out of Heaven and into Hell. By the time this legend was developed the Jews had come under Greek influence and they may have perhaps been swayed by Greek myths concerning the attempts by the Titans, and later the Giants, to defeat Zeus and assume mastery of the universe. Both Titans and Giants were defeated and imprisoned underground.

But whether Greek-inspired or not, the legend came to be firmly fixed in Jewish consciousness. Jesus refers to it at one point in the Gospel of St. Luke:

Luke 10.18. And he [Jesus] said ... I beheld Satan as lightning fall from Heaven.

It seemed natural to associate the legend with the Isaianic statement; indeed, that statement about Lucifer may have even helped give rise to the legend. In any case, the early Church fathers considered Isaiah's statement to be a reference to the eviction of the devil from Heaven, and supposed Lucifer to be the angelic name of the creature who, after his fall, became known as Satan. It is from this line of argument that our common simile "proud as Lucifer" arose.

—Isaac Asimov's Guide to the Bible, pp538-540

Sorry for the long quote, but a fascinating academic take by a prolific author. (Yeah, Asimov wrote a LOT more than the sci-fi he's famous for...)