Sauron was a Maia, and in his early career this meant that he had the ability to change shape at will: in the First Age, he seems to have had a generally human form (he's variously described as smiling, laughing, standing and so on), though in his battle with Huan he was able to shift freely between the forms of a wolf, a serpent and a flying vampire. In the Second Age, during the time when the Rings of Power were made, he took on the semblance of a noble teacher. However, his shape-changing abilities were not limitless, and after the Downfall of Númenor they were severely curtailed:
'...he was robbed now of that shape in which he had wrought so great an evil, so that he could never again appear fair to the eyes of Men...' (1)
After his defeat in the War of the Last Alliance, things became even worse for him, and he lost his physical form altogether:
'...he forsook his body, and his spirit fled far away and hid in waste places; and he took no visible shape again for many long years.' (2)
This was in the last year of the Second Age. The story of The Lord of the Rings is set more than three thousand years later, in the closing years of the Third Age, but that book includes only the slightest hints of a description of Sauron. No doubt Tolkien kept Sauron in the shadows for good dramatic reasons, leaving it to the reader's imagination to create something darker and more fearsome than a mere description could convey. This literary device has left behind something of a puzzle: what actually did Sauron look like? For that matter, did he have any kind of physical body at all?
The question is made more complicated by the recurrence of the image of the Red Eye, which often appears to signify Sauron. In some places, this is clearly used as a symbol, but in others it actually seems to be - at least partially - a physical description. This is the Eye that Frodo saw in the Mirror of Galadriel:
'The Eye was rimmed with fire, but was itself glazed, yellow as a cat's, watchful and intent, and the black slit of its pupil opened on a pit, a window into nothing.' (3)
This has led more than a few readers to interpret the Eye absolutely literally, to the point where Sauron is imagined simply as a burning eyeball. The most notable of these readers, of course, is Peter Jackson, who used the motif of the glowing eye throughout his trilogy of movies. Indeed, at one point in the movie The Fellowship of the Ring, Saruman explicitly states of Sauron that 'he cannot yet take physical form' (though nothing comparable to this appears in the book).
This isn't an unreasonable position to take. There are numerous references to the Red Eye as a symbol for Sauron, and in places it's difficult to tell where the symbol ends and the physical description begins. For example:
'The Red Eye will be looking towards Isengard.' (4)
On the other hand, there are several references to 'the red eye' that clearly have nothing to do with Sauron at all. There's an example of this in the following quote, where the 'red eye' is in the Tower of Cirith Ungol:
'Now the orc-tower was right above him, frowning black, and in it the red eye glowed.' (6)
Here, the idea of the red eye embodies the unsleeping watchfulness of Mordor, and has no direct relation to Sauron. In fact, there are reasons to think that the capitalised 'Red Eye' is simply an extension of this metaphor. For example, speaking of Sauron's fingers, Gollum - who has apparently seen the Dark Lord with his own eyes - says:
'He has only four on the Black Hand, but they are enough.' (5)
It follows that if Sauron has a Black Hand, he cannot merely be a Red Eye. There seems to be at least one metaphor in use here. Actually, there is quite a weight of evidence within The Lord of the Rings that Sauron had some kind of physical form:
'...if the Nameless One himself should come, not even he could enter here while we yet live.' (7)
'And the prisoner is to be kept safe and intact ... until He [Sauron] sends or comes Himself.' (6)
'He [Sauron] will not come save only to triumph over me when all is won.' (7)
The characters who say these lines (including his own Orcs) all seem to believe that Sauron could easily travel from one place to another. In particular, Aragorn seems to believe that Sauron not only has a physical form, but that it would be possible to do harm to that form:
'Let the Lord of the Black Land come forth! Justice shall be done upon him.' (8)
Curiously, this line was used word-for-word in the movie version of The Return of the King, despite the fact that Sauron was depicted there as an apparently immobile glowing eye.
If we move to Tolkien's letters, it becomes hard to avoid the conclusion that Sauron did, indeed, have a physical shape. For instance, there's a reference there to:
'...the year 1000 of the Third Age, when the shadow of Sauron began first to grow again to new shape.' (9)
The War of the Ring started in the year 3018 of the Third Age - it's hard to imagine that after more than two thousand years, Sauron still hadn't formed a new shape for himself. There's another reference among Tolkien's letters that goes even further, to the extent that it effectively settles the discussion:
'Sauron should be thought of as very terrible. The form that he took was that of a man of more than human stature, but not gigantic.' (10)
This is from a letter specifically discussing the events at the end of The Lord of the Rings, so there's really no question that it describes Sauron as Tolkien saw him then. In fact, it seems that the huge armoured Sauron that appears at the beginning of the movie The Fellowship of the Ring probably fits Tolkien's vision rather better than the image of a glowing eye.
Sources
1 The Silmarillion: Akallabêth
2 The Silmarillion: Of the Rings of Power and the Third Age
3 The Fellowship of the Ring II 7, The Mirror of Galadriel
4 The Two Towers III 9, Flotsam & Jetsam
5 The Two Towers IV 3, The Black Gate is Closed
6 The Two Towers IV 10, The Choices of Master Samwise
7 The Return of the King V 4, The Siege of Gondor
8 The Return of the King V 10, The Black Gate Opens
9 The Letters of J.R.R. Tolkien, No 144, dated 1954
10 The Letters of J.R.R. Tolkien, No 246, dated 1963
From the encyclopedia of Arda. Had to edit to give appropriate credit.
