How can a storm cloud in Middle-earth "move behind the sun"?

Question

This is probably the most trivial question I've ever asked here, and relates to something mentioned in passing. The passage occurs in The Two Towers, Book IV, "The Taming of Smeagol". While attempting to climb down from a mountain, Frodo and Sam are caught in a fast-moving storm caused by Sauron. Tolkien writes:

With that [Frodo] stood up and went down to the bottom of the gully again. He looked out. Clear sky was growing in the East once more. The skirts of the storm were lifting, ragged and wet, and the main battle had passed to spread its great wings over the Emyn Muil; upon which the dark thought of Sauron brooded for a while. Thence it turned, smiting the Vale of Anduin with hail and lightning, and casting its shadow upon Minas Tirith with threat of war. Then, lowering in the mountains, and gathering its great spires, it rolled on slowly over Gondor and the skirts of Rohan, until far away the Riders on the plain saw its black towers moving behind the sun, as they rode into the West. But here, over the desert and the reeking marshes the deep blue sky of evening opened once more, and a few pallid stars appeared, like small white holes in the canopy above the crescent moon.

Here are the relative positions of the cloud, Frodo, the Riders, the setting sun, and Mordor, as I understand them (obviously not to scale, but that doesn't matter). Note that the cloud never moves anywhere near the Riders - it is always east of them, and the sun is always west of them.

Unless I don't understand weather in Middle-earth, this doesn't make any sense. How can a storm cloud "move behind the sun"? Every storm cloud I have ever seen has been decidedly closer to me than to the sun. Storm clouds are in front of the sun, never behind it. What is Tolkien trying to describe here?

Answer 1

My reading of that line requires you to imagine the perspective of the Rohirrim. They're in the west, watching a huge, unnatural storm front move in from the east.

Assuming that it's around mid-day (or at least not first thing in the morning), the sun will have risen into the sky (from the east). But the storm front, like a wall of cloud, is now moving in from the east, seemingly chasing it across the sky.

Notice how many words in that passage are about movement. That whole paragraph is about watching the storm move, how it moves, where it moves. And, most importantly in the sentence you bolded, how it moves relative to the riders. They are riding west, but the storm clouds are closing in on them.

Until the cloud overtakes and blocks out the sun, it is "behind" the sun.

So essentially, the section that reads:

...until far away the Riders on the plain saw its black towers moving behind the sun, as they rode into the West.

could be "translated" as:

...until far away the Riders on the plain saw the black clouds swiftly overtaking the sun, as they rode into the West.

A famous example

You know the famous scene in Independence Day (and plenty of other scifi stories) where the giant spaceship slowly moves its shadow across the city?

That's what is being described here. The cloud front is "behind" the sun, moving quickly across the sky, darkening everything in its path, until it finally overtakes the sun and the world (or the viewer at least) is plunged into darkness.

Update with your sketch

Using the sketch you added to your question, I made my own notations to illustrate my interpretation.

By this moment in time, the Sun has already risen in the east and moved west across most of the sky. The clouds also started in the east and are moving west, but having only recently started, they haven't moved as far across the sky yet.

From the perspective of the Riders, the clouds seem to be chasing the sun across the sky. And because they haven't gotten as far, they seem to be "behind the sun" in the sense of one racer being "behind" another.

The clouds are chasing the sun, they're overtaking the sun. When they reach it, blot it out, and keep going, they will have passed the sun, but until they reach it, they are behind the sun.

Answer 2

As far as I can tell, Tolkien simply means "turning the sky dark by covering the sun, as if the clouds had come from behind the sun itself".

He uses the term in a variety of contexts throughout his writings but always to mean, essentially, that the sun appears to be in front of the sky:

"A ship then new they built for him of mithril and of elven-glass with shining prow; no shaven oar nor sail she bore on silver mast: the Silmaril as lantern light and banner bright with living flame to gleam thereon by Elbereth herself was set, who thither came and wings immortal made for him, and laid on him undying doom, to sail the shoreless skies and come behind the Sun and light of Moon." - FotR (Chapter 1)

and

Of Winter marching blue behind the sun
Of bright All-Hallows. Then their hour was done,
And wanly borne on wings of amber pale
They beat the wide airs of the fading vale,
And flew like birds across the misty meres. - The Book of Lost Tales

and

Yet many a time and oft a tiny star-ship of Varda [e.g. a shooting star] that has dipped into the Outer Seas, as often they will, is sucked through that Door of Night behind the Sun; and some track her galleon through the starless vast back unto the Eastern Wall, and some are lost for ever, and some glimmer beyond the Door until the Sunship issues forth again. Then do these leap back and rush up into the sky again, or flee across its spaces; and this is a very beautiful thing to see — the Fountains of the Stars. - The Book of Lost Tales

Answer 3

Think about it as if the sun (source of light) and the storm (source of darkness) are two concurrents. The one who wins goes in front of the other.

The author tries to make you imagine that storm emits darkness as the sun does for light.

(This is how I understand it, since it is obvious that logically the sun is behind everything but the stars.)

Answer 4

Note that the cloud never moves anywhere near the Riders - it is always east of them, and the sun is always west of them.

No, because:

it rolled on slowly over Gondor and the skirts of Rohan, until far away the Riders on the plain saw…

It's moving west, and eventually has moved so far west that it is indeed west of the Riders.

It might be moving "slowly" over Gondor, but "slowly" in the context of covering a large kingdom is in fact moving with great speed. (Consider that visible satellites can appear to an earthly observer to be moving slowly when they are travelling at tremendous speeds. The moon's 3,700 km/h barely seems to move at all when you look at it).

Of course, the other problem is that to move "behind the sun" is impossible for something that is terrestrially bound, but if we consider the sun to be a vessel to hold the radiance of the last fruit of Laurelin, guided in its path by the Maia Arien, that becomes possible too.

Answer 5

Having had a look at the quote and your drawing, it seems clear to me that Tolkien meant that, as the Sun moved from East to west, the storm clouds were moving in that direction to, being behind the Sun in its westward track.

Answer 6

I think that Tolkien's words have less to do with the laws of physics and more to do, as @Rand al'Thor states, that the sun is the source of light and the storm is the source of darkness (or a part thereof).

The storm going behind the sun (power of good) indicates that the sun has a position of prominence, and thus this is more of an allegorical use of elements of the physical world. Tolkien gives it another twist by making it an actual storm, likely created by Sauron.

This is similar to Tolkien's use of light and dark. The elements and characters of the dark cannot coexist with light and its penetrating revelation of the truth.