I have a bad habit, bad and lifelong, of not bothering to look up unknown words encountered when I'm reading. I make a guess based on the context, or perhaps just ignore the word altogether, figuring or at least hoping that it isn't that important, and press on. I have done that for years with "sempiternal." I suppose I've read Eliot's Four Quartets ten or twelve times, yet have always pressed on past the first two lines of "Little Gidding" very well aware that I did not know what one important word meant, experiencing it as a definite bump in the road, but still not bothering to look it up:

Midwinter spring is its own season
Sempiternal though sodden towards sundown....

I had supposed that it meant something involving the ideas both of surprise and transience. That's what spring in midwinter would in fact be, isn't it? But no; it comes close to being a synonym for "eternal," as in the presumably eternal things encountered by Dante in Paradise. The word occurs in Longfellow's translation: 

In such wise of those sempiternal roses
    The garlands twain encompassed us about
    And thus the outer to the inner answered.

--Paradiso, Canto XII

If I had come across that first line in isolation, it might not, to my ear, have instantly fallen into the iambic pentameter in which the translation is written. Some, sometimes much, of our perception of meter depends on our feeling a rhythm which has been clearly established and which we naturally expect to continue. Without that expectation, I might initially have wanted to accent "In" and "wise" and then not known what to do with the rest of the line, because I wouldn't have known how to pronounce "sempiternal."

In reading "Little Gidding" I had always supposed the stress to be on the second syllable: "sempiternal." The preceding line does not have a regular rhythm and so provides no guidance as to how the word should be accented (for those of us who did not already know). But Longfellow's meter is pretty consistent, and he surely means for us to read the line "In such wise of those sempiternal roses."

So what does it mean? Its use by Longfellow finally drove me to my big old Webster's New Twentieth Century Dictionary (Unabridged), which says it's "eternal in futurity...having beginning, but no end." The entry also includes as a second definition a simple synonym for "eternal." Various online dictionaries seem to treat it that way, too. The Latin etymology, semper and eternus, suggests "always eternal," which would seem to be redundant, somewhat along the same lines as "very unique," though not similarly illogical. 

Longfellow must have liked the word, as it occurs four times in his translation, all in the Paradiso. And in every case it seems to mean simply "eternal." That's how Anthony Esolen translates it. The lines above, for instance:

So the two garlands circled round us now,
wreathed of the roses of eternity,
the outer answering the inner bow.

("bow" is there because the garlands have been compared to a double rainbow, and because Esolen rhymes where he can, and it's a slant rhyme with "now.")

The context is one of Dante's very elaborate descriptions, in this case two circles of lights which are souls of the blessed. Neither in this nor in two of the the other occurrences of "sempiternal" is there a suggestion of something that has a beginning but no end. The one possible exception is only possible, maybe even a stretch: it's a reference to the "sempiternal justice" of heaven, which could be read as referring to God's justice toward mankind, which has a beginning insofar as mankind does. As I say, a stretch.

So why did Longfellow use it, if in Dante it means simply "eternal"? Well, one obvious possibility is that it suited his metrical need at those points. I can't blame him for that. But after reaching that conclusion it occurred to me (at last) to look at the Italian, even though I don't know Italian--I thought there might be a visual clue. And there it is, line 19:

...cosi di quelle sempiterne rose...

Google Translate renders that as "so of those everlasting roses." So that explains that. The entire stanza is:

...così di quelle sempiterne rose

volgiensi circa noi le due ghirlande
e sì l'estrema a l'intima rispuose.

 

...so the two wreaths of those eternal roses
circled all around us and, thus reflected,

and so the outermost to the innermost answered.

The second Triad, which is germinating
    In such wise in this sempiternal spring...
("in questa primavera sempiterna")

--Paradiso, Canto XVIII

This is another and even more elaborate description, which I will not try to summarize except to say that it also describes the glittering hierarchies of heaven. What caught my attention was not just the recurrence of "sempiternal" but "sempiternal spring." Surely, I thought, there must be a connection to Eliot here. He knew, loved and was deeply influenced by Dante--had he read Longfellow's translation, and was this an intentional allusion? That would be interesting, and mildly pleasing to me if it turned out that Eliot shared my good opinion of Longfellow's work.

Dante is mentioned frequently in Eliot's prose writings, but the only place I knew of where I would be sure to find him is the essay "What Dante Means To Me." And I can find no suggestion there that he had read Longfellow's translation. If there is any suggestion at all, it goes in the other direction, as Eliot mentions getting to know Dante by reading the Italian with a prose translation at hand. 

But it's not at all far-fetched to suppose that Dante's "sempiterne" influenced Eliot's choice of "sempiternal," given his great knowledge of Dante and the fact that "Little Gidding" includes what Eliot says is his only conscious attempt to imitate Dante (in section II).

So my almost-exciting discovery turned out to be no discovery at all. At least I now know what "sempiternal" means, and the glitch in the opening of "Little Gidding" is removed. But I still am not sure what Eliot meant by it, given that it cannot be literally true. That midwinter spring seems eternal, or a breath of the eternal, as you experience it, perhaps? Or that it will always recur?  Much of the poem deals with the intersection of time and eternity, so this is a description of just such an intersection, an eternal spring which now and then intrudes upon our time, in a wintry moment when perhaps we need it. 

If you read this far, thank you for your patience. I only meant to write a few paragraphs describing the failed attempt to connect Longfellow and Eliot. This all arose from my reading of Longfellow's translation, which I discussed a few weeks ago.