Case question regarding some text from Brahms' "Ein deutsches Requiem"

Question

I am a choral singer who is slowly trying to learn German. I have a question about why the dative case is being used in some text from Johannes Brahms' Ein deutsches Requiem.

In particular, there is a movement with the following text [1]:

Siehe, meine Tage sind einer Handbreit vor dir...

This comes from Psalm 39 and the phrase, as I understand it, would be roughly translated as:

Behold, my days are as a handbreadth before Thee...

I understand that this is a historical work (composed between 1865 and 1868), but I am confused why eine Handbreit here is declined in the dative case (at least that's what I assume the einer is marking). I understand that vor in this situation takes the dative case, which is why before Thee is translated as vor dir, but why is Handbreit also in dative? It seems like the nominative case would make more sense here, since we are using sein to say My days are...

At any rate, I assume I'm missing something, be it how vor actually works, or this is some historical artifact.

Vielen Dank!

[1] http://www.classical-music.com/article/brahmss-german-requiem-text (Full text, see movement 3 for the passage in question.)

Answer 1

The word handbreit is composed from two nouns originally: it comes from die Breite einer Hand or, with different word order as it was quite usual in that times: einer Hand Breite

From there the composit word handbreit evolved.

That explains why in your sentence the article is written in genitive (not dative) clause:

Siehe, meine Tage sind einer Hand Breit(e) vor dir...

Or (in more modern word order):

Siehe, meine Tage sind (um) die Breite einer Hand vor dir...

You can read more about this definition in the dictionary of the Brothers Grimm. There you even find another example from the bible with the same genitive clause emphasis of mine)

HANDBREITE, f. die breite einer hand: und eine leisten umb her, einer handbreit hoch. 2 Mos. 25, 25; ein rise .. sechs ellen und einer handbreit hoch.

Answer 2

Luther's translation of Psalm 39 reads:

Sihe / meine Tage sind einer hand breit bey dir¹

The adjective breit is the subject complement of ist, i.e.

Meine Tage sind breit.

In order to specify how broad, the genitive einer hand is added to the adjective breit. In New High German, such measurements are in the accusative.

einen Meter lang, breit, hoch
einen Kilometer weit
einen Monat lang

In Luther's time, the genitive and the accusative were in competition with each other. Luther also has the accusative.

Vier ecket sol es sein vnd zwifach / eine hand breit sol seine lenge sein²

In the end, the accusative won out. See also Paul's Deutsche Grammatik, III, § 244.

Some further examples with a genitive and an adjective from Luther's bible (all taken from the DWB):

Sollen denn die Weiber jres Leibs frucht essen / die jüngsten Kindlin einer spannen lang?³

Ein Fenster soltu dran machen oben an / einer ellen gros.⁴

Vnd gab jm kein Erbteil drinnen /auch nicht eines fusses breit⁵

Vnd es giengen Leisten herumb / hinein werds gebogen / einer Quehrhand hoch⁶