Why did the Romans perceive darkness, tenebrae, as a plural count noun? [Perseus corpus-search reference]
Or perhaps the better question is: what special nuance is conveyed by the plural tenebrae which would be lost were the singular tenebra employed in its stead? The situation somehow feels different than the distinction between using singular umbra for shadow versus plural umbrae for shadows. But I could well be wrong in that.
Lewis and Short mention that using tenebrae for darkness is “stronger than obscuritas”. Is the plural use somehow more abstract or figurative?
I ask because this semantically distinctive plural use for darkness has not merely survived in rare obscurity but indeed thriven in Latin’s daughters, where this special plural sense is still alive and well. This to me suggests that for the Romans tenebrae carried a shade of meaning powerful enough to withstand millennia of diachronic evolution essentially unchanged.
For example, in French, ténèbres is a plurale tantum having no singular at all. In only slight contrast to that situation, in both Portuguese and Spanish the singular does coëxist beside the plural, but only as an occasional straggler. The plural is much more common in both languages; it somehow seems to carry a heavier weight to my mind as a Spanish speaker.
The Spanish have always been conscious of the Latin plural’s sense: in the Quijote, Cervantes famously used the motto Post tenebras spero lucem in his title pages
Portuguese has trevas, where their dictionary notes that it’s more used in the plural. Brazilian Portuguese sometimes uses the older and less altered form tênebras in the same special way.
This same thing also happens in the Spanish descendant of tenebrae, which is tinieblas, famously occurring in the title of Pedro Almodóvar’s ground-breaking Spanish-language film, Entre Tinieblas.
Maybe the Roman Catholic Church's retention of the Latin sense for the proper noun Tenebrae (see footnote) is part a reason for the persistence of the plural, but it doesn't at all explain the origin for the stronger, abstract sense which we see in Classical Latin.
But what does?
The OED defines Tenebrae as:
The name given to the office of matins and lauds of the following day, usually sung in the afternoon or evening of Wednesday, Thursday, and Friday in Holy Week, at which the candles lighted at the beginning of the service are extinguished one by one after each psalm, in memory of the darkness at the time of the crucifixion. Also attributive.