enduring forever; of never-ending duration; eternal

This word appears in T. S. Eliot’s poem “Little Gidding.” Why he didn’t just use “eternal,” I have no idea. I thought that maybe it had something to do with the number of syllables per line, but other lines before and after the one with “sempiternal” have either nine or eleven syllables, while the “sempiternal” line has ten. Maybe Eliot was just showing off.