sempiternus denotes what is perpetual, what exists as long as time endures, and keeps even pace with it; aeternus, the eternal, that which is raised above all time, and can be measured only by aeons (αἰῶνες, indefinite periods . . .[T]he sublime thought, without beginning and end, is more vividly suggested by aeternus than by sempiternus, since the former has more direct reference to the long duration of the eternal, which has neither beginning nor end. Sempiternus is rather a mathematical, aeternus a metaphysical, designation of eternity
Qimmik wrote:It's possible that in classical Latin, aeternus acquired a more poetic, "sublime" flavor because sempiternus is a word that has a metrical shape (long short long, a "cretic") that can't fit in hexameter verse, and therefore it couldn't be used by poets such as Lucretius, Vergil, Ovid, etc. This may account, at least partly, for the distinction Lewis and Short purport to draw.
spa38 wrote:Latin meaning has been preserved in spanish.
Sempiternam means having no end, i.e. something that has a beginning, but will last forever.
Aeternam means having neither beginning nor end.