The Latin dictionary (on-line Lewis & Short) glosses sempiternus as existing day-by-day throughout all of time, whereas aeternus transcends time, or exists age-by-age.


Hmm. Both words are used in the Mass. It might be instructive to see in what context they are used. I know we have requiem aeternum in the Requiem Mass. I can't think of any specific instances of sempiternus but I'm sure it won't take too much research.