Basically, I believe you are correct.
Lewis & Short's Latin dictionary draws this distinction, attributing it to a 19th century German treatise on Latin synonyms:
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
However, I doubt whether this is a distinction that was consistently drawn in classical or medieval authors. At a superficial glance, L&S's citations of sempiternus
from Cicero (see link below) don't seem to confirm this distinction completely: cursus stellarum
, ignis Vestae
, nihil umquam nisi sempiternum et divinum animo volutare
; and even Terence: deorum vita sempiterna
It's possible that in classical
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.
Perhaps the similarity of aevum
to the Greek word αἰών, aeon (to which aevum
is apparently etymologically related) may also have played a part, as L&S suggests. (Aeternus
The Oxford Latin Dictionary doesn't attempt to draw a distinction between these two words.Sempiternus
occurs as early as Plautus and Terence (around 200 BCE); the earliest attested use of aeternus
) seems to be in Lucretius and Varro (early 1st century BCE).
Links to Lewis & Short:http://perseus.uchicago.edu/cgi-bin/philologic/getobject.pl?c.17:1505.lewisandshorthttp://perseus.uchicago.edu/cgi-bin/philologic/getobject.pl?c.0:1415.lewisandshort
Lewis & Short is a classical Latin dictionary, with a cut-off date somewhere in late antiquity, but I don't think medieval Latin usage should be different.