For I story I intend to write, should “Terran Empire” be translated into Latin as “Imperium Terrae” (which is what I’m planning to go with,) “Imperium Terra,” or some other adjectival form of the word?
Assuming your fictional society is translating a pre-existing English adjective “Terran” into Latin, that makes sense. “Terra” is originally a word meaning “(dry) land, earth, ground,” with “Earth” as a secondary meaning. I’d go with Imperium Orbis Terrae or Imperium Orbis Terrarum (orbis, orbis, “circle, orb”) or else Imperium Mundi (mundus, mundi, “world, universe”), myself.
I’d like to add that in this revolutionary map from Nicolaus Copernicus’ book De revolutionibus orbium coelestium, where he explained the heliocentric model, the Earth is not inscribed as Terra, but as Tellus (which is a feminine noun in Latin). Maybe this is due to the connotations of terra mentioned by Dr Drake.
[sub]Noticed the pun on “revolutionary?”[/sub]
Tellus again how clever puns are.
I’ll suggest imperium terrenum for “Terran Empire”, keeping the adjectival form.
Regarding tellus, the word was used in antiquity to refer to the “globe” of Earth in relation to the other heavenly bodies (you have to remember this is under the Ptolemaic system, so it’s not quite the same as a “planet” in modern parlance, but close). E.g. in the Somnium Scipionis (17), Scipio views the eight celestial globes (Stars, Saturn, Jupiter, Mars, Sun, Venus, Mercury, Moon), and completes the picture with Nam ea, quae est media et nona, Tellus, neque movetur et infima est, et in eam feruntur omnia nutu suo pondera. - “For the ninth, which is in the middle, Tellus, is the lowest and never moves, and all weights are drawn to it by their own gravity.” (nutus literally means “will”, but Cicero implies elsewhere it has properties much like what we would call gravitational force). IMO Copernicus was no doubt familiar with this passage, and labeled accordingly.