An Etymological Dictionary of Astronomy and Astrophysics

Lower in place or position; closer to the bottom or base.
Astro.: → inferior conjunction;
→ inferior culmination; → inferior planet.

Etymology (EN): From L. inferior “lower,” comp. form of inferus (adj.) “that is below or beneath,” from infra “below” (→ infrared), cognate with Pers. zir, as below.

Etymology (PE): Zirin, adj. from zir “below, down,” Mid.Pers. azêr “below, under,” êr “below, down; low, under,” adar “low;”
Av. aδara- (adj.), aδairi- (prep.) “below;” cf. Skt. ádhara- “lower;” L. infra (adv., prep.) “below, underneath, beneath,” inferus “lower;” O.E. under “under, among”);
PIE base *ndher.