An Etymological Dictionary of Astronomy and Astrophysics

Being in accordance with the actual state or conditions; e.g. → true anomaly; → true equinox; → true horizon; → true north; → true Sun.

Etymology (EN): M.E. trewe (adj. and adv.); O.E. triewe, treowe “trustworthy” (cf. O.Fris. triuwi, Du. getrouw, O.H.G. gatriuwu, Ger. treu, O.N. tryggr, Goth. triggws “faithful, trusty”).

Etymology (PE): Râstin, from râst “right, true; just, upright, straight;” Mid.Pers. râst “true, straight, direct;” Soghdian rəšt “right;” O.Pers. rāsta- “straight, true,” rās- “to be right, straight, true;” Av. rāz- “to direct, put in line, set,” razan- “order;”
cf. Skt. raj- “to direct, stretch,” rjuyant- “walking straight;” Gk. orektos “stretched out;” L. regere “to lead straight, guide, rule,” p.p. rectus “right, straight;” M.E.; O.E. reht, riht; cf. O.H.G. reht, Ger. recht, O.N. rettr, Goth. raihts; PIE base *reg- “move in a straight line,” hence, “to direct, rule.”