An Etymological Dictionary of Astronomy and Astrophysics

1. A portion of a circle cut off by an arc and two radii at 45°, one-eighth of the area of a circle.
   

2. A instrument incorporating a telescope, the predecessor of the → sextant, used to measure angular distances, mainly in nautical astronomy. It spanned an angular range of 45°.

Etymology (EN): From L.L. octans “eighth part of a circle,” from → octa-, oct- + -ans, as in quadrans; → quadrant.

Etymology (PE): Haštakân, from haštak “one-eigth,” from hašt “eight” (Mid.Pers. hašt, O.Pers.*aštahva- “eighth;” Av. ašta; cf. Skt. astā; Ossetic ast; (Buddhist) Sogdian ‘št; Gk. okto, L. octo (Fr. huit; Sp. ocho);
P.Gmc. *akhto(u) (O.E. eahta, æhta, E. eight, O.N. atta, Ger. acht, Goth. ahtau); PIE base *oktô(u)

• -ak, contraction of yak “one,” (Mid.Pers. êwak (Proto-Iranian *aiua-ka-); O.Pers. aiva- “one, alone;” Av. aēuua- “one, alone” (cf. Skt. éka- “one, alone, single;” Gk. oios “alone, lonely;” L. unus “one;” E. one) + -ân nuance suffix.