An Etymological Dictionary of Astronomy and Astrophysics

An instrument used with a telescope for accurately measuring small angular separations between two celestial bodies (as between binary
stars). The instrument consists of two parallel fine wires with one wire being fixed and the other movable by means of a finely threaded screw.

Etymology (EN): Filar, from L. fil(um) “a thread” (see the paragraph below)

• -ar variant of the adjective-forming suffix → -al;
  → micrometer.

Etymology (PE): Kehsanj, → micrometer; zehi adj. of zeh “string, bow-string;” Mid.Pers. zih “bow-string,” zig “string; astronomical table” (loaned into Ar. as zij);
Av. jiiā- “bow-string;” cf. Skt. jiyā- “bow-string;” Gk. bios “bow;” L. filum “thread;” Arm. jil “string, line;” Lith. gijà “thread;” Russ. žica “thread;” PIE base *g^whi- “thread, tendon.”