An Etymological Dictionary of Astronomy and Astrophysics

1. Math.: A finite or infinite collection of objects in which order has no significance. Members of a set are often referred to as elements and the notation a ∈ A is used to denote that a is an element of a set A. The study of sets and their properties is the object of set theory.
   

2. To pass below the horizon.
   → moonset; → sunset.
   

3a) (tr.v.) To put (something or someone) in a particular place.

3b) To adjust a device to a desired position.

Etymology (EN): 1) M.E. sette, from O.Fr. sette “sequence,” variant of secte, from M.L. secta “religious group, sect,” from L. secta “manner, following, school of thought,” literally “something to follow, pathway, course of conduct, school of thought,” from sectari “to pursue, accompany,” “a way, road,” from sequi “to follow,” → sequence.

2. M.E. setten, O.E. settan “cause to sit, put in some place, fix firmly;”
   cf. O.N. setja, O.Fris. setta, Du. zetten, Ger. setzen.

Etymology (PE): 1) Hangard, from Mid.Pers. hangart “whole, complete,” hangartik “complete,” hangartênitan “to collect, assemble,” from *hamkard- literally “created, cut together,” from han- variant of ham- “together,” cognate with L.L. insimul “at the same time,” from in- intensive prefix + simul “together, at the same time” (cf. Gk. homos “same,” Mod./Mid.Pers.
ham- “together, with,” O.Pers./Av. ham-, Skt. sam-; also O.Pers./Av. hama- “one and the same,” Skt. sama-; originally identical with PIE numeral *sam- “one,” from *som-).

(→ com-),

• gard, variant of kard “created, cut,” Mid.Pers. kirrēnītan, kirrēn- “to cut, create,” cf. fragard “section, chapter,” Av. karət- “to cut,” kərəntaiti “cuts, breaks,” with fraca- “to create, bring forth,”
  karəta- “knife, dagger” (Mid.Pers. kârt “knife;” Mod.Pers. kârd “knife”), Skt. kart- “to cut, split, break,” krti- “sword, knife;” PIE base *(s)kert- “to cut;” Hitt. kartae- “to cut;” Arm. kherthem “to skin;” L. cortex “bark of a tree,” corium “skin, leather;”
  O.H.G. scrinden “to split;” Lith. kertu “to fell, cut down.”
  

2. Forušodan, literally “to descend, go down,” from foru- “down, downward; below; beneath” (Mid.Pers. frôt “down, downward;” O.Pers. fravata “forward, downward;” cf. Skt. pravát- “a sloping path, the slope of a mountain”)

• šodan “to go, to pass; to become, to be, to be doing” (Mid.Pers. šudan, šaw- “to go;” Av. š(ii)auu-, šiyav- “to move, go,” šiyavati “goes,” šyaoθna- “activity; action; doing, working;” O.Pers. šiyav- “to go forth, set,” ašiyavam “I set forth;” cf. Skt. cyu- “to move to and fro, shake about; to stir,” cyávate “stirs himself, goes;” Gk. kinein “to move;” Goth. haitan “call, be called;” O.E. hatan “command, call;” PIE base *kei- “to move to and fro”).
  

3. Beštidan, from bešt past stem of Tabari/Mâzandarâni beštan “to place, put, set,” from intensive prefix be- + eštan, from heštan “to place, put,” → intercalate.