An Etymological Dictionary of Astronomy and Astrophysics

1a) Proceeding in a straight course or line without deviation or interruption.

1b) Math.: Varying in the same manner as another quantity, especially increasing if another quantity increases or decreasing if it decreases.

1c) Astro.: Designating west-to-east motion of a planet in the same direction as the Sun’s movement against the stars.

2. To administer; manage; supervise; to serve as a director in the performance or production of.

Etymology (EN): From L. directus “straight,” p.p. of dirigere “set straight,” from → dis- “apart” + regere “to guide;” cognate with Pers. râst, as explained below.

Etymology (PE): Râst “right, straight” (râšt in afrâštan); Mid.Pers.
râst; O.Pers. rāsta- “straight, true,” rās-
“to be right, straight, true,” rād- “to prepare;” Av. rāz-
“to direct, put in line, set,” razišta- “straightest, most correct,”
erezu- “correct, straight,” rāzayeiti “directs,” razan- “order;”
Skt. raj- “to direct, stretch,” rjuyant- “walking straight;”
Gk. orektos “stretched out;” L. regere “to lead straight, guide, rule,”
p.p. rectus “right, straight;” Ger. recht; E. right;
PIE base *reg- “right, just; to move in a straight line.”
Sarrâst, from sar here an intensive element meaning “side,” originally “head” (Mid.Pers. sar; Av. sarah- “head;” Skt. siras- “head;” Gk. kara “head,” keras “horn;” Mod.Pers. sarun “horn;” L. cerebrum “brain;” PIE *ker- “head, horn”) + râst, as above.
Râštidan, infinitive from râšt, variant râst “direct.”