Old English deorcnysse "absence of light," from dark (adj.) + -ness. The 10c. Anglo-Saxon treatise on astronomy uses þeostrum for "darkness." Figurative use for "sinfulness, wickedness" is from early 14c. From late 14c. as "obscurity," also "secrecy, concealment," also "blindness," physical, mental, or spiritual.
c. 1400, elongacioun, in astronomy, "angular distance of a planet from the sun as it appears from the earth;" early 15c., "extension, spreading," from Medieval Latin elongationem (nominative elongatio), noun of action from past-participle stem of Late Latin elongare "remove to a distance," from assimilated form of Latin ex "out" (see ex-) + longus "long" (see long (adj.)).
1560s, "relating to or connected with the science of optics; pertaining to vision," from optic + -al (1). Of abstract art, from 1964. In astronomy, in reference to double stars that appear so only because they lie in the same line of sight from earth, by 1868. Optical illusion is attested by 1757. Related: Optically.
1640s, in astronomy, of a star or planet, "come to or be on the highest point of altitude; come to or be on the meridian," from Late Latin culminatus past participle of culminare "to top, to crown," from Latin culmen (genitive culminis) "top, peak, summit, roof, gable," also used figuratively, a contraction of columen "top, summit" (from PIE root *kel- (2) "to be prominent; hill"). Figurative sense in English of "reach the highest point" is from 1660s. Related: Culminant; culminated; culminating.
late 14c., declinacioun, in astronomy, "distance of a heavenly body from the celestial equator, measured on a great circle passing through the body and the celestial pole," from Old French declinacion (Modern French déclinaison) and directly from Latin declinationem (nominative declinatio) "a bending from (something), a bending aside; the supposed slope of the earth toward the poles; a turning away from (something), an avoiding," noun of action from past-participle stem of declinare (see decline (v.)). From c. 1400 as "a bending or sloping downward." Related: Declinational.
late 14c., in astronomy, "imaginary point of the celestial sphere vertically opposite to the zenith of the sun; the inferior pole of the horizon," from Medieval Latin nadir, from Arabic nazir "opposite to," in nazir as-samt, literally "opposite direction," from nazir "opposite" + as-samt "road, path" (see zenith). Transferred sense of "lowest point" of anything is recorded by 1793.