Etymology
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planetary (adj.)

1590s, "of or pertaining to a planet;" see planet + -ary. Perhaps from or based on Late Latin planetarius "pertaining to a planet or planets," but according to OED this is attested only as a noun meaning "an astrologer." Planetary nebula, so called for its shape as seen through a telescope, attested from 1785.

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Plutonian (adj.)

1660s, "pertaining to the god Pluto," from Latin Plutonius, from Greek Ploutōnius, from Ploutōn "pertaining to Pluto" (see Pluto). Geological sense is from 1828 (see plutonic). Planetary sense by 1952.

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eccentricity (n.)
1540s, of planetary orbits; 1650s, of persons (an instance of eccentricity); 1794, of persons (a quality of eccentricity); from eccentric (adj.) + -ity or from Modern Latin eccentricitatem, from eccentricus. Related: Eccentricities.
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node (n.)

early 15c., "a knot or lump," from Latin nodus "knot" (from PIE root *ned- "to bind, tie"). Originally borrowed c. 1400 in Latin form, meaning "lump in the flesh." Meaning "point of intersection" (originally in astronomy, of planetary orbits with the ecliptic) is recorded from 1660s.

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quincunx (n.)

1640s, originally astrological, of planetary alignments at a distance of five signs from one another, from Latin, literally "five twelfths" (especially "five unciae," that is, "five-twelfths of an as," the basic unit of Roman currency), from quinque "five" (from PIE root *penkwe- "five") + uncia "ounce; a twelfth part (of anything)," related to unus "one" (from PIE root *oi-no- "one, unique").

From 1650s as "arrangement of five objects in a square, one at each corner and one in the middle" (like the five pips on a playing card or spots on dice). Also applied, especially in garden design, to arrangements in two sets of oblique rows at right angles to one another (1660s), a sense also in the Latin word. Related: Quincuncial; quincuncially.

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coitus (n.)

"copulation, sexual intercourse," 1848, scientific use of Latin coitus "a meeting together; sexual union," past participle of coire "to come together, meet," from assimilated form of com "together" (see co-) + ire (past participle itus) "to come, to go," (from PIE root *ei- "to go").

In Middle English nativized as coite (early 15c.). Coitus was used in English in general senses of "meeting, uniting," and also in reference to magnetic force, planetary conjunction, etc., before the sexual sense came to predominate.

Coitus interruptus, "sexual intercourse in which the penis is voluntarily withdrawn from the vagina before ejaculation, for the purpose of avoiding conception," is from 1886 (from 1885 in German publications). Coitus reservatus in reference to prolonged copulation by deliberate control is from 1890 in English (1880 in German).

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week (n.)

Old English wucu, wice, etc., from Proto-Germanic *wikō(n)- (source also of Old Norse vika, Old Frisian wike, Middle Dutch weke, Old High German wecha, German woche), probably originally with the sense of "a turning" or "succession" (compare Gothic wikon "in the course of," Old Norse vika "sea-mile," originally "change of oar," Old English wican "yield, give way"), from PIE root *weik- (2) "to bend, to wind." The vowel sound seems to have been uncertain in Old and Middle English and -e-, -i-, -o-, -u-, -y-, and various diphthongs are attested for it.

"Meaning primarily 'change, alteration,' the word may once have denoted some earlier time division, such as the 'change of moon, half month,' ... but there is no positive evidence of this" [Buck]. No evidence of a native Germanic week before contact with the Romans. The seven-day week is ancient, probably originating from the 28-day lunar cycle, divisible into four periods of seven day, at the end of each of which the moon enters a new phase. Reinforced during the spread of Christianity by the ancient Jewish seven-day week.

As a Roman astrological convention it was borrowed by other European peoples; the Germanic tribes substituting their own deities for those of the Romans, without regard to planets. The Coligny calendar suggests a Celtic division of the month into halves; the regular Greek division of the month was into three decades; and the Romans also had a market week of nine days. Phrase a week, as in eight days a week recorded by 1540s; see a- (1).

Greek planetary names [for the days of the week] ... are attested for the early centuries of our era, but their use was apparently restricted to certain circles; at any rate they never became popular. In Rome, on the other hand, the planetary names became the established popular terms, too strongly intrenched to be displaced by the eccl[esiastical] names, and spreading through most of western Europe. [Carl Darling Buck, "A Dictionary of Selected Synonyms in the Principal Indo-European Languages," 1949]
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