Etymology
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regress (v.)

1550s, "to return to a former state or place, go back," from Latin regressus "a return, retreat, a going back," noun use of past participle of regredi "to go back," from re- "back" (see re-) + gradi "to step, walk" (from PIE root *ghredh- "to walk, go").

In astronomy, "appear to move in a backward direction," by 1823. The psychological sense of "to return to an earlier stage of life" is attested from 1926. Related: Regressed; regressing.

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culmination (n.)

1630s, in astronomy/astrology, "position of a heavenly body when it is on the meridian," from French culmination, noun of action from past participle stem of Late Latin culminare "to top, to crown," from Latin culmen (genitive culminis) "top, peak, summit, roof, gable," also used figuratively, contraction of columen "top, summit" (from PIE root *kel- (2) "to be prominent; hill"). Figurative sense of "highest point or summit" is from 1650s.

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trivium (n.)

by 1751, from Medieval Latin trivium (9c.) "grammar, rhetoric, and logic," the first three of the seven liberal arts, considered initiatory and foundational to the other four (arithmetic, geometry, astronomy, and music). From Latin trivium, in classical Latin "place where three roads meet; a frequented place; public street, highway," from tri- "three" (see three) + via "road" (see via). Compare trivia and also see quadrivium.

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artist (n.)

1580s, "one who cultivates one of the fine arts," from French artiste (14c.), from Italian artista, from Medieval Latin artista, from Latin ars (see art (n.)).

Originally especially of the arts presided over by the Muses (history, poetry, comedy, tragedy, music, dancing, astronomy), but also used 17c. for "one skilled in any art or craft" (including professors, surgeons, craftsmen, cooks). Since mid-18c. especially of "one who practices the arts of design or visual arts."

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quadrivium (n.)

"arithmetic, music, geometry, astronomy" (the four branches of mathematics, according to the Pythagoreans), by 1751, from Latin quadrivium, which meant "place where four roads meet, crossroads," from quadri- "four" (from PIE root *kwetwer- "four") + via "way, road, channel, course" (see via). Compare liberal arts, and also see trivium.

The adjective quadrivial is attested from mid-15c. in English with the sense of "belonging to the quadrivium," late 15c. with the sense of "having four roads, having four ways meeting in a point."

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Muse (n.)

late 14c., "one of the nine Muses of classical mythology," daughters of Zeus and Mnemosyne, protectors of the arts; from Old French Muse and directly from Latin Musa, from Greek Mousa, "the Muse," also "music, song," ultimately from PIE root *men- (1) "to think." Meaning "inspiring goddess of a particular poet" (with a lower-case m-) is from late 14c.

The traditional names and specialties of the nine Muses are: Calliope (epic poetry), Clio (history), Erato (love poetry, lyric art), Euterpe (music, especially flute), Melpomene (tragedy), Polymnia (hymns), Terpsichore (dance), Thalia (comedy), Urania (astronomy).

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mobile (n.)

early 15c. in astronomy, "outer sphere of the universe" (the primum mobile), from mobile (adj.); the artistic sense "abstract sculpture consisting of parts suspended so as to move," associated with Alexander Calder, is by 1939, perhaps a shortening of mobile sculpture (1936). Now-obsolete sense of "the common people, the rabble" (1670s, short for Latin mobile vulgus) led to mob (n.). Middle English had moble, moeble (mid-14c.) "movable goods, personal property," from Old French moble, meuble, from the Latin adjective, but in 16c. this was replaced by furniture.

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apogee (n.)
"point at which the moon is farthest from the earth," 1590s, from French apogée or directly from Latin apogaeum, from Greek apogaion (diastema) "(distance) away from the earth," from apogaion, neuter adjective, "from land," here in a specialized sense "away from the earth," from apo "off, away" (see apo-) + gaia/ge "earth" (see Gaia).

Figurative sense "climax, culmination" is from 1640s. A term from Ptolemaic astronomy, which regarded the earth as the center of the universe and applied the word to the sun and planets; for these bodies it was displaced in the Copernican system by aphelion. Adjective forms are apogeal, apogean, apogeic.
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primum mobile (n.)

"the first source of motion," mid-15c., from Medieval Latin (11c.), literally "the first movable thing;" see prime (adj.) + mobile.

In the Ptolemaic system of astronomy, the tenth or outermost of the revolving spheres of the universe, which was supposed to revolve from east to west in twenty-four hours, and to carry the others along with it in its motion; hence, any great or first source of motion. [Century Dictionary]

A translation of Arabic al-muharrik al-awwal "the first moving" (Avicenna). Englished by Chaucer as the firste Moeuyng (c. 1400). Old science also had primum frigidum "pure cold: an elementary substance, according to the doctrine of Parmenides."


 

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Ophiuchus 

ancient constellation (representing Aesculapius), 1650s, from Latin, from Greek ophioukhos, literally "holding a serpent," from ophis "serpent" (see ophio-) + stem of ekhein "to hold, have, keep" (from PIE root *segh- "to hold").

Translated in Latin as Anguitenens and Serpentarius. Milton's "Ophiuchus huge in th' Arctick Sky" ("Paradise Lost") is a rare (minor) lapse for a poet who generally knew his astronomy; the constellation actually straddles the celestial equator, and, as its modern boundaries are drawn, dips into the zodiac: The sun passes through Ophiuchus from about Nov. 30 to Dec. 18. The serpent now is treated as a separate constellation.

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