Etymology
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astronomy (n.)

c. 1200, "astronomy, astrology, scientific or occult study of heavenly bodies," from Old French astrenomie "astronomy, astrology," from Latin astronomia, from Greek astronomia, abstract noun from astronomos, literally "star-regulating," from astron "star" (from PIE root *ster- (2) "star") + nomos "arranging, regulating; rule, law," from PIE root *nem- "assign, allot; take." Perhaps originally with reference to mapping the constellations or movements of planets.

Þer wes moni god clarc to lokien in þan leofte, to lokien i þan steorren nehʒe and feorren. þe craft is ihate Astronomie. [Layamon, "The Brut," c. 1200]

In English, astronomy is earlier than astrology and originally included the senses now distributed over both words; the gradual differentiation was complete by late 17c. and astronomy came to mean exclusively "the scientific study of the heavenly bodies." See astrology.

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astronomer (n.)

"one versed in the laws of the heavenly bodies," late 14c., from astronomy (q.v.) + -er (1). It replaced French import astronomyen (c. 1300), which, had it survived, probably would have yielded *astronomian. For sense differentiation, see astrology and compare astrologer.

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astronomical (adj.)
1550s, "pertaining to astronomy," from astronomy + -ical. Popular meaning "immense, concerning very large figures" (as sizes and distances in astronomy) is attested from 1899. Astronomical unit (abbreviation A.U.) "mean distance from the Earth to the Sun," used as a unit of measure of distance in space, is from 1909. Related: Astronomically.
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astrology (n.)

late 14c., "calculation and foretelling based on observation of heavenly bodies," from Latin astrologia "astronomy, the science of the heavenly bodies," from Greek astrologia "astronomy," literally "a telling of the stars," from astron "star" (from PIE root *ster- (2) "star") + -logia "treating of" (see -logy).

Originally identical with astronomy and including scientific observation and description. The special sense of "astronomy applied to prediction of events" was divided into natural astrology "the calculation and foretelling of natural phenomenon" (tides, eclipses, dates of Church festivals, etc.), and judicial astrology "the art of judging occult influences of stars and planets on human affairs."

In Latin and later Greek, astronomia tended to be more scientific than astrologia.  In English, the differentiation between astrology and astronomy began late 1400s and by late 17c. this word was limited to the sense of "reading influences of the stars and their effects on human destiny."

And consequently, an Astrology in the World before Astronomy, either in Name or Science. so that (Non obstante whatever any Astronomer shall oppose to the contrary) Astrology hath the right of Primogeniture. And all the Sober and Judiciously Learned must needs acknowledge the Truth hereof.—Howbeit, it were to be wished that the Astrologer understood Astronomy, and that the Astronomer were acquainted with Astrology: Although I do in truth despair of ever finding many to be so happily Accomplished. [John Gadbury, introduction to "Ephemerides of the Celestial Motions," London, 1672]
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*nem- 
Proto-Indo-European root meaning "assign, allot; take."

It forms all or part of: agronomy; anomie; anomy; antinomian; antinomy; astronomer; astronomy; autonomous; autonomy; benumb; Deuteronomy; economy; enumerate; enumeration; gastronomy; heteronomy; innumerable; metronome; namaste; nemesis; nimble; nim; nomad; nomothetic; numb; numeracy; numeral; numerator; numerical; numerology; numerous; numismatic; supernumerary; taxonomy.

It is the hypothetical source of/evidence for its existence is provided by: Greek nemein "to deal out," nemesis "just indignation;" Latin numerus "number;" Lithuanian nuoma "rent, interest;" Middle Irish nos "custom, usage;" German nehmen "to take."
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*ster- (2)

Proto-Indo-European root meaning "star." Buck and others doubt the old suggestion that it is a borrowing from Akkadian istar "venus." The source of the common Balto-Slavic word for "star" (Lithuanian žvaigždė, Old Church Slavonic zvezda, Polish gwiazda, Russian zvezda) is not explained.

It forms all or part of: aster; asterisk; asterism; asteroid; astral; astro-; astrobiology; astrobleme; astrognosy; astroid; astrolabe; astrolatry; astrology; astromancy; astronaut; astronomy; AstroTurf; constellation; disaster; Estella; Esther; instellation; interstellar; lodestar; star; stardust; starfish; starlet; starlight; starry; stellar; stellate.

It is the hypothetical source of/evidence for its existence is provided by: Sanskrit star-; Hittite shittar, Greek aster "star," with derivative astron; Latin stella, Breton sterenn, Welsh seren "star."

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sub-giant (n.)
also subgiant, in astronomy, of stars, 1937, from sub- + giant (n.).
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lucida (n.)
in astronomy, "star easily seen by the naked eye," also "brightest star in a constellation or group," 1727, from Modern Latin lucida (stella) "bright star," from fem. of Latin lucidus "light, bright, clear" (see lucid). Plural lucidae. Astronomy has used lucid for "visible to the naked eye" since 1690s.
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circumpolar (adj.)

"surrounding one of the celestial or terrestrial poles," 1680s in astronomy; 1690s in geography, from circum- "around" + polar.

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