Origins
The word 'astronomy' derives from Greek 'astronomía' (ἀστρονομία), a compound of 'ástron' (ἄστρον, star) and 'nómos' (νόμος, law, arrangement, distribution), making its literal meaning 'the arrangement of the stars' or 'star-law.' The first element traces to PIE *h₂ster- (star), one of the oldest reconstructable words in the Indo-European family, which also produced Latin 'stella' (star, with an l-form from *h₂ster-l-), Germanic '*sternō' (whence English 'star' and German 'Stern'), and Sanskrit 'stṛ' (star). The second element derives from PIE *nem- (to assign, to allot), which also gave Greek 'némein' (to distribute, to manage), 'nomás' (wandering — one who moves to distribute pasture, whence 'nomad'), and 'oikonomía' (household management, whence 'economy').
The word 'astronomía' is attested in Greek from at least the 6th century BCE. It coexisted with 'astrología' (ἀστρολογία, star-reasoning), and for most of antiquity the two terms were used more or less interchangeably. Both Plato and Aristotle used 'astrología' for what we would call astronomy. The semantic separation — astronomy as the scientific study of celestial bodies and astrology as the divination of earthly events from stellar positions — developed gradually. Ptolemy, writing in the 2nd century CE, distinguished between the mathematical study of celestial motions and the art of predicting their influence on human affairs, but he practiced both. The clean split came only with the Scientific Revolution of the 16th and 17th centuries, when Copernicus, Kepler, and Galileo established astronomy as a discipline based on observation, mathematics, and physical theory, while astrology was increasingly marginalized as pseudoscience.
The word passed from Greek through Latin 'astronomia' and Old French 'astronomie' into English around 1200. In medieval English, 'astronomy' and 'astrology' continued to overlap in meaning, and Chaucer used both terms in ways that blur the modern distinction. The 'Treatise on the Astrolabe' (c. 1391), written by Chaucer for his son Lewis, is fundamentally an astronomical text — a manual for using an instrument to determine the positions of celestial bodies — but it bears no sharp line between astronomy and astrology as we would draw it today.
Proto-Indo-European Roots
The PIE root *h₂ster- (star) has an unusually stable form across the Indo-European languages, suggesting that it was established very early and transmitted with remarkable fidelity. English 'star,' German 'Stern,' Dutch 'ster,' Swedish 'stjärna,' Latin 'stella,' Greek 'ástron/astḗr,' Welsh 'seren,' Armenian 'astł,' and Persian 'setāre' all descend from this single root. The word's stability may reflect the importance of stars to early Indo-European peoples as navigational aids and timekeepers — knowledge too vital to let the word drift.
The related word 'disaster' (from Italian 'disastro,' literally 'ill-starred' — 'dis-' (bad) + 'astro' (star)) preserves the ancient belief that catastrophic events were caused by unfavorable positions of the stars. Similarly, 'consider' (from Latin 'cōnsīderāre,' literally 'to examine the stars together' — 'con-' + 'sīdus/sīderis,' star) originally meant to make a decision by consulting the stars. And 'influenza' (from Italian 'influenza,' influence — originally the 'influence' of the stars believed to cause epidemics) carries the same astrological thinking. These words are linguistic fossils of a worldview in which the stars governed human destiny.
The distinction between 'ástron' (star) and 'astḗr' (star) in Greek is minor — both derive from the same root and were used somewhat interchangeably, though 'astḗr' was more common in the singular and 'ástron' (originally a neuter form) more common in the plural or in compounds. English inherits both: 'astronomy' and 'astrology' use the 'astro-' form, while 'asteroid' (star-like), 'asterisk' (little star), and 'aster' (the flower, named for its star-shaped blossoms) use the 'aster-' form.
Greek Origins
The compound 'astronaut' (star-sailor) was coined in 1929 by the science fiction writer and engineer J.J. Harper, though it did not enter wide use until the American space program adopted it in the late 1950s. The Soviet equivalent, 'cosmonaut' (universe-sailor, from Greek 'kósmos' + 'naútēs'), reflects a different conceptual emphasis: Americans sailed to the stars, while Soviets sailed through the cosmos. The Chinese term 'tàikōngrén' (太空人, space-person) is more straightforward, avoiding both the stellar and cosmic metaphors in favor of a simple description.