Origins
The word 'astrology' records in its very structure a time when looking at the stars and reasoning about their influence on human life were considered a single intellectual activity. Greek 'astrologia' compounds 'astron' (star) with '-logia' (study of, discourse about, reasoning concerning), and for most of Western history, this word and 'astronomia' were used interchangeably to describe the study of celestial bodies.
Greek 'astron' (star) descends from PIE *h₂ster-, one of the most securely reconstructed nouns in comparative linguistics. The root appears in virtually every Indo-European branch: Latin 'stella' (from earlier *sterla), English 'star' (from Proto-Germanic *sternō), German 'Stern,' Old Norse 'stjarna,' Welsh 'seren,' Sanskrit 'stṛ,' Avestan 'star-,' Hittite 'haster-,' and Armenian 'astl.' The reconstructed PIE form is supported by an unusually wide distribution, suggesting that the word is of great antiquity.
The suffix '-logia' comes from Greek 'logos' (word, reason, discourse), from the verb 'legein' (to speak, to reason, to gather). This suffix became one of the most productive in European intellectual vocabulary, appearing in 'biology,' 'geology,' 'theology,' 'psychology,' 'etymology,' and hundreds of other discipline names.
Middle English
In Greek antiquity, Ptolemy's 'Tetrabiblos' (second century CE) was the foundational text of astrology, and he saw no conflict between mathematical astronomy and astrological interpretation. The same person who calculated planetary orbits also interpreted their influence on earthly events. This unity persisted through the medieval period: Chaucer, who wrote the 'Treatise on the Astrolabe' (a guide to an astronomical instrument), used 'astrology' and 'astronomy' as synonyms.
The separation of astrology from astronomy was a gradual process driven by the Scientific Revolution. Kepler (1571-1630) practiced both but increasingly distinguished between the mathematical science of planetary motion and the interpretive art of predicting earthly events from celestial positions. By the late seventeenth century, 'astronomy' had claimed the mantle of legitimate science while 'astrology' was increasingly marginalized as superstition. The two words, once synonymous, became antonyms of intellectual respectability.
The word family built on Greek 'astron' is substantial. 'Astronomy' uses the suffix '-nomia' (from 'nomos,' law or arrangement) — the laws of the stars. 'Astronaut' combines 'astron' with 'nautes' (sailor) — a star-sailor. 'Asteroid' adds the suffix '-eides' (resembling) — a star-like thing. 'Astral' is the adjective form. 'Asterisk' is a 'little star' (the Greek diminutive).
Later History
More surprisingly, 'disaster' belongs to this family. Italian 'disastro' (ill-starred event) combines 'dis-' (bad, unfavorable) with 'astro' (star), preserving the astrological belief that catastrophes resulted from malign stellar configurations. When we call a flood or an earthquake a 'disaster,' we are, without knowing it, blaming the stars.
The Latin cognate 'stella' (from earlier *sterla, with a diminutive suffix) produced its own English family: 'stellar,' 'constellation' (stars together), 'stellate' (star-shaped), and the name 'Stella' itself.
The persistence of 'astrology' as a living practice, despite its divorce from mainstream science, is remarkable from a linguistic perspective. The word retains its Greek structure ('star-reasoning'), its medieval associations (horoscopes, zodiac signs, planetary influences), and its cultural vitality (millions of people read daily horoscopes, and astrology has experienced a significant cultural revival in the twenty-first century). Whether understood as science, pseudoscience, or cultural tradition, the practice of reading the stars and finding meaning in their positions is among the oldest human intellectual activities, and the word 'astrology' — built from a PIE root that may be six thousand years old — carries that long history in its syllables.