The fixed stars do not move relative to one another.

“Fixed stars” originally meant something more observationally specific than “stars are nailed permanently to the universe.” The planets visibly wandered against the stellar background; the stars, by contrast, kept essentially the same patterns and separations over ordinary human timescales. Ptolemy explicitly explained the term this way in the Almagest: they were called fixed because they appeared to preserve their figures and relative distances.

That apparent stability made sense within ancient and medieval stellar astronomy. Ptolemy’s second-century Almagest compiled a sophisticated mathematical astronomy and star catalogue and remained a basic astronomical text for more than a thousand years. The constellations really are stable enough that ancient catalogues remained useful reference points.

Ironically, those same old catalogues eventually helped expose the limitation. Edmond Halley compared eighteenth-century stellar positions with ancient and early-modern measurements and argued in 1717/1718 that several bright stars had moved. Modern reanalysis shows that Halley’s evidence was not statistically secure. Jacques Cassini’s 1738 work on Arcturus provides a cleaner first significant proper-motion detection.

Modern astrometry turns this tiny proper motion into a vast map of stellar movement. Stars move through the Milky Way at different velocities and distances; their constellations only look fixed because stellar distances are enormous and human lifetimes are short.