By 1900, astronomers had measured the distances to only about 60 stars
Pinning down exactly where a star sits sounds dull until you learn that nobody managed a single stellar parallax until Friedrich Bessel. Astrometry, the science of star positions, has since climbed from the 850-odd stars Hipparchus listed to a satellite charting a billion of them with a hundredfold sharper aim.
Around 190 BC Hipparchus compared his observations with the older catalogue of Timocharis and Aristillus and found that Earth's axis slowly wobbles, the effect called precession. Along the way he devised the brightness scale astronomers still use and listed at least 850 stars. Ptolemy's Almagest later gave 1,022. In the 10th century the Persian astronomer al-Sufi recorded positions, magnitudes and colours and drew each constellation in his Book of Fixed Stars, while Ibn Yunus in Egypt logged over 10,000 solar positions with an astrolabe nearly 1.4 metres across. The Timurid ruler-astronomer Ulugh Beg catalogued 1,019 stars in the 15th century, accurate, like his predecessors, to about 20 arcminutes.
Tycho Brahe's huge wall-mounted instruments cut errors to between 15 and 35 arcseconds in the 16th century, and Taqi al-Din timed star positions in Constantinople with a clock of his own design. James Bradley set out in 1729 to detect stellar parallax, failed because the shift was too small, and discovered the aberration of light and the nodding of Earth's axis instead. Friedrich Bessel, often called the father of modern astrometry, refined Bradley's catalogue of 3,222 stars from 1807 and finally measured a parallax: 0.3 arcseconds for 61 Cygni.
Progress then came from technology. Photographic plates sped work in the early 1900s, and the unfinished Carte du Ciel project made photography routine. Computers and automated plate readers arrived in the 1960s. In the 1980s charge-coupled devices pushed errors down to a milliarcsecond and made the field affordable for amateurs.
Space removed the atmosphere's blur. ESA's Hipparcos, operating from 1989 to 1993, fixed positions, distances and motions for 118,218 stars, and its companion Tycho catalogue covered over a million. Gaia, launched in 2013, improved precision a hundredfold and charted a billion stars. Precise positions feed the cosmic distance ladder, help keep UTC in step with Earth's rotation, and can reveal planets by the tiny wobble they tug into their stars.
Source: Astrometry
