What Is a Star Catalogue? The Sky Written Down and Checked Later

A catalogue lists objects with their positions in a defined coordinate system, together with brightness and whatever else the instruments of the time could determine. Position is the hard part, because the sky appears to move, so any measurement must be referred to a frame and an epoch, meaning a specified date, since positions change slowly for several reasons. The most important is precession, the slow wobble of the Earth's axis over about twenty-six thousand years, which shifts all coordinates steadily and must be corrected for when comparing catalogues from different centuries. Beyond that, each star has its own proper motion across the sky, tiny but measurable over long baselines, and each shows parallax, an annual apparent shift caused by the Earth's own orbit, which is the measurement that yields distance. A modern catalogue therefore records not just where an object is but how fast it is moving and how far away it is, and the progression from listing positions to listing motions to listing distances is the history of the whole enterprise.

A handful of compilations mark the stages, each enabled by a new instrument:

• •Hipparchus in the second century before the common era, whose catalogue is lost but survives through later transmission, and who is credited with discovering precession by comparing his positions with older Babylonian records
• •Ptolemy's Almagest, listing over a thousand stars, which transmitted Greek astronomy through the Islamic world into medieval Europe and dominated for fourteen centuries
• •Ulugh Beg's catalogue from Samarkand in 1437, the first major re-observation rather than a copy, produced at a large purpose-built observatory
• •Tycho Brahe's naked-eye measurements in the late sixteenth century, accurate to about an arcminute, which were precise enough for Kepler to derive elliptical orbits from them
• •The Bonner Durchmusterung and its successors in the nineteenth century, cataloguing hundreds of thousands of stars visually
• •Photographic surveys from the 1880s onward, which recorded far more stars than anyone could measure by eye and created an archive that has been mined ever since

The scientific value of catalogues lies in the differences between them. Halley compared his own positions with Ptolemy's in 1718 and found that three bright stars had moved substantially, which established that stars are not fixed and introduced proper motion. Stellar parallax was finally measured in 1838 after centuries of failure, giving the first direct distances to stars and settling definitively that the Earth moves. Comparing photographic plates from different years revealed variable stars, novae, moving objects and eventually the expansion implied by galaxy redshifts. The photographic archives have repeatedly been re-examined to establish the past behaviour of objects that only became interesting later, which is why plate collections are preserved and digitised despite the images being over a century old. That is the defining property of a catalogue: its value increases with age, because the baseline for comparison lengthens, and no amount of modern observation can substitute for a measurement that was actually made in 1890.

Space-based astrometry transformed precision by removing the atmosphere. The Hipparcos mission in the late 1980s measured over a hundred thousand stars to milliarcsecond accuracy, revising the cosmic distance scale. Its successor Gaia has measured positions, motions and parallaxes for well over a billion objects at microarcsecond precision, which is equivalent to resolving the width of a human hair at a distance of a thousand kilometres, and it has produced a three-dimensional map of a substantial part of the galaxy including the motions of stars, which has revealed structures, streams and past mergers that were invisible before. Ground-based surveys complement this at other wavelengths and with repeated imaging designed to catch things that change. The change in scale has forced a change in method, since no human examines the data, and catalogues are now databases queried by software with the science emerging from statistical analysis of billions of rows rather than from anyone noticing an individual star behaving oddly.

Recording positions precisely enough for later comparison is what turned looking at the sky into evidence, and it required correcting for precession before catalogues centuries apart could be compared at all. Halley found stars had moved since Ptolemy, which killed the fixed sky. A catalogue's value grows with age because the baseline lengthens, which is why century-old photographic plates are still mined for answers.