
The study and analysis of old astronomical photographs and plates for scientific discovery.Historical astrophotography is the practice of mining old astronomical photographs and glass plates for new scientific discoveries. These archives, some dating back over a century, contain a unique record of the sky as it appeared long before modern digital detectors. Because they capture the positions and brightnesses of celestial objects at specific moments in the past, they serve as a time machine for studying changes that occur over decades or even a century — timescales that are otherwise inaccessible to contemporary observations.
The systematic use of photographic plates in astronomy began in the late 19th century, with major sky surveys like the Carte du Ciel project (1887) and the Harvard College Observatory's collection, which accumulated over 500,000 plates. The scientific exploitation of these archives for new discoveries gained momentum in the late 20th century as digitization projects made them accessible to the broader community.
Old plates are digitized and compared with modern images of the same regions. Astronomers measure shifts in stellar positions (proper motions), changes in brightness (for variable stars or transients), or the appearance of new objects that were not present in the old images. The key is careful calibration: aligning the plates with modern astrometric catalogs and correcting for distortions in the original optics and emulsions.
Used to discover high-proper-motion stars, identify long-period variable stars, find pre-discovery images of supernovae or novae, and constrain the orbits of objects like Kuiper Belt bodies or even hypothetical Planet Nine.
The Harvard plate collection includes images taken by women 'computers' like Henrietta Swan Leavitt, whose analysis of variable stars on these plates led to the cosmic distance ladder.








