Astronomical Photographic Plates: More than a Century of Service

From approximately 1880 through 1980, photographic plates were the only mechanical detector available to astronomers for recording their observations. Fortunately, the several million such plates that have been preserved from that era have retained their information faithfully for such a time-span. Because all celestial objects change—most on very slow, but some on rapid, time-scales—those plates have bequeathed to astronomy a unique historical record of how these objects move and evolve. Once digitized, shared, re-examined and combined with modern electronic observations, plates continue to extend the available time-span for research. Time-domain astronomy in the 21st century is thus facilitated by photographic records that extend back to the 19th century. The creation, maintenance, and analysis of photographic plates required an entire ecosystem of observatory activities. Through the annotations found on plates, envelopes, and storage boxes, and in logbooks, historians can trace the scientific processes that underlie our current knowledge of the physical universe, the people who facilitated astronomical discovery, and the networks that disseminated astrophysical knowledge. The plates themselves are evidence of evolving technologies and research priorities. Astronomical imagery in many media, including photographic plates, captivates the public imagination. In photographs, the night sky is rendered available at any time and in greater detail than the naked-eye perceives. In the early 20th century, direct images made from photographic plates graced lantern slides for talks and illustrated publications for both technical and amateur audiences. In the 21st century, artists continue to draw inspiration from photographic plates, both as celestial imagery and as historical artifacts. Although a vibrant legacy of scientific observations and historical activity, photographic plates are fragile connections to the past. The International Astronomical Union Working Group on the Preservation and Digitization of Photographic Plates advocates for their scientific use. The International Glass Plates Group is a community of astronomers, librarians, and archivists working together to maintain and promote multiple uses of these archives. We can never see the same sky again and cannot know what new questions future generations researchers will ask. With care, these photographic plates will continue to be a source of scientific investigation, historical exploration, and creative inspiration for years to come.