Photographic plates were the primary data recording medium in astronomy from the late 1800's until the the 1980's.  It is estimated that over three million of these plates exist in observatories and universities around the world.

Many of these observational records are still important as they provide the only way to study the long-term changes of astronomically interesting objects. However, astronomy's 130-year record of the sky is in danger of being lost.  The previous lack of a systematic process for preserving astronomical photographs is leading to these irreplaceable observations being discarded.  The International Astronomical Union passed a resolution in 2001 that observatories should make efforts to preserve plates, but most institutions find it hard to justify an investment of staff time or space for keeping them when they are not easily accisible in digital format.  Furthermore,  the astronomers most familiar with the nature of this material are now retiring.  In a number of recent cases photograph collections have simply been tossed out.

The Astronomical Photographic Data Archive has been established at PARI as a means to preserve this valuable astronomical record and make the collections readily available.  APDA is intended as a national archive for astronomical plates.  PARI provides climate controlled storage space and facilities for examining and measuring plates.  About 50,000 plates have been received since the archive's inception in 2006.  On-line catalogues are made available.  Scanning of selectied with an off-the-shelf tranmission scanner can be done.  But, eventually, full digitization of the collections are envisioned.  A logical extension is to include the historic photographic material in the Virtual Observatory, a major astronomical initiative to establish on-line links to a large number of data-bases of observations.

There are a number of fundamental scientific reasons for needing access to astronomy's past observations. Basically, all astronomical objects are changing, but often only very slowly.  Detecting and understanding these changes offers insight into the nature of the objects.  Some questions can only be answered by studying historical data, by exploring Time Domaiun Astronomy.

Studies of historic photographic material have often produced compelling, often definitive, data to assist the interpretation of astrophysical phenomena. Table 1, based on astronomy literature, illustrates the variety and scope of such researches. Click to see the table.  The use of the older photographic material seems to be growing.  This can probably be attributed to a rising awareness of the potential of plate archives.  Easier availability of these observations would surely increase their use.
 
ScanIt: The IAU Working Group for the Preservation and Digization of Photographic Plates Newsletter, Editor: E. Griffin (DAO) is a newsletter with topics about astronomical photographic plate preservation.