Short-term changes can be studied through new observations, but many changes happen slowly and need to be observed for years, even decades, to understand the variation.
Fortunately, astronomical observations spanning more than 130 years were archived and stored on photographic glass plates and film all around the world. There are an estimated one to two million plates in North America alone.
In 2007, a group of 32 international scientists gathered at PARI for a “Workshop on a National Plan for preserving Astronomical Photographic Data” made possible by PARI and the National Science Foundation. During the workshop, participants adopted and recommended that PARI establish a national plate repository that would be known as the Astronomical Photographic Data Archive (ADPA). This library would be dedicated to the task of collecting, preserving and storing the data and to establish an online plate archive database accessible by global community of scientist, researchers and students.