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 31 international scientists gathered at PARI to develop a plan for the preservation of this astronomical photographic data. They established the Astronomical Photographic Data Archive (APDA) which is dedicated to the task of collecting, restoring, preserving and storing the data—which includes scanning each image and establishing a database that can be accessed via the Internet by the global community of scientists, researchers and students.