If you poke around any observatory that’s been around a while, you’re likely to find a room filled with treasure: pictures of the sky shot on glass plates. Many of the plates are old and brittle. Some have been damaged by water or other hazards. But almost all of them still have scientific value. And astronomers are trying to preserve them in both physical and digital form. The first glass-plate images were shot in the mid-1800s. Over the next century or so, they were the main way to record the universe. Some are just a few inches per side, while many others are close to a foot across. Today, astronomers use those plates to study how the universe has changed. They can watch the expansion of clouds of gas and dust ejected from dying stars, for example. They can extend their plots of the motions of binary stars, asteroids, and many other objects, providing better calculations of their orbits. And they can see how stars have changed brightness, helping understand what caused the changes i
