Everyone knows the experience: a pile of old photos in a dusty attic — or more likely now on a dusty disk drive — with no indication of who is in them or when they were taken. As long as the subjects are astronomical features, though, there's now an answer at hand.

Astrometry.net is an open-source software project, run out of the University of Toronto in Canada and New York University, which aims to recognize any starscape and place it in its proper coordinates within seconds — specifying not just which patch of the sky is shown, but when.