Fast Geometric Hashing for Automated Astrometry is a fascinating presentation about a solution to the problem of examining a photo of a section of the sky, and figuring out what part of the sky it is. It involves geometry, and algorithms, a clever hashing algorithm for that returns the same result independent of rotation and scale, mapping, search trees, and so on.
I'm not much of an astronomer: I didn't realize for example, that the catalog of the sky included roughly a billion objects! I'd never heard of HEALPix, a scheme for dividing spheres into uniform grids (pixelizing them). And kd-trees are a data structure for dividing up space, kind of like a 3D tree map.
Plenty of goodies for nerds of all sorts.
PS: the whole thing has been hooked up with the flickr API to automatically identify objects in astronomy pictures on flickr. Very cool.