What geotagging is to ordinary photography, astrotagging is to astrophotography — embedding machine-readable data that identifies the location of a photo. But astrophotography is harder: orientation, field of view and pixel scale come into play if you want to map a photo on Google Sky or WorldWide Telescope. And it’s not like GPS can help you here. Here’s one way of doing it. Astrometry.net’s robot takes photos uploaded to the Flickr astrometry group or the Astronomy Photographer of the Year group and looks for matching star patterns. Having identified the image, it then adds tags for the image’s location (right ascension, declination), pixel scale, orientation and field size, as well as tags for any interesting catalogue objects like galaxies or nebulae.

Astrotags used in this manner are explained here, here and here; see this page on how to access Astrometry.net’s tags through the Flickr API.

Via Ogle Earth.