On the face of it, telescopes and data bases sound like very different things. Telescopes sit on the top of mountains, and are pointed at the skies; data bases sit on computer hard disks, humming away and going nowhere. Yet they have something in common; both allow astronomers to explore the universe. Modern telescopes are highly automated pieces of machinery equipped with digital sensors that produce reams of observational data. Such data can be stored, processed and distributed just like other digital information. This means it is no longer necessary for astronomers actually to visit a telescope to make observations. ‘Data bases sit on computer disks, humming away’ implies —