Here's a video clip of Pink Floyd playing "Astronomy Domine." According to the file name, it's from a 1967 performance at the Beat Club. I must have downloaded this during my Soulseek days. Now you can download it, too, because I gave in and uploaded the file to RapidShare.
Postscript. There's an extended version on YouTube that includes some insightful commentary and analysis by Hans Keller.