No, you're correct... it is difficult. It can be done manually with various mechanical contraptions (cheaper, sometimes) or it can be done with computerized systems. With even the simplest of mechanical trackers (such as a "barn door tracker"
the rotation of the earth can be accounted for well enough to do reasonably long exposure times (a few minutes or more) at lesser focal lengths. With a top-notch computerized tracker, people can do exposure times for as long as their camera allows and at longer focal lengths.
Everything I just said is an oversimplification, but my point is to confirm that it is quite difficult to do astrophotography using ultra-long exposures. Stacking helps eliminate the "trailing" by allowing for the stacking of exposures rather than using one, longer exposure, but it's not a perfect solution. I'd like to get into the hobby someday, and I'll probably build a barn door tracker just to gain more appreciation for the astrophysics behind the necessity for using one.
*edit*
It just struck me that you were referring to the original video and the physical panning of the camera. Yes, that's also difficult, and works in similar principal to what I described with the astrophotography... only the camera is the Earth and Yosemite is the cosmos, heh. It's generally done using a mechanized dolly and an expensive, highly precise tripod head.
Edited 1 time(s). Last edit at 01/26/2012 07:15PM by dqniel.