>>4475272
Basically if the earth is a spinning top, you're using the polar alignment scope (that runs through the center of the mount's head, perpendicular to the axis of rotation) to create a parallel spinning top that spins at an equal rate but reverse direction (which has the effect of freezing the night sky while the earth rotates underneath you from the perspective of a camera sitting on top).

It has a motor in it that matches the rate of rotation for you, but the hard part is trying to perfectly align this spinning top you're setting up with the earth's own axis of spin. The extent that you get this wrong is the extent that the two rotations (yours and earths) will no longer be in sync and your photos will blur/streak in proportion to how long your exposure is.

Everyone solves this through either short exposures + stacking, or through setting up an autoguiding system where a camera will lock onto stars and compensate for any alignment/rate errors. I was just surprised/happy I got a fairly clean image for that long of an exposure using a dummy system that won't compensate for my errors.