They move cuz the axis of the earth moves - we be on a wobbly planet.
This is so cool - I want to see this phenomenon in person before I die.
I have seen it, in northern Michigan. Put it together with the numbing cold, the layers and layers of clothing, the fog of your breath, the absolute pristine snow and the deafening silence of winter at night, and you have a memory that will stay with you forever.
CC