This video shows what happens when a Photoshop CS5 stacking action runs to take 54 one minute exposures and combine them into one stacked image.

Notice that the shadow doesn't move when stacking - because the first frames show the lightest portions anyway.

For more information and description about how this all works, see BLOG.StarCircleAcademy.com

And in particular these articles:
blog.starcircleacademy.com/2011/02/automated-stacking-of-star-trails-in-ps-cs5/
and
blog.starcircleacademy.com/2011/03/star-trail-creation/

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