Star Trails In Long Exposure Photography

I have often admired images like these, and I think it's pretty cool how you can tell how long the exposure is from the length of the trails ("The stars complete one full rotation in 24 hours, or 15 degrees every hour") as well as get a sense of the earth spinning over time. http://www.photographymad.com/pages/view/long-exposure-star-trail-photography

Star trails reflected in a lakeSharp star trails