How did Christian Sasse create this image? Read on …

Article by Christian Sasse, originally published at his blog, The Cosmic Clock. Used with permission.
Star trail photography has gained popularity in recent years. What are star trails?
The Earth rotates or spins on its axis about every 24 hours, causing an apparent movement of the stars overhead by about one-quarter of a degree per minute. If we leave a camera in a fixed position and point upwards, open the shutter in bulb mode and let the Earth rotate under the stars, we will create an image with star trails. Similarly we could take shorter exposures and superimpose (stack) the images with image processing software.