Originally posted by UofM.Physicist
The typical shutter speed on a handheld digital camera is 1/3 second.
Not in dark conditions. The "shutter" will stay open for as long as it takes the camera to record what it thinks is a good image.
Now, if the camera happens to move in that period of time, you'll get shadowing that looks like a double exposure. With the photo in question, the person opened the shutter for short period of time, then moved it to better match the path of the thing in the sky.