A star trail is a type of photograph that uses long-exposure times to capture the apparent motion of stars in the night sky due to Earth's rotation. A star-trail photograph shows individual stars as streaks across the image, with longer exposures resulting in longer arcs. Typical exposure times for a star trail range from 15 minutes to several hours, requiring a "Bulb" setting on the camera to open the shutter for a period longer than usual.
Capturing star trail images
Star trail photographs are captured by placing a camera on a tripod, pointing the lens toward the night sky, and allowing the shutter to stay open for a long period of time. Star trails are considered relatively easy for amateur astrophotographers to create. Photographers generally make these images by using an SLR camera with its lens focus set to infinity. A cable release allows the photographer to hold the shutter open for the desired amount of time. Typical exposure times range from 15 minutes to many hours long, depending on the desired length of the star trail arcs for the image. Even though star trail pictures are created under low-light conditions, long exposure times allow fast films, such as ISO 200 and ISO 400. Wide-apertures, such as f/5.6 and f/4, are recommended for star trails.
Because exposure times for star trail photographs can be several hours long, camera batteries can be easily depleted. Mechanical cameras that do not require a battery to open and close the shutter have an advantage over more modern film and digital cameras that rely on battery power. On these cameras, the Bulb, or B, exposure setting keeps the shutter open. Another problem that digital cameras encounter is an increase in detector noise with increasing exposure time.
American astronaut Don Pettit recorded star trails with a digital camera from the International Space Station in Earth orbit between April and June, 2012. Pettit described his technique as follows: "My star trail images are made by taking a time exposure of about 10 to 15 minutes. However, with modern digital cameras, 30 seconds is about the longest exposure possible, due to electronic detector noise effectively snowing out the image. To achieve the longer exposures I do what many amateur astronomers do. I take multiple 30-second exposures, then 'stack' them using imaging software, thus producing the longer exposure."
Rotation of the Earth
Star trail photographs are possible because of the rotation of the Earth on its axis. The apparent motion of the stars is recorded as streaks on the film or detector. For observers in the northern hemisphere, aiming the camera towards the north creates an image with concentric circular streaks centered on the north celestial pole (very close to Polaris). For observers located in the southern hemisphere, this same effect is achieved by aiming the camera south. In this case, the streaks are centered on the south celestial pole. Aiming the camera towards the east or west creates straight-line streaks that are angled with respect to the horizon. The size of the angle depends on the photographer's latitude.
Astronomical site testing
Star trail photographs can be used by astronomers to determine the quality of a location for telescope observations. Star trail observations of Polaris have been used to measure the quality of seeing in the atmosphere, and the vibrations in telescope mounting systems. The first recorded suggestion of this technique is from E.S. Skinner's 1931 book A Manual of Celestial Photography. 
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