Long Exposure and Time-Lapse Star Trails is a method of capturing the movement of Stars over the heavens. Star-Trail photography shows circles, lines or arced lines moving over the heavens depending where you point your camera. You get nice circular Star-Trails when you point your camera to the poles, but you can get straight lines as well when you point your camera away from the poles.
Time-Lapse is the process where you take multiple short exposure photographs with the help of a cable-release and stack them together to get the majestic star trails.
Star Trails reflects the rotation of Earth on its axis, in relation to the stars, and capturing the “movement” of the stars onto ling exposure or time-lapse photo techniques. One full rotation takes approximately 24 hours (give or take a couple of minutes), but from our point of view it would be difficult to capture a full rotation of stars due to luminescence of the day.
Note: The Earth rotates at approximately 15 degrees per hour, and you should be able to capture this depending on the lens and camera you are using. The stars moves from east to west, just like the sun rises in east and sets in the west. The position of the stars is not always the same as the Earth is spinning at an inclination of between 22.1 and 25.5 degrees and depends as well of the position of earth around the sun.
15 Seconds: Capturing star trails are relatively easy with the right camera settings (as discussed in other tutorials). But the primary thing to note that star trails usually start at between 10 and 15 seconds exposure.
Length: The perception of length of the Star Trails lines is dependent on the lens you are using, for example using an 18mm lens would generate short lines, and by using a 300mm lens would generate extremely long lines. The downside again of using a long lens though is that you lose some of your foreground objects.
Interesting fact: Star Circles are formed at the celestial poles and stars that make a full circle (not crossing the horizon are called circumpolar stars.