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Star Trails – How Do They Get Formed?
Understanding how star trails are formed is crucial to nailing the composition. This is also important to consider when selecting a location. Star trails are essentially formed by the (apparent) motion of the stars in the sky due to the rotation of the earth. Every 4 minutes, the earth rotates exactly 1° (360° in 24 hours). Thus, for a viewer on earth, it would seem as though the stars track 1° of their complete arc through the sky every 4 minutes. This may not seem like much, but over the long exposures needed for star trails, those minutes and degrees add up.
It is essential to figure out the (apparent) motion of the stars themselves. The earth rotates around its polar axis. Thus, for a viewer on earth, the (apparent) motion of stars would be around the same axis. This means that the stars track a circle around this axis. How do you find the center point of rotation of an imaginary axis?
Conveniently for earth’s Northern Hemisphere, there is a star called Polaris (the Pole Star) whose location is just about in line with that imaginary axis. This means that this particular star does not appear to move a whole lot in the sky. If you can find this star, it will become your new reference to visualize the star trails, as this star will be the center of rotation for all the other stars. This article goes into more details about the apparent motion of stars around the polar axis.
Unfortunately, the Southern Hemisphere does not have a similar star. A photographer down under will have to rely on locations of other prominent constellations, such as the Southern Cross, to estimate the center of rotation in the Southern Hemisphere.
There are many ways to find the Pole Star. Perhaps the easiest method is to track the 2 stars that define the front edge of the bowl of the “ladle” formed by the Big Dipper constellation.
Now that we have found the center of rotation, how much will the stars rotate? As I mentioned earlier, stars track 1° of motion in the sky every 4 minutes. For example, 2 hours of stacked star trail shots are worth about 40° of motion, and would track 1/9th of a full circle in the sky.
In what directions will the stars appear to rotate? All the stars in the sky trace concentric circles around both the north and south polar axes except for the Pole Star, but their curvatures appear to change on either side of the equatorial plane.
This brings me to the next point: the equatorial plane. Finding it can be tricky and usually requires some approximations and trial exposures. The equatorial plane of your star trails is an extension of the equatorial plane of the earth. Its stars lie halfway between the opposite tips of the Polar axis in relation to the earth.
Using the example above, the 40° arcs closer to each pole in your star trail imagery will have a shorter length than the 40° arcs further away from the axis because the circles they describe are smaller than those closer to the equatorial plane. Likewise, the curvature of these arcs flattens out the closer they are to the equatorial plane. On this imaginary line of the equatorial plane, all the stars seem to move in a straight line, and on either side of it, the stars curve away in opposite directions toward the north and south poles. Image #2 illustrates the location of the equatorial plane.
The main advantage of shooting this plane is that the motion of stars close to this plane is rather fast and they create rather long trails even for short exposures.
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