An Introduction to Star Trail Photography
What Are Star Trails?
As photographers, we are all keenly aware that the sun moves through the sky over the course of the day, dramatically changing the light from hour to hour. We know this daily march from East to West is caused by the Earth rotating beneath us, forcing us to face towards and away from the sun in turn over the course of 24 hours.
What is less familiar to our daily experience is that the stars also move, and it happens for the same reason.
The Earth spins around its axis, an imaginary line that connects the North Pole to the South Pole. If you follow this line out from both ends out of the Earth, it points you towards specific areas of the sky we refer to as the "Celestial Poles". The North Star (Polaris) sits almost exactly on top of the North Celestial Pole, which is why it is sometimes also referred to as the "Pole Star".
Figure 1. The Earth and its rotational axis. An observer standing at the North Pole looking straight up would see the North Celestial Pole (and the North Star). An observer at the South Pole looking straight up would see the South Celestial Pole.
Because Polaris sits on the same line that the Earth rotates around, an observer in the northern hemisphere would notice that the North Star - alone of all the objects in the sky - does not move over the course of the night. It would be very much like looking up at a light attached to a pole in the center of a merry-go-round. If you turn to face the light, it'll appear stationary even though everything around it wheels about.
Observers in the southern hemisphere unfortunately do not have a comparably bright Pole Star of their own near the South Celestial Pole, but the principles are exactly the same. Namely, as the night progresses, all the stars will appear to move in concentric circles, with the relevant Celestial Pole at their center:
Figure 2. The expected paths of each of the stars in the Little Dipper, with Polaris, the North Star, at the center, stationary.
If one were to make an exposure for some fraction of the night, the resulting image would show a series of arcs appearing to rotate around the Celestial Pole:
Figure 3. Simulation of star trails formed over 2 hours looking at the Little Dipper in the northern hemisphere.
These are our star trails.
At this point you should note two things that will be important compositional considerations:
- The stars move "east" around their Celestial Pole (counter-clockwise in the northern hemisphere; clockwise in the southern)
- The further from the Celestial Pole a star is, the longer its trail will be for a given exposure time
