Sky Watch: Capture 'star trails' on camera

Most people know that Earth's rotation causes the sun to rise in the east and set in the west. Of course, the same is true for the moon, planets and stars.

To understand what's happening, try this experiment. Stand in the middle of a room and, keeping your feet on the same spot, turn slowly from right to left. While the Earth's axis of rotation cuts through our planet from the South Pole to the North Pole, yours runs from your feet to your head.

While turning slowly, pay attention to the room around you. You'll notice that objects will appear on your left, drift across your field of view toward the right, and disappear there, just like much of the sky we see.

Now, while continuing to turn slowly, look above your North Pole -- your head. (Be careful not to fall over while doing this!) Watch the ceiling as you turn, and you'll find that the spot directly over your "north" axis remains still, and those nearby seem to circle it without ever "rising" or "setting."

The same is true for the stars.The point directly above the Earth's North Pole is known as the north celestial pole; it's marked by the North Star, Polaris, and all northern stars appear to wheel continuously around it. Most of us have never stayed out for a full night to watch this grand procession, but you can effectively capture it in a photograph.

The easiest way to shoot star trails is with a film camera. What you'll need is a camera that you can set to manual modes (exposure and focus), a normal or wide-angle lens, a locking remote control or cable release, a solid tripod and some fast (ISO 200 or 400) color film.

From a dark, non-light-polluted area, aim your camera toward Polaris, set your lens to around f/5.6 or so, focus on infinity (the figure-8 symbol in your camera) and take several exposures at "B" (bulb mode) -- say, 10, 30 or 60 minutes. When your film is processed, you'll discover that the stars appear trailed in concentric circles around the north celestial pole, and you'll also notice just how many airplanes cross the sky during the night!

With digital, however, one can take long exposures only with high-end cameras, so after you compose and focus, make some test shots of the scene to determine a good exposure (e.g., f/2.8, ISO 800, 30 seconds). Then, with a fully charged battery and your camera set to "continuous shooting" mode, lock your remote "on" and let the images crank out automatically for as long as you like. When you're finished, you will likely have dozens or hundreds of images.

Here's where the fun begins. If you have a PC, visit StarTrails.de and download the free software called Startrails. In two clicks of your mouse, you'll create a perfectly exposed star trail image.