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Star Trails As the Earth Rotates

The ancients, observing the sky, reasoned that, since the Earth beneath their feet was solid and unmoving, the heavens must rotate about the land. Now we know that's an illusion. The stars are "fixed" and the Earth rotates. Star trail photos are striking images that provide visual proof of the relative movement of Earth and sky.

Star Trail Photography
- With a camera not pointed to the pole, short arcs can look like straight lines.
  
  Star trail pictures are produced by pointing a camera at a fixed spot in space and keeping the shutter open for a while. "A while" can be anything from 15 minutes to hours. During that time, the rotation of the Earth will move the axis of the camera's field of view relative to the stars. If the camera is pointed to the North (or South) Pole, the exposed image will look like a series of concentric arcs centered on the pole. If pointed elsewhere, the arc may not be that evident. The method is pretty straightforward, but some attention to detail is required.
A Traditional Camera
- A traditional camera recording on physical film can easily get over- or under-exposed. High-speed film, like ISO 800 or 1000 could be a good choice to pick up even faint star trails, but it will also be more sensitive to background and stray light, so some astrophotographers use ISO 100 film. The F/# of the lens also needs to be adjusted properly. If you select a fast F/#, like F/2.4, starlight will record a clear trail, but the background light may wash out the trails. On the other hand, a slow F/#, like F/11, will cut down on the background light, but will also not record many star trails.
Digital Cameras
- Digital cameras offer different advantages, but their own set of concerns. Because the images are recorded digitally they can also be modified digitally -- including being added together. This means the exposure can be set to a moderate time, say 30 seconds. The F/# can be adjusted to collect light from relatively dim stars and still not collect too much background light. Then take one photo after another. If taking an exposure for more than a few minutes, this can rapidly grow tedious, so a remote release timer becomes a helpful accessory. The timer will allow you to set the exposure length and the overall number of exposures. After the images are collected, put them in a digital photo editor -- some programs are available for specifically processing star trails -- and assemble the individual images into the star trail exposure.
Using Star Trails
- In a star trail image, anything that doesn't follow the same kind of track is not a star -- like the meteor seen here.
  
  As mentioned in the first section, the result of a star trail exposure is a series of arcs representing the path of the stars across the sky. But when an image is created for such a long time, there are likely to be other objects that cross the field. A star trail can capture meteors, planets, comets, and asteroids -- all of which will not follow the same arcing paths as the surrounding stars. Modified star trail techniques have been used to detect similar objects, and not just for the pretty pictures.

Astronomy