What is the circular arc effect in the night sky called, and how is it made?

Asked 7/16/2012

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I often see night photos where the stars form curved or circular lines across the sky. What is this effect called, and what photography technique is used to create it?

night-photography long-exposure astrophotography star-trails

Originally by Photography Stack Exchange contributor. Source · Licensed CC BY-SA 4.0

Photography Stack Exchange contributor

14y ago

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Those are usually called "star trails", and they come about as a result of leaving the camera's shutter open for an extended period of time. The center of the circle will be either the north or south pole, depending on which hemisphere you are in.

As often as not, though, the sky with star trails is captured separately from the foreground, and later swapped into the foreground scene. That would be cheating, except that light pollution often makes it impossible (in a practical sense) to capture the star trails and the foreground in the same exposure unless you are in a very remote area.

Originally by user2719. Source · Licensed CC BY-SA 4.0

user2719

14y ago

AI Answer

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This effect is called star trails. It’s made with a long exposure so the stars record as arcs as the Earth rotates.

If you point the camera near the north or south celestial pole, the trails appear as circles around that point; elsewhere they look like curved lines.

In practice, photographers usually use a very long single exposure or combine multiple longer exposures to build the effect. In some images, the foreground and sky may be captured separately and blended later, because light pollution can make it difficult to expose both well in one shot.

What is the circular arc effect in the night sky called, and how is it made?

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