The streaks stars leave on a photo taken from a fixed tripod, because of the Earth’s rotation. Unwanted in a sharp image, they become an artistic effect when hours of exposures are stacked: the stars then draw circles around the celestial pole.
Related terms
- NPF rule
A formula by Frédéric Michaud (Société astronomique de France) for the longest exposure without star trails on a fixed tripod: t (s) = (35 × N + 30 × p) ÷ f, where N is the f-number, p the pixel size in µm and f the focal length in mm. At f/2.8 with 4 µm pixels and a 24 mm lens it gives about 9 s. It is more reliable than the old 500 rule with today’s sensors.
- Celestial poles
The two points in the sky around which all stars seem to turn: the extension of the Earth’s axis. The north celestial pole lies very close to Polaris; the south pole has no bright star. The pole’s height above the horizon equals your latitude.
- Tracking
The motion that cancels the Earth’s rotation to keep an object in view. At the sidereal rate the mount makes one turn in 23 h 56 min, about 15 arcseconds per second. At 200× with a 50° eyepiece, an untracked star near the celestial equator crosses the field in about a minute.
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