Star Trailing
Star trailing is stars rendering as short streaks instead of clean points in a photograph, caused by Earth's rotation during an exposure long enough for that apparent motion to become visible. It happens even with a perfectly stationary, correctly focused camera, since the sky itself is what's moving relative to the ground.
How quickly trailing becomes visible depends on focal length (longer lenses show it sooner), sensor pixel pitch (smaller pixels reveal it sooner), and the target's declination (stars near the celestial equator trail fastest, stars near the celestial pole trail slowest). The 500 rule and NPF rule both estimate the maximum exposure time before trailing appears, from different levels of accuracy. Deliberately embracing trailing, rather than avoiding it, is its own genre — star-trail photography intentionally uses long or stacked exposures to capture the visible arcs.
Frequently Asked Questions
Is star trailing always undesirable?
No — avoiding it is the goal for pinpoint Milky Way and deep-sky photography, but star-trail photography deliberately embraces it, using long or stacked exposures to render the sky's rotation as visible arcs around the celestial pole.
Does a star tracker eliminate star trailing entirely?
Yes, for practical purposes — it physically rotates the camera to counteract the sky's apparent motion, removing the trailing constraint that limits stationary-tripod exposures to seconds rather than minutes.