Star Trails
Arcs traced by stars across the frame as the Earth rotates during a very long exposure or a stacked sequence.
Formula / rule
15° per hour of rotation
Measured in
minutes
In depth
A single multi-hour exposure accumulates dark current and light pollution, so the modern method is to shoot many short frames on an intervalometer and combine them with a lighten blend. Pointing at the celestial pole yields concentric circles; pointing east or west yields long diagonal streaks.
Examples in practice
Two hundred frames of 30 seconds each, stacked, give a two-hour trail without any of the noise a single exposure would show.