Star Trail Stacking Calculator
Star trails are built by stacking many short exposures so the stars draw continuous arcs across the frame. To plan a session you need to know how many frames to shoot for the trail length you want and how long the whole sequence will take. This calculator returns the frame count, the arc length the stars will sweep, and the total session time, from your target trail duration, sub-exposure length, and the gap between frames. Stars rotate at the sidereal rate of about 15.04 degrees per hour.
Star trail formula
Cycle = sub-exposure + gap (seconds)
Frames = ceil( trail seconds / cycle )
Arc = trail hours * 15.041 degrees per hour
Session = frames * cycle
The 15.041 figure is 360 degrees divided by one sidereal day of 23 hours 56 minutes 4 seconds. Keeping the gap small avoids visible breaks in the trails.
Worked example
A 60 minute trail with 30 second sub-exposures and a 1 second gap: cycle = 31 seconds; frames = ceil(3,600 / 31) = ceil(116.13) = 117. Arc = 1 hour * 15.041 = 15.04 degrees. Session = 117 * 31 = 3,627 seconds = 60.45 minutes.
Star trail stacking: frequently asked questions
How many frames do I need for a star trail?
Divide the total trail duration you want by the cycle time of one frame (exposure plus the gap before the next shot). For a 60 minute trail with 30 second exposures and a 1 second gap, each cycle is 31 seconds, so you need about 117 frames.
How long an arc will the stars trail?
Stars appear to rotate 360 degrees in about 23 hours 56 minutes (one sidereal day), which is 15.041 degrees per hour. Multiply that rate by your total trail duration in hours to get the arc length. A 1 hour trail spans about 15 degrees.
Why stack many short frames instead of one long exposure?
Short sub-exposures keep noise and light pollution under control and let you discard frames spoiled by planes or clouds. Stacking software combines them with a lighten blend so the trails join into continuous arcs. Keep the gap between frames small to avoid dashed trails.
Sources
- US Naval Observatory: Astronomical Applications Department (sidereal time).
- The 15.041 degrees per hour rotation rate is 360 degrees per sidereal day (23 h 56 m 04 s).
Reviewed by the CalculatorHub team, edited by James Graham, 19 June 2026. See our methodology.