Star trail calculator
Plan an interval star trail shoot before you sit in the cold: total run time, real open-shutter exposure, gaps, storage and how many degrees of arc the finished stack will cover.
Shoot plan
Seconds the shutter is open for each frame.
Time from the start of one frame to the start of the next. Equal to the exposure = no gap.
Optional extras
Average RAW (or RAW+JPEG) file size, to estimate total storage and give a sense of card and battery pressure.
Used to show how much of the frame width the star arc sweeps (assumes a full-frame sensor).
Your sequence
Trails in the sky
Storage (estimate)
How it works
A star trail sequence is usually shot as an intervalometer capture: many identical exposures, each slightly later than the last, then stacked to show the stars’ movement as lines. Because the Earth rotates at a steady 15° per hour (about 0.00417° per second), every open second adds a fixed slice of arc to the final trail.
The timeline maths is simple: the run lasts from the first frame start to the last frame end, so capture duration = (frames − 1) × interval + exposure. The open-shutter time is frames × exposure; everything else is gap time.
One honest note on gaps: a gap between frames becomes a small dark dot in every finished trail. In practice the dots are invisible once a stack is compressed into a video or a modest JPEG, and most cameras do need a beat between frames to write the file and let the sensor settle. The often-repeated claim that a “1-second gap prevents sensor heat trails” is an over-simplification - keep gaps small because continuous lines look cleaner, not because of some magic threshold.
Example
A classic plan is 120 frames at 30 s with a 2 s gap (interval 32 s). That runs for about 1 h 4 min of real time, with 1 hour of open-shutter time (the sensor is actually exposing 94% of the run) and roughly 15° of star arc in the finished stack. At ~45 MB per RAW frame that is about 5.4 GB of files to store and handle later.
Planning checklist
- Battery: an hour of continuous shooting plus live view and intervalometer drains batteries faster than the shutter time alone suggests.
- Card speed: keep the buffer clear so no frame is dropped - a fast card lets you use near-zero gaps.
- Warm-up / dew: long shoots at night mean lens dew and sensor heat; a short gap and a lens heater help.
- Test exposure first: shoot one frame and check focus and composition before committing to an hour of captures.
Related tools
Balance the long exposure against your lens with the long exposure calculator, and for non-trailed single frames check the astro exposure calculator. If you would rather shoot a single clean milky way frame, start with the milky way planner.
Frequently asked questions
What does "interval" mean here?
The interval is the time between the start of consecutive frames. If the interval equals the exposure length there is no gap between frames; a longer interval means a gap. Gaps show up as dotted breaks in the finished star trails, so most photographers keep them small (1–3 seconds).
Why are the gaps mentioned if they are so short?
Even a 1–2 second gap creates a small dark dot in the trail every frame. When the stack is compressed into a video or a long JPEG the dots are usually invisible, but for a single stills stack where you want perfectly continuous lines, zero or near-zero gaps matter. Some cameras need a short gap so the sensor can cool and the buffer can write.
Does my lens focal length change how fast stars move?
No. Every star moves across the sky at the same 15° per hour (about 0.00417° per second) because the Earth rotates. Focal length only decides how much of that arc fits across your frame - a wide lens shows the arc as a short line, a telephoto shows a long sweep across the frame.
How many frames do I need?
It depends on how long a trail you want and your exposure per frame. As a rough guide, 100–150 frames at 30 seconds each gives about an hour of open-shutter time - roughly 15° of arc. More frames simply extends the trail.