Free calculator · runs in your browser

Star Trail Calculator

How long a star trail you'll draw over an exposure — the sky turns ~15° an hour, but a star's declination sets how long its arc is. The same engine the Viewpoint app uses, in your browser.

Trail arc length—
Sky rotation (any star)—
In the app

Find the dark, plan the rotation

Star trails need a dark sky and the right framing. Viewpoint finds the night-sky window for your location, maps where the celestial pole sits, tracks the moon's brightness, and works offline under the stars. The trail math here is the same the app runs.

Start free trial Open the web app

How star trails work

The whole sky appears to rotate about the celestial pole at roughly 15° per hour (360° in ~24 h). A star near the pole (high declination) traces a tight, short arc; a star near the celestial equator (declination 0°) sweeps the longest arc. This calculator gives the arc length a star at your chosen declination draws over the exposure, plus the total sky rotation for that duration.

Tips

  • Point near the pole (Polaris in the north) for concentric circles; aim at the equator for long, dramatic streaks.
  • Stack many shorter frames rather than one very long one to control noise and plane/satellite trails.

Related

For single-frame star sharpness, see the astro shutter (500-rule / NPF) calculator. Both are in the Viewpoint app.

For the settings and stacking workflow, read How to shoot star trails.

More calculators

Depth of fieldNear & far focus limits and total depth of field. Hyperfocal distanceThe focus distance that's sharp to infinity. Exposure value (EV)A single value from aperture, shutter and ISO. ND & long exposureYour shutter time with a neutral-density filter. Astro shutterSharp stars via the 500 and NPF rules.