Star Trail Calculator — Plan Your Star Trail Photography

Stars sweep across the sky at about 15° per hour. For short arcs of 15-30°, allow roughly 1-2 hours (1 hour gives 15°, 2 hours gives 30°). A full night of 6-8 hours produces long sweeping arcs of around 90-120°; a complete 360° circle would need a full ~24-hour sidereal day and cannot be captured in a single night. Most impressive star trail images are 1-3 hours long, creating arcs of 15-45 degrees.

Stacking multiple shorter exposures (typically 15-30 seconds each) is strongly recommended. It reduces noise, prevents overexposure from light pollution, allows you to remove frames affected by aircraft or clouds, and avoids the battery drain and sensor heating of very long single exposures.

Keep individual stacked frames within the 500 rule so each one stays pinpoint sharp: maximum seconds ≈ 500 ÷ (focal length × crop factor). A 24 mm lens on a full-frame body allows about 21 seconds; the same lens on an APS-C body (1.5×) allows about 14 seconds. This calculator shows the 500-rule limit for your focal length and sensor so you can set a sub-exposure that keeps stars as points within each frame before they are joined into the trail.

Most cameras need 1-3 seconds between shots for the shutter to reset and the image to write to the card. This gap creates small breaks in the trails, though these are usually invisible at normal viewing sizes. An intervalometer automates the timing.

For circular trails, point towards the celestial pole (Polaris in the northern hemisphere). For straight trails, point east or west along the celestial equator. For a mix of curved and straight trails, point roughly north-east or north-west.