Star Trail Calculator
Enter your lens, camera and interval timer settings to see how long the trails will be in the finished stack and whether the gaps will show.
Star trails
How far the stars travel across your frame during a star trail sequence.
0° gives the longest, straightest trails. Polaris is near +89°.
Interval timer delay plus the time the camera takes to save.
Trail length in the stack
6,308 px
31° 4.8′ on the sky over 2 h 4 min.
- Total session time
- 2 h 4 min
- Trail on the sky
- 1,864.8′
- Trail on the sky (degrees)
- 31.08°
- Rotation around the pole
- 31.1°
- Trail in one frame
- 25.4 px
- Movement during each gap
- 0.85 px
- Pixel scale
- 17.74″/px
Stars move less than a pixel between frames, so the stacked trails should look continuous.
Stars circle the pole at about 15° per hour. A full circle needs close to 24 hours, so in one night you capture an arc.
How this is calculated
- 1
Session length
Total time is frames × exposure + (frames - 1) × gap. 240 frames of 30 s with a 1 s gap take 7,439 s, just over 2 hours.
- 2
Trail length on the sky
Stars move at 15.041″ per second of time multiplied by cos(declination). Over 7,439 s a star on the celestial equator travels about 111,900″, or 31°.
- 3
Trail length in pixels
The pixel scale is 206.265 × pixel size (µm) / focal length (mm), so 4.3 µm pixels behind a 50 mm lens give 17.7″ per pixel. Dividing the trail length by the pixel scale gives the length on the sensor.
- 4
Rotation and gaps
The sky turns about the pole by 15.041″ × time, which is about 15° per hour. A star moving more than about a pixel during each gap leaves a visible break, which usually means keeping gaps to 1 s or less.
Frequently asked questions
How long does it take to get full circle star trails?
A complete circle takes one sidereal day, 23 h 56 min, so it is impossible in a single night. A 4 hour session gives arcs of about 60°, which already looks dramatic around the pole.
What interval should I use for star trails?
Set the timer so the gap between frames is 1 s or less. Longer gaps leave dotted trails, especially with longer lenses where the stars move more pixels per second.
What exposure is best for star trails?
Usually 20 to 60 s at ISO 400 to 1600. Under city skies, such as in Lahore or Karachi, shorter exposures stop the sky glow from washing out the frame, and the stacking software joins them into one trail.
Why are star trails shorter near Polaris?
Every star turns through the same angle, but stars near the pole travel round a small circle. The arc length scales with cos(declination), so a star at +80° moves less than a fifth as far as one on the equator.