Star Trail Calculator

Star trail photography captures the apparent circular movement of stars across the sky caused by Earth’s rotation. The length and curvature of the trails depend on exposure time and the star’s angular distance from the celestial poles.

Formulas: Trail Length (degrees) = Exposure Time (hours) × 15°/hour Trail Length (mm on sensor) = Trail Length° × (π ÷ 180) × Focal Length Stacked Exposures Method: Number of Shots = Total Duration (min) ÷ Single Exposure (sec) × 60

Why 15 degrees per hour: Earth rotates 360° in ~24 hours: 360 ÷ 24 = 15°/hour = 0.25°/minute = 15 arcseconds/second.

Stars near Polaris (the North Star) trace tight circles; stars near the celestial equator trace nearly straight, long arcs.

Single long exposure vs. image stacking:

• Single long exposure (30+ minutes): Requires dark skies, no light pollution changes, and a camera that handles long exposures well. Risk of amp glow and noise.
• Image stacking (recommended): Take many short exposures (e.g., 20–30 seconds at ISO 1600–3200) and blend them in software (Startrails, Sequator, or Photoshop). Reduces noise, avoids sensor heating, and lets you remove airplane trails.

Worked example: You want trails at least 20° long to be visually dramatic. Using a 24mm lens.

Required exposure = 20° ÷ 15°/hr = 1.33 hours = 80 minutes

Using the stacking method with 25-second exposures: Number of shots = (80 × 60) ÷ 25 = 4800 ÷ 25 = 192 shots

Trail length on sensor = 20 × (π ÷ 180) × 24mm = 0.349 × 24 = 8.4mm

Camera settings guide:

• Aperture: f/2.8 or wider to gather as much light as possible
• ISO: 800–3200 depending on light pollution
• Focus: Manual focus to infinity (use Live View and zoom to a bright star)
• Interval: 1–2 seconds between shots for seamless stacking

Best nights: New moon phase, clear skies, minimum humidity. Aim the camera toward Polaris (Northern Hemisphere) for concentric circular trails.