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How star trails are made
Stars look fixed in the sky, but the Earth turns beneath them at about fifteen degrees per hour. Leave a camera shutter open long enough and each star draws a curved line across the frame. The problem is knowing how long to leave the shutter open: too short and the trails are tiny dashes, too long and the sky overexposes or the battery dies mid-frame. This calculator gives you both answers, the single long exposure and the stacked alternative.
The trail speed depends on where in the sky you point. Stars on the celestial equator move fastest, at the full fifteen degrees per hour. Stars near the celestial pole crawl, because they circle a small ring around the pole. That is why the calculator asks for declination: a thirty degree arc near the equator takes about two hours, while the same arc near the pole takes far longer. Pointing at the pole gives circles, pointing at the equator gives long straight streaks, and pointing between gives graceful arcs.
Single exposure or stacking
The classic method is one very long exposure, sometimes two or three hours. It is simple and it produces smooth, unbroken trails. It also has real risks. One passing car headlight, one airplane, or one cloud can ruin the entire frame, and hours of work vanish. Sensor noise builds up on long exposures too, and the battery may not survive the whole shot. For those reasons most night photographers today stack: they shoot many shorter exposures, often thirty seconds each, and blend them in software afterwards.
Stacking gives the same total arc with far less risk. If one frame has an airplane in it, you delete that frame and the trail barely notices. Noise stays low because each frame is short, and you can check progress as the night goes on. The calculator shows both: the single exposure time for the full arc, and the per-frame time when you split the same arc across your chosen number of frames. Sixty frames of two minutes each and one frame of two hours draw the same arc, but only the stacked version lets you go home with something if the weather turns.
A worked example
Suppose you want thirty degree arcs from stars at forty-five degrees declination. The trail speed there is fifteen degrees per hour times the cosine of forty-five degrees, about 10.6 degrees per hour. Thirty degrees divided by 10.6 gives about 2.8 hours, or roughly two hours and fifty minutes. As a single exposure that is a very long shot. Split across sixty stacked frames, each frame is about two minutes and fifty seconds, which most cameras handle comfortably.
Now try the same arc near the pole, at eighty degrees declination. The cosine of eighty degrees is only 0.17, so the trail speed drops to about 2.6 degrees per hour, and thirty degrees takes over eleven hours. That is longer than a winter night in most places, which tells you something useful: near the pole, you should aim for shorter arcs or accept that full circles need the whole night. The calculator makes these trade-offs visible before you spend a night outside.
Planning a star trail night
Pick a dark site first. Light pollution washes out faint stars and turns the sky orange on long exposures, so get away from towns and check a dark-sky map. Pick a moonless night, or shoot while the moon is below the horizon, because moonlight has the same effect as light pollution. Compose with something in the foreground: a tree, a rock, or a building gives the trails scale and stops the image looking like an abstract pattern.
Focus manually on a bright star using live view, then lock the focus so it cannot drift. Shoot in manual mode with a fixed white balance, and take a few test frames to set the exposure for a single frame before starting the sequence. For stacking, an intervalometer that fires frames back to back with no gap is essential: gaps between frames break the trails into dashes. Finally, bring warm clothes and a spare battery, because a three hour sequence in the dark feels much longer than it sounds.
Frequently asked questions
Why do my trails have gaps? Gaps come from pauses between stacked frames. Set the intervalometer so the next frame starts the moment the last one ends, and keep exposures short enough that the camera can write each file before the next begins.
Do I need a star tracker for star trails? No, and in fact a tracker defeats the purpose: it follows the stars and stops them trailing. Trackers are for sharp stars, not trails. For trails you want the camera fixed to the Earth.
What about the Milky Way in the same shot? The Milky Way needs short, sharp exposures, while trails need long ones. They are different techniques, so choose one for the night or shoot both and combine them deliberately, not by accident.
Star-trail figures are starting points: your sensor’s pixel density and your print size change how much trailing the eye accepts, so treat the calculator’s times as a plan, test with a short sequence, and adjust. Use the tool above to plan the arc, choose single or stacked, and head out with a number you trust.
Related calculators
- Spanish version
- Time-Lapse Shoot Duration Calculator – Total Camera Time for Your Target Clip
- 500 Rule Calculator – Classic Maximum Exposure Before Stars Start to Trail