Key takeaways
- The sky turns about 15 degrees every hour, so each star draws an arc of 15 degrees in an hour and 60 degrees in four.
- Take many exposures of 20 to 30 seconds with a gap of one second, for one to three hours, and combine them with a lighten blend.
- Pointing north with Polaris in the frame gives concentric circles, while east or west gives diagonal streaks and south gives gentle curves.
- A foreground such as a tree or a boat house gives the arcs a place to rise from. Take one extra frame at dusk to expose it well.
To photograph star trails, take many short exposures of 20 to 30 seconds with a gap of about one second, let the camera run for an hour or more, and combine the frames with a lighten blend. The stars draw arcs because the sky appears to turn 15 degrees every hour.
Star trails are among the most satisfying pictures a beginner can make, because the equipment is the same as for any night photograph, and the result is dramatic. Each star draws a bright arc across the frame, and the pattern shows the rotation of the earth in a single glance.
The sky turns fifteen degrees every hour
The earth rotates once in a day, so the stars appear to move at 15 degrees per hour. That is easy to remember, and it tells you how long to shoot.
| Time | Length of each arc | Frames needed |
|---|---|---|
| 30 minutes | 7.5° | About 70 |
| 1 hour | 15° | About 140 |
| 2 hours | 30° | About 280 |
| 3 hours | 45° | About 415 |
| 4 hours | 60°, a sixth of a circle | About 550 |
| 6 hours | 90° | About 830 |
The arcs are centred on the celestial pole, which in the north lies very close to Polaris, less than a degree away. At the latitude of Finland, Polaris stands high above the horizon, at an altitude equal to your latitude, so a wide lens that points north and includes the star will give you concentric circles.
| Pointing | The trails look like |
|---|---|
| North, with Polaris in the frame | Concentric circles around the pole |
| East or west | Straight, diagonal streaks |
| South | Gentle curves |
Why not one long exposure?
You could in principle keep the shutter open for two hours. In practice a single exposure of that length would fill with heat noise from the sensor, brighten the sky to a pale grey and ruin the whole result if a car's headlights swept across it or a passing person tripped over a tripod leg. A series of short exposures lets you throw away the damaged ones and keeps the background dark.
| Feature | One long exposure | Many short exposures |
|---|---|---|
| Sensor noise | Builds up as the sensor warms | Stays low in every frame |
| The sky | Brightens to a pale grey | Stays dark |
| A disturbance such as headlights | Ruins the whole picture | Spoils a frame or two, which you discard |
| Battery | Drained for a single result | Easier to manage, and you can swap it between frames if needed |
| The foreground | Hard to treat separately | Easy to blend in from a dusk frame |
A recipe that works
- Compose. Choose a foreground and point a wide lens north with Polaris in the frame for circles, on a steady tripod.
- Focus. Focus on a star in live view, switch to manual focus and leave it alone.
- Set the exposure. Use a wide aperture, 20 to 30 seconds and ISO 800 to 1600, and turn off long exposure noise reduction.
- Start the sequence. Set continuous shooting or an interval timer with a one second gap, and check that the battery will last, using a spare or a power pack in the cold.
- Let it run. Leave the camera for one to three hours, and discard any frames spoiled by headlights or passers-by.
- Combine the frames. Stack the frames with a lighten blend so that every pixel keeps its brightest value, and blend in a dusk frame for the foreground.
Turn off long exposure noise reduction, since it inserts a long gap after each frame. The settings are explained further in your first night with a camera and a tripod.
Combining the frames
Free programs stack the frames with a "lighten" blend, which keeps the brightest value of every pixel from all frames. The stars add up to trails, while the sky stays at the level of a single exposure. If you see tiny gaps in the trails, they come from the pause between frames, which is why the delay should be as short as your camera permits. This is a different kind of stacking from the averaging that cleans up a faint nebula, which is described in why a hundred frames beat one.
Composition makes the picture
Trails are only half of the picture. A tree, a boat house or a rocky shoreline in the foreground gives the viewer a place to stand, and the arcs seem to rise from it. Take one frame while there is still dusk light to expose the foreground well, and blend it in afterwards.
The arcs are the rotation of the earth, drawn in light.
Frequently asked questions
How long should star trail exposures be?
Each frame can be 20 to 30 seconds, and the total time is what makes the trails. One hour gives arcs of 15 degrees, and two or three hours give long, dramatic curves. Take as many frames as you need to fill that time.
Why are there gaps in my star trails?
The gaps come from the pause between frames. Use continuous shooting or an interval timer with the shortest delay your camera allows, usually about one second, and turn off long exposure noise reduction, which adds a long pause after each frame.
Do I need a star tracker for star trails?
No, and a tracker would defeat the purpose, because it moves the camera with the sky and keeps the stars as points. A fixed tripod lets the sky turn past the camera and draw the trails.
Which direction should I point for the best star trails?
North, with Polaris in the frame, gives concentric circles and is the classic choice from Finland. East or west gives diagonal streaks, and south gives gentle curves.
What ISO should I use for star trails?
Between 800 and 1600 is a good start with a wide aperture and 20 to 30 seconds. Choose the lowest value that still records the stars clearly while keeping the sky background dark.
Sources and further reading
AstroNight is now on the App Store. The free planner and coach for astrophotography works on iPhone and iPad with iOS 17 or later, offline, with no account. Get it on the App Store.
