RigTonightCivil, nautical and astronomical twilight: when is it dark enough for astrophotography?
The sky doesn't go dark at sunset. Sunlight keeps scattering through the upper atmosphere until the sun is well below the horizon, and for deep-sky imaging even a faint trace of it matters. Astronomers split twilight into three stages, defined by how far the sun is below the horizon.
The three twilights
| Stage | Sun below the horizon | What it looks like |
|---|---|---|
| Civil twilight | 0° to 6° | Bright enough to read outdoors; only the brightest stars and planets show |
| Nautical twilight | 6° to 12° | The horizon fades; most naked-eye stars appear |
| Astronomical twilight | 12° to 18° | Looks dark to the eye, but the sky is still measurably brighter, especially toward the sun |
| Astronomical darkness | More than 18° | No sunlight left in the sky |
The same stages happen in reverse before dawn. At mid-latitudes each stage lasts about 30–40 minutes, longer in summer and further from the equator.
Why 18° matters for imaging
Astronomical twilight looks dark, but a camera can tell. The sky background is still brighter than true night, with a gradient toward the horizon where the sun is. For faint galaxies and broadband nebulae, wait for full astronomical darkness. Frames taken earlier will have brighter, uneven backgrounds and lower contrast.
The summer problem
The further you are from the equator, the shallower the sun's path below the horizon in summer. Around the June solstice:
- North of about 48.5° latitude (most of the UK, northern Europe and southern Canada), the sun never gets 18° below the horizon. There is no astronomical darkness at all for weeks either side of midsummer.
- North of about 54.5°, it doesn't even reach nautical darkness.
The same applies in the southern hemisphere around the December solstice. If you live at these latitudes, early summer is a good time for maintenance, processing and the brighter targets below.
What you can shoot in twilight
Twilight isn't wasted:
- Narrowband nebulae. An Hα filter blocks most of the twilight glow, so emission nebulae work through astronomical and even nautical twilight.
- Bright clusters, double stars and planets. Bright enough to stand out.
- Setup jobs. Polar alignment, focusing and framing are perfect twilight tasks. Then you're ready to start imaging as soon as it's truly dark.
- Flat frames. The even glow of the sky in civil twilight works as a light source for flats.
Twilight in RigTonight
The Tonight tab works out sunset, each stage of twilight and sunrise for your location, and shows darkness hour by hour in the forecast strip. Its dark window starts at astronomical darkness; when there isn't any, as in high-latitude summers, it falls back to nautical darkness, then to the end of civil twilight. Target rankings count only the time each object spends above your minimum altitude while it's dark.