Astronomical twilight: when does real night begin?
You have prepared everything. Telescope set up, camera connected, thermos filled. The Sun disappears behind the horizon, and the sky keeps glowing merrily. Twenty minutes later, still. After an hour, slowly a little less. What is going on?
The answer is twilight. And there is not just one but three stages of it. Each with its own name, its own angles and its own message to the astrophotographer: Not yet. Soon. Now.
The three stages of twilight
What matters is not whether the Sun is visible, but how far it is below the horizon. The deeper, the less light the atmosphere scatters into the sky and the darker it gets. Astronomers define three clear phases for this:
Civil twilight (0° to -6°)
Civil twilight begins right after sunset. The name sounds like a trip to the town hall, and it is about as useful for astrophotography. The sky is still clearly brightened, bright planets like Venus or Jupiter flash out, but most stars stay invisible. Anyone imaging now gets a blue gradient as a background: nice for landscape photos, poor for galaxies.
Nautical twilight (-6° to -12°)
When the Sun is 6 to 12 degrees below the horizon it gets noticeably darker. The name comes from sailors, who could sight stars against the still-visible horizon in this phase. Practical for navigation, less practical when you want to image the Orion Nebula. First fainter stars appear, but the sky background stays too bright for deep-sky objects. Bright planets and star clusters can already be captured well here.
Astronomical twilight (-12° to -18°)
Now it gets serious. When the Sun is between 12 and 18 degrees below the horizon, almost all stars are visible. But "almost" is the key word. The background is not yet fully black. Faint nebulae and distant galaxies sink into the brightened ground. You can already sense them, but not really see them.
Real night begins at -18°
Only when the Sun has disappeared more than 18 degrees below the horizon does astronomical twilight end and real night begin. The sky background is now as dark as it can be without light pollution and moonlight. Faint nebulae, far-off galaxies and delicate star clusters only now come into their own.
In Germany it often takes up to 90 minutes after sunset in summer before this point is reached, sometimes longer. In winter it goes much faster, but it is colder. You cannot have everything.
North of about 48° latitude there are even nights in the summer months in which the Sun does not sink deep enough at all. Then there is simply no astronomical night, no matter how long you wait. Anyone living in Hamburg knows this phenomenon from hard-won experience.
How does SkyGrade compute the weather window?
SkyGrade computes, for each location and date, the exact moment when astronomical twilight ends and when it begins again in the morning. This happens purely mathematically, without an external API, directly from the position of the Sun.
The weather window in the result shows exactly this period: the hours in which real night truly prevails. Cloud cover, seeing and transparency are rated only for this window, not for the whole night. Because what use is a cloud-free sky at 9 pm if you have to wait until 11:30 pm anyway?
Practical tips
- Plan your observing night from the end of astronomical twilight, not from sunset
- In summer in Germany this often means: earliest from 11 pm, sometimes only after midnight
- Use the twilight time sensibly: set up gear, align the mount, drink coffee
- Bright objects like the Moon, planets or open star clusters can be imaged well even during nautical twilight
- In winter real night starts earlier, but your fingers hurt sooner
Further reading
- What is seeing? – when the night is finally dark, seeing decides whether the images come out sharp.
- The Bortle scale: how dark is your sky really? – not every dark night is equally dark. The Bortle scale makes the difference measurable.
- Transparency: why a cloudless sky can still be poor – clear sky does not automatically mean good clarity.