The short answer
Astronomical twilight is the last stage of dusk (and the first stage of dawn), when the Sun is between 12° and 18° below the horizon. To the casual eye the sky already looks dark, but a faint glow from the Sun still brightens it. When the Sun sinks more than 18° below the horizon, astronomical twilight ends and true night begins: the sky is as dark as your location allows, which is what you want for faint galaxies, nebulae and the Milky Way.
The three stages of twilight
Twilight is divided by how far the center of the Sun is below the horizon. The same stages happen in reverse before sunrise.
| Stage | Sun below horizon | What the sky looks like |
|---|---|---|
| Civil twilight | 0° to 6° | Bright enough to read outside; Venus, Jupiter and the brightest stars appear |
| Nautical twilight | 6° to 12° | The horizon fades; most bright stars and constellations show |
| Astronomical twilight | 12° to 18° | Looks dark to most people, but faint objects are still washed out |
| Night | More than 18° | Fully dark; the sky’s background is set only by light pollution, the Moon and airglow |
The names come from what each stage was useful for. Civil twilight is roughly when outdoor activities need no artificial light. Nautical twilight is when sailors could still see the horizon to take star sightings. Astronomical twilight is the stage that matters to astronomers, because the faintest objects only appear after it ends.
How long twilight lasts
The time from sunset to the end of astronomical twilight depends on your latitude and the date, because both affect the angle at which the Sun sinks below the horizon.
- Near the equator, the Sun sets nearly straight down, and full darkness typically arrives roughly an hour and ten to an hour and a quarter after sunset, all year round.
- At mid-latitudes, such as most of the United States and southern Europe, it typically takes around an hour and a half in spring and autumn, and longer near the summer solstice.
- At higher latitudes, such as northern Europe or Canada, the Sun slides down at a shallow angle. In summer, twilight can last for hours.
Because these times shift from week to week, it’s easiest to look them up for your place. The astronomical twilight calculator gives the start and end of each stage for any date.
When the sky never gets fully dark
If the Sun never gets 18° below the horizon, astronomical twilight never ends and the night never becomes fully dark. In the Northern Hemisphere this happens around the June solstice at latitudes above roughly 48.5° north, which includes cities such as Paris, London and Vancouver. The further north you are, the more weeks it lasts. The same happens around the December solstice at southern latitudes beyond about 48.5° south.
That doesn’t mean summer stargazing is pointless at those latitudes. Bright stars, planets, the Moon and double stars are unaffected, and many star clusters still show well. It’s faint galaxies and the Milky Way that suffer. Further north still, beyond about 60°, the Sun stays within 6° of the horizon near midsummer and even civil twilight lasts all night.
Morning twilight
Before sunrise the sequence runs backwards. Astronomical twilight begins when the Sun rises to 18° below the horizon, and from then on the sky slowly brightens even though it still looks dark. If you plan to observe faint objects before dawn, finish before morning astronomical twilight begins. Planets low in the east, such as Venus or Mercury when they are morning objects, are often best seen during nautical and civil twilight, when they are as high as they’ll get before the sky grows too bright.
Using twilight times to plan
Different targets need different stages of darkness. A simple plan for an evening:
- Civil twilight: look for Venus, Jupiter and the brightest stars as they appear. Mercury, if it is an evening object, is often only visible now, low in the west.
- Nautical twilight: learn constellations while only the brighter stars are showing, and look at the Moon and planets.
- After astronomical twilight: move on to faint targets, such as galaxies and nebulae from the deep-sky catalog, if the Moon is down.
Twilight is only one part of darkness. A bright Moon can wash out the sky long after astronomical twilight ends; see how moonlight affects stargazing. And light pollution sets a floor on how dark the sky can get; see the Bortle scale. For the full picture of timing, read when is the best time to stargaze?