Civil, nautical, astronomical — types of twilight

The interval between sunset and full darkness lasts longer than most people realize. Depending on your latitude and the season, it can be anywhere from forty minutes to two hours. During that interval, the sky goes through three named phases, each of which marks a specific level of darkness with practical meaning. Most people don’t know the names, but everyone notices the transitions.

The three are civil, nautical, and astronomical twilight, defined by where the Sun is below the horizon.

The progression

Sunset is the moment the top edge of the Sun crosses below the horizon. That’s the start of the sequence.

Civil twilight is the period from sunset until the Sun is six degrees below the horizon, which takes between 25 and 60 minutes depending on latitude and season. During civil twilight you can still read a newspaper outdoors without a light. Bright planets — Venus, Jupiter, sometimes Mars — are visible if you know where to look. A handful of first-magnitude stars start to appear. This is the “blue hour” that photographers love.

Nautical twilight runs from six degrees below horizon to twelve degrees. Named for sailors who used a sextant to measure star angles for navigation; the conjunction of “horizon still visible” with “stars now visible” was the navigator’s sweet spot for fixing position. By the end of nautical twilight, several dozen stars are visible and the constellations are becoming recognizable.

Astronomical twilight is the deepest twilight, from twelve degrees to eighteen degrees below horizon. Faint stars are appearing, and to the casual eye it looks fully dark. But there’s still a measurable glow from scattered sunlight in the upper atmosphere — too faint to see directly, but enough to wash out the faintest objects like nebulae and galaxies.

Past 18 degrees, the Sun’s contribution to sky brightness is effectively zero. The remaining light pollution is from artificial sources, the Moon, or natural airglow. This is true night, and it’s what SkyMinute uses as the reference point for our “best moment tonight” calculation.

How long the whole sequence takes

Near the equator, the Sun rises and sets nearly vertically through the horizon, so it crosses through twelve degrees of sky quickly. Total time from sunset to astronomical night, at low latitudes, is around an hour.

At higher latitudes, the Sun crosses the horizon at a shallow angle, so it spends more time near it before dropping deeply. In Prague at 50°N, the full sunset-to-night sequence takes roughly 90 minutes in summer and 70 minutes in winter.

Above about 48° latitude, an interesting thing happens in summer: the Sun never gets 18 degrees below the horizon, and so true astronomical night never arrives at all. The sky stays in some level of twilight all night, with the brightest stars visible but the Milky Way faint and the deep sky inaccessible. This is sometimes called “white nights” in Russian and Scandinavian usage; in midsummer in Saint Petersburg or Stockholm you can quite literally read a book outdoors at midnight.

The same problem in reverse hits the Antarctic, and to a lesser extent the southernmost cities like Punta Arenas or Hobart. Mid-latitudes, between about 30 and 48 degrees, get the best of both worlds: proper darkness every night of the year.

What this means for you

The practical takeaway is simple. Sunset is not “dark.” If you want to look at planets and the Moon, you can step outside almost immediately after sunset and see them. If you want to look at stars or constellations, wait until well into nautical twilight, which is roughly 40 to 70 minutes after sunset depending on where you are. If you want to look at the Milky Way or deep-sky objects, wait for astronomical night to begin, which is 60 to 100 minutes after sunset.

The corollary: if you live above 50° latitude, your best stargazing window is autumn through spring. From late May to early August at those latitudes, you may not see proper darkness at all. People in Stockholm or Edinburgh or southern Alaska know this in their bones and adjust their observing schedules around it.

The other corollary: the Moon overrides all of this. Even at full astronomical night, a bright moon washes out the faintest objects. The cycle from new moon to full moon and back is independent of the sunset-twilight-night cycle, but it has a similar effect on what you can see. A new moon at full astronomical night is when the sky is darkest. A full moon at astronomical night is much brighter than astronomical twilight at new moon.

Why SkyMinute waits

When we compute “best moment tonight” for your city, we take the moment astronomical twilight ends and add about 30 minutes. That gives the sky a few extra minutes to settle into its darkest state of the night, and it puts you at a time when the most objects are visible at once. We then pick the direction based on which planets or the Moon happen to be highest at that time.

If you’re in a high-latitude location during summer when there’s no true astronomical night, we fall back to the end of civil twilight plus 30 minutes — the best the night will get. The page will tell you the sky isn’t going to get fully dark; we don’t pretend otherwise.