How dark is dark enough?

The three twilights

After the Sun sets, the sky doesn't switch off. Sunlight still lights the air high above us, and the sky fades in three steps that astronomers name by how far the Sun is below the horizon.

Civil twilight ends when the Sun is 6 degrees down. Outdoor games get hard to play, the first planets and the brightest stars appear, and the western sky is still glowing. Nautical twilight ends at 12 degrees: the horizon over the sea becomes hard to see, which is where the name comes from. Astronomical twilight ends at 18 degrees: from then on the sky is as dark as it will get.

How long each takes

In the middle of the United States in winter, civil twilight lasts about half an hour: our sky table has Denver's sunset at 4:35 pm on 21 December 2026 and dark enough by 5:05 pm. The full dark comes roughly an hour after that. In summer, and the farther north you live, every step takes longer, which is why a June night in Alaska barely gets dark at all.

Why the card picks civil twilight

For a family at bedtime, waiting an hour and a half for full darkness is too long, and it isn't needed: the Moon and the five planets you can see with your eyes all show well against a twilight sky. So Sky Tonight says "dark enough by" for the end of civil twilight, and picks a time to look half an hour after that, never before 7 pm and never after 10 pm.

For a meteor shower, darker is better: the faint ones only show once the sky is fully dark and the Moon is down. Each shower page says what an early bedtime still gets.

Your eyes need dark too

Even when the sky is dark, eyes take about twenty minutes to adjust. A bright phone screen undoes it in a moment, which is what Sky Tonight's red Outside mode is for.

1. Check the card's dark-enough time.
2. Go out a few minutes before it and watch the first stars appear.
3. Count them every five minutes: the number grows as the sky darkens.