Sunset and Astronomical Twilight Calculator: When It Gets Dark

What each time means

The U.S. Naval Observatory defines these events by the position of the Sun's center relative to the horizon:

After astronomical twilight ends in the evening, and before it begins in the morning, the sky is as dark as it gets from sunlight. That's the window for the Milky Way, faint galaxies and the best meteor counts.

Worked example

For Denver, Colorado (39.74° N, 104.99° W) on November 11, 2026, at UTC−7, the calculator gives sunset at about 4:48 p.m., civil twilight ending about 5:16 p.m., nautical about 5:49 p.m. and astronomical about 6:21 p.m. USNO's own data for that day gives sunset at 4:47 p.m. and the end of civil twilight at 5:16 p.m.

How the calculation works

The calculator uses a standard solar position formula (the kind published for almanac use) to find when the Sun's center reaches a set angle below the horizon: 90.833° from the zenith for sunrise and sunset, and 96°, 102° and 108° for civil, nautical and astronomical twilight, the zenith distances USNO gives for computation. It's accurate to about a minute or two for most places, which is plenty for planning a night out. USNO itself notes that rise and set times can be off by a minute or more because refraction varies, and that hills and observer height matter more; accuracy also drops at high latitudes.

Seasons and latitude

• Near the equator, twilight is short year round because the Sun drops steeply.
• At mid-latitudes, twilight lasts longer in summer than in late autumn. In Denver it is about an hour and a half from sunset to full darkness in November, by this calculator.
• At high latitudes in summer, the Sun may never reach 18° below the horizon. London around the June solstice has no astronomical darkness, and the calculator says so.

Don't forget the Moon

Full darkness from the Sun doesn't mean a dark sky if the Moon is up. The calculator shows the Moon's estimated brightness for the evening; the Moon phase calendar gives exact phase times. The darkest skies come when astronomical twilight has ended and the Moon is below the horizon or a thin crescent.

Tips for a dark sky night

• Arrive before the end of astronomical twilight so your eyes adapt as the sky darkens.
• Use a dim red light sparingly, as the American Meteor Society suggests.
• Check the weather and pick a site away from city lights.

Common questions

How long after sunset is it dark?

It depends on latitude and season. In Denver on Nov 11, 2026 it is about 93 minutes from sunset to the end of astronomical twilight. Near the equator it is quicker; at high latitudes in summer it can take hours or never happen.

What is the difference between the twilights?

USNO says that after civil twilight ends (Sun 6° below the horizon) artificial light is normally needed for ordinary outdoor activities. During nautical twilight (to 12°) the horizon is still visible, so mariners can take star sights. After astronomical twilight ends (18°), scattered sunlight is fainter than starlight and other natural light.

Why does the calculator say there is no full darkness?

In summer at high latitudes, the Sun never gets 18° below the horizon, so astronomical twilight runs all night.

How accurate are the times?

Usually within a minute or two of U.S. Naval Observatory values for typical locations. Hills, buildings and weather change what you see.