Night Sky Darkness Calculator

Twilight Phases

After the sun sets, the sky doesn't become fully dark immediately. Twilight is divided into three phases based on how far the sun is below the horizon:

• Civil Twilight — Sun is 0° to 6° below the horizon. There is enough light for most outdoor activities without artificial lighting.
• Nautical Twilight — Sun is 6° to 12° below. The horizon is still visible at sea and bright stars appear, but it's too dark for most outdoor activities.
• Astronomical Twilight — Sun is 12° to 18° below. The sky is nearly dark but faint light from the sun still affects sensitive observations.

Astronomical Darkness

True astronomical darkness occurs when the sun is more than 18° below the horizon. This is the optimal window for deep-sky observations, astrophotography, and viewing faint objects like galaxies and nebulae.

Solar Position

This calculator uses the solar position algorithm to determine the sun's altitude throughout the night. Key events (sunset, twilight boundaries) occur when the sun crosses specific altitude thresholds.

Moon Influence

Even during astronomical darkness, moonlight can significantly reduce sky quality. A full moon illuminates the sky comparable to nautical twilight. The calculator shows moon phase and illumination to help you plan around moonlight interference.

Polar Regions

At high latitudes during summer, the sun may never dip below −18°, resulting in no astronomical darkness. In extreme cases (midnight sun), the sun doesn't set at all. The calculator handles these special cases and will notify you when full darkness is not available.