Astronomical Twilight

Astronomical twilight represents the final transition phase between twilight and true night, occurring when the sun sits between 12° and 18° below the horizon. Unlike civil and nautical twilight, which are defined by practical visibility thresholds for everyday activities and navigation, astronomical twilight is defined by the threshold at which the sky becomes dark enough for astronomical observations. During this period, the sun's light no longer directly illuminates the atmosphere at observable altitudes, though a faint glow may still linger near the horizon, particularly in the direction where the sun sits below. For photographers, astronomical twilight marks the beginning of conditions suitable for serious astrophotography. Most stars, constellations, and Milky Way features become clearly visible, though the very faintest deep-sky objects may still be slightly washed out by residual sky glow compared to full astronomical darkness.

Landscape astrophotographers often prefer late astronomical twilight or early astronomical dawn for images that include both landscape and stars, as the faint horizon glow provides just enough light to render foreground details without requiring separate exposures or artificial lighting. The sky during astronomical twilight displays a subtle gradient from near-black at the zenith to dark blue or purple near the horizon—a characteristic that can add visual interest to astrophotography compositions. Cityscape astrophotographers may also use astronomical twilight to capture star fields or the Milky Way above urban skylines while retaining some color in the sky rather than pure black.

The duration of astronomical twilight varies significantly by latitude and season. At equatorial latitudes, it lasts roughly 25-30 minutes, providing a predictable window for shoot preparation. At higher latitudes during summer months, astronomical twilight may last several hours, or the sun may never drop below 18° below the horizon, meaning true astronomical darkness never occurs—a phenomenon common in locations experiencing white nights. During winter at these same latitudes, astronomical twilight may be very brief or the sun may remain below 18° for extended periods, providing long windows for astrophotography. PhotoWeather precisely calculates astronomical twilight timing using solar position algorithms and integrates current atmospheric conditions like visibility, cloud cover, and light pollution to help astrophotographers plan their sessions for optimal sky darkness.