Civil, Nautical & Astronomical Twilight Explained

After the Sun dips below the horizon, the sky doesn’t go dark instantly — it fades through three named stages of twilight, each defined by how many degrees the Sun sits below the horizon. Each stage marks a real, usable threshold, not just a poetic description of dusk.

Civil twilight

From sunset until the Sun is 6° below the horizon. There’s still enough natural light for most outdoor activity without artificial lighting, and the horizon is clearly visible. This is roughly the window photographers call the "blue hour," and it’s usually when streetlights and headlights become necessary.

Nautical twilight

From 6° to 12° below the horizon. The horizon is still faintly visible at sea, which historically let sailors take star sightings against it for celestial navigation — hence the name. On land, it’s too dim for most activity without a light source, and the brightest stars and planets are becoming visible.

Astronomical twilight

From 12° to 18° below the horizon. The sky still carries a faint natural glow that’s enough to wash out the dimmest stars and deep-sky objects, even though it looks fully dark to the naked eye. Only once the Sun passes 18° below the horizon is the sky considered truly, fully dark.

Why the distinction matters

Photographers plan around civil twilight’s soft, even light. Astronomers wait for astronomical twilight to end before deep-sky observing, since even faint residual sky glow can drown out dim targets. Sailors and pilots have historically relied on nautical twilight’s visible horizon for navigation. The three thresholds turn "getting dark" into something precise enough to plan around.