CLAREA · LEARN · SOLAR GEOMETRY
Understanding Twilight
The hour between worlds
In short
Twilight is the 20 to 40 minutes when the sun is just below the horizon and the sky turns violet-blue. Most people never look up at it.
Twilight is the brief window when the sun is below the horizon but its light still reaches the upper atmosphere. As the sun descends from 0° to 18° below horizontal, the sky moves through three named phases: civil, nautical, and astronomical twilight.
The spectrum during this window is distinctly violet-blue — the result of high-altitude scattering with the lower atmosphere already in shadow. It is the briefest light environment of the day, often lasting only 20–40 minutes. Most modern people never look up at it.
Sources
- Rise, Set, and Twilight Definitions — U.S. Naval Observatory, aa.usno.navy.mil