When a strong blue light dominates the sky at dusk and dawn before and after the dark night, it is often referred to as the blue hour. Many people feel emotionally connected to this phenomenon. Writers and poets have repeatedly described and sung about the Blue Hour, especially during and since the Romantic period.

However, the blue hour is also physically special. During this period, when the sun is about 4 to 8° below the horizon as seen by the observer, the blue has a different spectral...