Blue hour

Period of twilight in the morning or evening

The blue hour (from French l'heure bleue; pronounced [lœʁ blø]) is the period of twilight (in the dawn or dusk, around the nautical stage) when the Sun is at a significant depth below the horizon. During this time, the remaining sunlight takes on a mostly blue shade. This shade differs from the colour of the sky on a clear day, which is caused by Rayleigh scattering.

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