![]() ![]() In fact, the blue hour occurs when the Sun is far enough below the horizon so that the sunlight's blue wavelengths dominate due to the Chappuis absorption caused by ozone. The still commonly presented incorrect explanation claims that Earth's post-sunset and pre-sunrise atmosphere solely receives and disperses the sun's shorter blue wavelengths and scatters the longer, reddish wavelengths to explain why the hue of this hour is so blue. In places closer to the poles, the illumination and twilight periods are longer as the sun rises and sets at shallower angles. The blue hour is shorter in regions near the equator due to the sun rising and setting at steep angles. Rather, blue hour refers to the state of natural lighting that usually occurs around the nautical stage of the twilight period (at dawn or dusk). Since the term is colloquial, it lacks an official definition such as dawn, dusk, or the three stages of twilight. The blue hour occurs when the Sun is far enough below the horizon so that the sunlight's blue wavelengths dominate due to the Chappuis absorption caused by ozone. ![]() ![]() This shade differs from the colour of the sky on a clear day, which is caused by Rayleigh scattering. During this time, the remaining sunlight takes on a mostly blue shade. The blue hour (from French l'heure bleue pronounced ) is the period of twilight (in the morning or evening, around the nautical stage) when the Sun is at a significant depth below the horizon. Period of twilight in the morning or evening Blue hour at the Old Cathedral of the Holy Name of Jesus, Bragança in Portugal ![]()
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