Blue hour

For other uses, see Blue hour (disambiguation).
Blue hour at the Old Cathedral of the Holy Name of Jesus, Bragança in Portugal

The blue hour (from French l'heure bleue;[1][a] pronounced [lœʁ blø]) 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. 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.

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.[2] Since the term is colloquial, it lacks an official definition such as dawn, dusk, or the three stages of twilight. 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).[3]

The blue hour is shorter in regions near the equator due to the sun rising and setting at steep angles. In places closer to the poles, the illumination and twilight periods are longer as the sun rises and sets at shallower angles.[4]

  1. ^ Verfaillie, Roland (2011). L'heure Bleue. San Francisco: Purple Onion Press. p. 5. ISBN 9780978708559.
  2. ^ Hoeppe, Götz (2007). Why the Sky Is Blue: Discovering the Color of Life. Princeton: Princeton University Press. pp. 249–53. ISBN 978-0-691-12453-7.
  3. ^ "The Blue Hour". timeanddate.com. Retrieved 6 April 2019.
  4. ^ Wu, Jennifer; Martin, James (13 March 2014). Photography: Night Sky: A Field Guide for Shooting after Dark. Mountaineers Books. p. 139. ISBN 978-1-59485-839-0.


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