The term blue hour refers to the special coloring of the sky during the time of twilight after sunset and before the onset of nighttime darkness , while the sun is about 4 to 8 degrees below the horizon. The term was particularly coined by writers and poets who often associate it with melancholy feelings. The same coloration can also be seen at dawn, although the term is used less often in this context. The blue of the sky has a different spectral composition because it is due to a different physical cause than during the day. During the blue hour, this deep blue sky has about the same brightness as the artificial light from building and street lighting. Not least because of this, the blue hour plays a special role in photography .
Physical explanation of the coloring
The sky blue during the day is caused by Rayleigh scattering , due to which more light of shorter wavelength - i.e. blue light - is scattered. If, however, the sunlight falls more and more obliquely in the course of twilight and takes a longer path through the earth's atmosphere, more and more light is already scattered out before it can reach the sky above the place of twilight. Since this applies in particular to the blue light, it would mean that the sky at the zenith after sunset would be more yellowish or greenish. With the inclined incidence of light, the light also has a longer path through the ozone layer at an altitude of 15 to 30 kilometers, which is why the Chappuis absorption comes into play, which is not noticeable during the day because it is significantly weaker than the Rayleigh scattering .
As early as 1880, the French chemist James Chappuis had recognized that ozone colors visible light blue because it absorbs light in the yellow, orange and red spectrum, which means that the blue light is left over, so to speak. However, it was considered that the blue of the sky was already perfectly explained by the Rayleigh scattering known at the time, so that no importance was attached to this effect in this regard. It was not until 1952 that the American geophysicist Edward Hulburt (1890–1982) recognized this connection. He came across it when he measured the intensity and color of the light during twilight and compared it with the theoretically predicted values. Due to the magnitude of the deviations, measurement errors could be excluded. It was not until Hulburt, who was aware of the absorption effect of ozone, that the Chappuis absorption was included in his calculations, that theory and experimental data could be reconciled.
As the path through the earth's atmosphere becomes longer and longer at dusk, the importance of Rayleigh scattering for the coloring of the sky at the zenith decreases, while the importance of Chappuis absorption increases. At sunset, only one third of the coloration is due to Rayleigh scattering and two thirds to Chappuis absorption; in the later course of twilight, the latter becomes the only significant effect.
Hulburt himself was amazed at the discovery of this seemingly completely coincidental harmonic interplay of two very different physical effects and expressed this in a letter of thanks for an award he received for his research.
The unsuspecting observer lying on his back and looking upward at the clear sky during sunset sees only that the overhead sky which was blue before, remains the same luminous blue color during sunset and throughout the darkening period of twilight. He is not aware that in order to produce this apparently simple and satisfactory result nature has dipped quite freely into her best bag of optical tricks.
“The unsuspecting observer, lying on his back at the clear sky during sunset, sees only that the sky above him, which was blue before sunset, maintains the same brilliant blue as the sun sets and then during dusk getting darker and darker. He is not aware that nature, in order to produce this apparently so obvious and obvious result, has quite generously reached deep into its optical bag of tricks. "
The duration in Central Europe is between 30 ( equinox ) and 50 minutes ( solstice ). The length of twilight in relation to the latitude plays an important role. The length of the apparition is 20 minutes in the tropics and up to five hours in the white nights . At the poles, the blue hour lasts (theoretically) two weeks.
The blue hour in photography
In photography , the blue hour is used for available-light shots and night photography . Compared to recordings in complete darkness, the surroundings are still slightly brighter and more visible at this time. In the image obtained, the contrasts between light and dark are softened and the images have a special mood. The lighting inside buildings comes in the same photographic contrast range of the non-artificially illuminated facade and surroundings and the color contrast to street lighting and building lighting provide photographic incentives. The different color temperatures (blue of the sky, orange of the light bulbs, turquoise of the fluorescent tubes) make such photos unusually colorful. The color temperature of the sky is between 9000 K and 12000 K. It corresponds roughly to that of the daytime sky seen from the shade, so the different spectral composition due to the Chappuis absorption is not significantly noticeable in the color temperature.
The blue hour in literature
- Oskar Loerke : Blue Evening in Berlin (poem, 1911)
- A poem by Gottfried Benn is entitled Blue Hour .
- A poem by Ingeborg Bachmann is entitled The Blue Hour .
- A novel by William Boyd from 1994 bears the German translation entitled The Blue Hour (Original: The Blue Afternoon ).
- A novel by Joan Didion from 2012 has the translation title The Blue Hours (Original: Blue Nights).
- The novel Tauben im Gras by Wolfgang Koeppen shows in the melancholy consciousness of the protagonist Philipp the “heure bleue” in the spiritual, political and social structures of the crisis situation of 1951: “Philipp loved the hour. In Paris it was the current bleue, the hour of dreaming, a span of relative freedom, the moment of being free from day and night. "
The blue hour in music
- A chanson by Herbert Nelson and Rudolf Nelson , known in the recording by Greta Keller , is titled A Blue Hour.
- A song by the band Faun from 2014 is entitled Blue Hour .
- In the song Odyssey , Udo Lindenberg referred to the blue hour.
- A song by Funny van Dannen from 2002 is entitled Blue Hour .
- An album by the classical composer Federico Albanese from 2016 is entitled "The Blue Hour".
- EO Hulburt : Explanation of the Brightness and Color of the Sky, Particularly the Twilight Sky. Journal of the Optical Society of America 43 (2), 1953, pp. 113-118, doi : 10.1364 / JOSA.43.000113 (English).
- Götz Hoeppe: Why the Sky is Blue. Discovering the Color of Life. Princeton University Press, Princeton 2007, ISBN 0-691-12453-1 .
- Time tables of the blue hour for freely selectable locations
- Götz Hoeppe: sky light. Mirror image of the earth's climate.
- Blue hour: Calculator & tips for twilight photos. Retrieved October 13, 2017 .
- Angelika Lochmann, Angelika Overath: The blue book. Readings of a color. Greno Verlag, Nördlingen 1988, ISBN 3-89190-432-0 , p. 208 f.
- Götz Hoeppe: Why the Sky is Blue. Discovering the Color of Life. Princeton University Press, Princeton 2007, ISBN 0-691-12453-1 , pp. 236 f. ( Google books )
- Götz Hoeppe: Why the Sky is Blue. Discovering the Color of Life. Princeton University Press, Princeton 2007, ISBN 0-691-12453-1 , pp. 249-253 ( Google books )
- P. Hautefille, J. Chappuis: Sur la liquéfaction de l'ozone et sur la couleur à l'état gaseux. In: Comptes rendus de l'Académie des sciences , Volume 91 (1880), pp. 552-525, German in the reports of the German Chemical Society , Volume 13 (1880), p. 2408.
- EO Hulburt: Some Recent Papers in the Journal of the Optical Society of America. In: Journal of the Optical Society of America. Volume 46, p. 9, 1956, doi : 10.1364 / JOSA.46.000005 .
- Harald Tedesco: Panorama Photography. Franzis Verlag, Haar near Munich 2016, ISBN 3-645-60437-5 , p. 222 ( Google books ).
- Oskar Loerke: Blue evening in Berlin .
- Stefanie Golisch : Hour of Illusion - Hour of Truth on Anthologie.de (text of Benn's poem and interpretation)
- Stefanie Golisch : The hour between reality and possibility in Fixpoetry (text of the poems by Benn and Bachmann and interpretation).
- Wolfgang Koeppen: Pigeons in the grass. Hamburg 1951, episode 74.
- Federico Albanese | Music. Retrieved November 11, 2019 .