The blue hour (from French ; ) is the period of

twilight Twilight is daylight illumination produced by diffuse sky radiation when the Sun is below the horizon as sunlight from the upper atmosphere is scattered in a way that illuminates both the Earth's lower atmosphere and also the Earth's surf ...

(in the

morning Morning is either the period from sunrise to noon, or the period from midnight to noon. In the first definition it is preceded by the twilight period of dawn, and there are no exact times for when morning begins (also true of evening and nigh ...

evening Evening is the period of a day that begins at the end of daylight and overlaps with the beginning of night. It generally indicates the period of time when the sun is close to the horizon and comprises the periods of civil, nautical and astronom ...

, around the nautical stage) when the

Sun The Sun is the star at the centre of the Solar System. It is a massive, nearly perfect sphere of hot plasma, heated to incandescence by nuclear fusion reactions in its core, radiating the energy from its surface mainly as visible light a ...

is at a significant depth below the

horizon The horizon is the apparent curve that separates the surface of a celestial body from its sky when viewed from the perspective of an observer on or near the surface of the relevant body. This curve divides all viewing directions based on whethe ...

. During this time, the remaining

sunlight Sunlight is the portion of the electromagnetic radiation which is emitted by the Sun (i.e. solar radiation) and received by the Earth, in particular the visible spectrum, visible light perceptible to the human eye as well as invisible infrare ...

takes on a mostly blue

shade Shade, Shades or Shading may refer to: * Shade (color), a mixture of a color with black (often generalized as any variety of a color) * Shade (shadow), the blocking of sunlight * Shades or sunglasses * Shading, a process used in art and graphic ...

. This shade differs from the colour of the sky on a clear day, which is caused by

Rayleigh scattering Rayleigh scattering ( ) is the scattering or deflection of light, or other electromagnetic radiation, by particles with a size much smaller than the wavelength of the radiation. For light frequencies well below the resonance frequency of the scat ...

. 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 Ozone () (or trioxygen) is an Inorganic compound, inorganic molecule with the chemical formula . It is a pale blue gas with a distinctively pungent smell. It is an allotrope of oxygen that is much less stable than the diatomic allotrope , break ...

. Since the term is

colloquial Colloquialism (also called ''colloquial language'', ''colloquial speech'', ''everyday language'', or ''general parlance'') is the linguistic style used for casual and informal communication. It is the most common form of speech in conversation amo ...

, it lacks an official definition such as

dawn Dawn is the time that marks the beginning of twilight before sunrise. It is recognized by the diffuse sky radiation, appearance of indirect sunlight being Rayleigh scattering, scattered in Earth's atmosphere, when the centre of the Sun's disc ha ...

dusk Dusk occurs at the darkest stage of twilight, or at the very end of astronomical twilight after sunset and just before nightfall.''The Random House College Dictionary'', "dusk". At predusk, during early to intermediate stages of twilight, enoug ...

, or the three stages of twilight. Rather, blue hour refers to the state of

natural lighting Nature is an inherent character or constitution, particularly of the ecosphere or the universe as a whole. In this general sense nature refers to the laws, elements and phenomena of the physical world, including life. Although humans are part ...

that usually occurs around the nautical stage of the twilight period (at dawn or dusk). The blue hour is shorter in regions near the

equator The equator is the circle of latitude that divides Earth into the Northern Hemisphere, Northern and Southern Hemisphere, Southern Hemispheres of Earth, hemispheres. It is an imaginary line located at 0 degrees latitude, about in circumferen ...

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.

Explanation and times of occurrence

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 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. When the

sky The sky is an unobstructed view upward from the planetary surface, surface of the Earth. It includes the atmosphere of Earth, atmosphere and outer space. It may also be considered a place between the ground and outer space, thus distinct from ...

is clear, the blue hour can be a colourful spectacle, with the indirect sunlight tinting the sky yellow, orange,

red Red is the color at the long wavelength end of the visible spectrum of light, next to orange and opposite violet. It has a dominant wavelength of approximately 625–750 nanometres. It is a primary color in the RGB color model and a seconda ...

, and

blue Blue is one of the three primary colours in the RYB color model, RYB colour model (traditional colour theory), as well as in the RGB color model, RGB (additive) colour model. It lies between Violet (color), violet and cyan on the optical spe ...

. This effect is caused by the relative

diffusibility Diffusion is the net movement of anything (for example, atoms, ions, molecules, energy) generally from a region of higher concentration to a region of lower concentration. Diffusion is driven by a gradient in Gibbs free energy or chemical p ...

of shorter

wavelengths In physics and mathematics, wavelength or spatial period of a wave or periodic function is the distance over which the wave's shape repeats. In other words, it is the distance between consecutive corresponding points of the same '' phase'' on ...

(bluer rays) of

visible light Light, visible light, or visible radiation is electromagnetic radiation that can be perceived by the human eye. Visible light spans the visible spectrum and is usually defined as having wavelengths in the range of 400–700 nanometres (nm ...

versus the longer wavelengths (redder rays). During the blue "hour", red light passes through space while blue light is scattered in the atmosphere, and thus reaches Earth's surface. Blue hour usually lasts about 20–96 minutes right after sunset and right before sunrise. Time of year, location, and air quality all have an influence on the exact time of blue hour. For instance in Egypt (every 21st of June), when

sunset Sunset (or sundown) is the disappearance of the Sun at the end of the Sun path, below the horizon of the Earth (or any other astronomical object in the Solar System) due to its Earth's rotation, rotation. As viewed from everywhere on Earth, it ...

is at 7:59 PM: blue hour occurs from 7:59 PM to 9:35 PM. When

sunrise Sunrise (or sunup) is the moment when the upper rim of the Sun appears on the horizon in the morning, at the start of the Sun path. The term can also refer to the entire process of the solar disk crossing the horizon. Terminology Although the S ...

is at 5:54 AM: blue hour occurs from 4:17 AM to 5:54 AM. Golden hour occurs from 5:54 AM to 6:28 AM and from 7:25 PM to 7:59 PM.

Art

Max Klinger (1857-1920) l' Heure bleue,1890,huile sur toile 191,5 x 176 cm, Musée des Beaux-Arts de Leipzig.jpg , ''Die blaue Stunde'' (1890), painting by

Max Klinger Max Klinger (18 February 1857 – 5 July 1920) was a German artist who produced significant work in painting, sculpture, prints and graphics, as well as writing a treatise articulating his ideas on art and the role of graphic arts and printmakin ...

Pôr-do-Sol (1897) - António Cândido da Cunha.png , ''Sunset'' (1897), painting by António Cândido da Cunha Tull-Twilight.jpg , ''Twilight'' (1897), painting by Edmund Tull (Ödön Tull) Taj-Mahal, twilight - CWB (monogram) ; Charles W. Bartlett. LCCN2018646015.jpg , ''Taj-Mahal, twilight'' (1920), woodcut by Charles W. Bartlett Tōkyō jūnidai, Kiba no yūgure by Kawase Hasui.jpg , ''Evening at the Lumber Yards of Kiba'' (1920), woodcut by Kawase Hasui

Photography

Many artists value this period for the quality of the

soft light Soft may refer to: * Softness, or hardness, a property of physical materials Arts and entertainment * ''Soft!'', a novel by Rupert Thomson, 1988 * Soft (band), an American music group * ''Soft'' (album), by Dan Bodan, 2014 * ''Softs'' (album), ...

. Although the blue hour does not have an official definition, the blue color spectrum is most prominent when the Sun is between 4° and 8° below the horizon. Photographers use blue hour for the tranquil mood it sets. When photographing during blue hour it can be favourable to capture subjects that have artificial light sources, such as buildings, monuments, cityscapes, or bridges. File:Super moon over City of London from Tate Modern 2018-01-31 4.jpg, The

City of London The City of London, also known as ''the City'', is a Ceremonial counties of England, ceremonial county and Districts of England, local government district with City status in the United Kingdom, city status in England. It is the Old town, his ...

during the blue hour File:Seascape after sunset denoised.jpg, Praia da Ursa, Cabo da Roca,

Portugal Portugal, officially the Portuguese Republic, is a country on the Iberian Peninsula in Southwestern Europe. Featuring Cabo da Roca, the westernmost point in continental Europe, Portugal borders Spain to its north and east, with which it share ...

. A wide-angle view of the seascape during the blue hour File:CH.ZG.Zug View from Guggi 01 16x9+R 8192x4608 Br058 gp denoising Br075.jpg,

Zug Zug (Standard German: , Alemannic German: ; ; ; ; )Named in the 16th century. is the largest List of cities in Switzerland, town and capital of the Swiss canton of Zug. Zug is renowned as a hub for some of the wealthiest individuals in the wor ...

Switzerland Switzerland, officially the Swiss Confederation, is a landlocked country located in west-central Europe. It is bordered by Italy to the south, France to the west, Germany to the north, and Austria and Liechtenstein to the east. Switzerland ...

during the blue hour Sree Padmanabhaswami Temple of Kerala 3.jpg, Padmanabhaswami Temple of Kerala during the blue hour

Notes

References

External links

Blue hour mobile application (iOS, Iphone/iPad)

bluehoursite.com: Everything about Blue Hour and Night Photography (news, articles, tips and calculator)

Twilight Calculator, Golden Hour/Blue Hour table
{{DEFAULTSORT:Blue Hour Earth phenomena Parts of a day Visibility Night Atmospheric optical phenomena Photography

Explanation and times of occurrence

Art

Photography

See also

Notes

References

External links