Beloved by outdoor photographers, the blue and golden hours are periods near sunset or sunrise when the sky is suffused with either a deep blue glow or soft, gilded light. The terms are colloquial, but they describe the effects of specific atmospheric phenomena. Here are a few facts about how they happen and when to catch them.
What Is the Blue Hour?
The blue hour doesn’t refer to a specific hour on the clock. It often doesn’t even last an hour. The blue hour is an atmospheric condition in which the sun lies below the horizon while its rays still illuminate the sky, casting a bluish light across the landscape. It happens before sunrise or after sunset. In clear skies during the blue hour, you might also see a pinkish band of light hovering above the horizon in the eastern sky (opposite the sun), known as the “belt of Venus,” which is caused by the reflection of the sunrise or sunset on the atmosphere. Anti-crepuscular rays — shafts of light punctuated by the shadows of clouds — might also be visible in the east.
How the Blue Hour Happens
In a 1953 paper published in the Journal of the Optical Society of America, geophysicist Edward O. Hurlburt identified why the sky remains blue after sunset. He realized that the ozone layer, a component of the stratosphere between nine and 18 miles above Earth, acts as a giant filter of sunlight. After the sun dips below the horizon, the ozone layer blocks its red and orange wavelengths, while the blue wavelengths pass through.
Without the ozone layer, twilight would look completely different. The daytime sky is blue due to the effect of Rayleigh scattering, in which the sun’s light is scattered by atmospheric particles; the blue wavelengths of light remain visible to us. At twilight, the sun’s light has “traversed the atmosphere on their long paths, losing their blue, short-wavelength components on the way to scattering,” anthropologist Götz Hoeppe wrote in Why the Sky is Blue: Discovering the Color of Life. As a result, as Hurlburt wrote in 1953, an ozone-less sky “would be a grayish green-blue at sunset becoming yellowish in twilight.”
What Is the Golden Hour?
As evidenced by the more than 12 million Instagram posts tagged #goldenhour, the warm tones of sunlight before sunrise or after sunset are popular for outdoor photography. In the morning, the golden hour occurs during a period called civil dawn — the final phase of the three phases of twilight before sunrise — when the sun is between zero and six degrees below the horizon. In the evening, the golden hour occurs during civil dusk, the first phase of twilight after sunset. Generally, the blue hour occurs before the golden hour in the morning, and after it in the evening, according to the distance of the sun below the horizon.
Why the Golden Hour Glows
The special light of the golden hour is due to the low angle of the sun. During civil dawn or dusk, the sun’s rays have to penetrate more of Earth’s atmosphere to reach the planet’s surface. The rays pass through dust, humidity, and pollution, which block the intensity of the light and give it a softer, hazier look. The intense golden hue results from the light bouncing off those particles; the blue wavelengths are scattered and diffused, allowing more of the red, orange, and yellow wavelengths to come through and reflect off Earth’s atmosphere.
Art Inspired by the Golden Hour
Like today’s photographers, 19th-century painters made use of the golden hour to suffuse scenes with emotion and drama. British painter Joseph Mallord William Turner manipulated golden light to illuminate harbors, cityscapes, and railroads, enveloping the sights of the industrial revolution in a pastoral veil. American landscape painter Thomas Cole used the golden hour to establish the visual vocabulary of the Hudson River School, America’s first major art movement. And Albert Bierstadt and Thomas Moran, students of Cole’s aesthetic, painted monumental works of the American West bathed in gold light meant to evoke the sublime power of nature.