Witness the breathtaking phenomenon of a green flash at sunset, captured on December 8, 2025, from the majestic Cerro Pachón in Chile. As the sun gracefully sets behind the Chilean Andes, it emits a fleeting emerald flare, a captivating display of nature's optical wonders. This captivating image, snapped by Petr Horálek, an NOIRLab Audiovisual Ambassador, showcases a rare atmospheric occurrence known as the green flash.
But what exactly is this mesmerizing sight? It's a result of Earth's rotation and the sun's descent towards the horizon, causing its light to pass through a thick slice of the atmosphere. This atmospheric prism bends and separates sunlight based on its wavelength, with shorter wavelengths (blue and green) bending more than longer ones (red and orange).
Just before sunset, the sun's disk is mostly hidden, creating a stack of slightly displaced images. The lower colors fade first, leaving a brief moment where the uppermost layer, dominated by green, forms a thin glowing band at the sun's edge, known as the green rim. When conditions are ideal, this rim appears as a small, detached green spark, the famous green flash.
Interestingly, a green rim is present at every sunset, but it's usually too thin and brief for the human eye to detect. Specialized cameras, high-quality lenses, and rapid image bursts are essential tools for capturing this fleeting phenomenon.
The location of this photograph, Cerro Pachón in Chile, is significant. Telescopes on this mountain and others like it observe through the same atmosphere that creates the green flash. This unique atmospheric effect causes different colors to bend by varying degrees, slightly smearing starlight into a miniature rainbow. Atmospheric dispersion correctors are designed to counteract this, and understanding this phenomenon helps astronomers enhance their observations of distant stars and galaxies.
The scientific community's interest in atmospheric optics like the green flash is profound. The shape, height, and duration of a green flash depend on temperature, pressure, and density variations with altitude. Warm and cool air layers act as stacked lenses, creating mirage effects and distorting the sun's image. By meticulously modeling and measuring green flashes, scientists can evaluate our understanding of the vertical atmospheric structure near the horizon.
For those eager to learn more, resources are available to explore the green flash and Earth's atmosphere, offering a deeper understanding of this captivating phenomenon.
