Astronomy Picture of the Day

Discover the cosmos! Each day a different image or photograph of our fascinating universe is featured, along with a brief explanation written by a professional astronomer.

2023 March 28
A distant sunset is seen between an orange sky and
dark clouds. A close look at the Sun shows it is topped 
with several green strips, each known as a green flash.
Please see the explanation for more detailed information.

A Multiple Green Flash Sunset
Image Credit & Copyright: T. Slovinský & P. Horálek (IoP Opava); CTIO, NOIRLab, NSF, AURA

Explanation: Yes, but can your green flash do this? A green flash at sunset is a rare event that many Sun watchers pride themselves on having seen.  Once thought to be a myth, a green flash is now understood to occur when the Earth's atmosphere acts like both a prism and a lens. Different atmospheric layers create altitude-variable refraction that takes light from the top of the Sun and disperses its colours, creates two images, and magnifies it in just the right way to make a thin sliver appear green just before it disappears. Pictured, though, is an even more unusual sunset. From the high-altitude Cerro Tololo Inter-American Observatory in Chile one day last April, the Sun was captured setting beyond an atmosphere with multiple distinct thermal layers, creating several  mock images of the Sun.  This time and from this location, many of those layers produced a green flash simultaneously. Just seconds after this multiple-green-flash event was caught by two well-surprised astrophotographers, the Sun set below the clouds.

Tomorrow's picture: dolphin vs cloud


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