Green flash at sunset

Settled in 1958 by D.J.K. O'Connell (comprehensive photographic and theoretical monograph, building on earlier 19th-century refraction theory).

Sailors and polar explorers reported a brief green flash at sunrise or sunset for generations, and by the mid-19th century physicists including Auguste Bravais had begun explaining it via the atmosphere's differential refraction, which spreads the sun's disk into a stack of colored images with green (and sometimes blue) at the top. Doubts persisted because the effect is fleeting and easy to confuse with camera artifacts or eye fatigue, until Jesuit astronomer D.J.K. O'Connell undertook a systematic photographic campaign at the Vatican Observatory, publishing over 80 color photographs and a full theoretical treatment in his 1958 monograph. His work definitively established the green flash as a real, predictable consequence of atmospheric refraction and dispersion (with inversion-layer mirages producing the most dramatic, prolonged versions), settling the basic physics even though rare exotic variants are still studied.

A fleeting green spark that sailors swapped stories about for centuries turns out to be sunlight itself being sorted by color as it grazes the curve of the Earth's atmosphere.

Explanations that circulated

• The folk belief that the green flash is an afterimage or optical illusion generated by the eye after staring at the bright setting sun, rather than a real, photographable atmospheric-refraction effect
• The claim that a green flash requires a perfectly flat sea horizon and cannot be seen over land — genuine flashes, sometimes enhanced by temperature-inversion mirages, have also been documented over mountain and desert horizons

References

• The Green Flash and Other Low Sun Phenomena, D. J. K. O'Connell, Vatican Observatory / North-Holland Publishing Company (monograph), 1958