Neptune’s wink? A sign of luck? As Summer Solstice approaches and we long for those lingering dusks watching the sun dip over the horizon, we turn our thoughts to the magnificent, fleeting and often disputed Green Flash. If you look closely enough, what might a Cornish sunset reveal to you? Our five-minute guide to the Green Flash preps you to be eyes wide open next time you find yourself near a clear horizon at sundown…
At a brief moment between glowing sunset and the dark blue of night, the sun’s light can hit the atmosphere at such an angle as to glow bright green. Elusive, mystical, the so-called ‘Green Flash’ is a trick of the light, caused by “light refracting in the atmosphere, and the behaviour of different wavelengths within the spectrum of light”, explains Met Office meteorologist and broadcaster Clare Nasir.
“The Green Flash happens when blue and violet light are scattered by the atmosphere, while red, orange and yellow are absorbed. During the last few seconds before the sun sets below or rises above the horizon, the green light is the most visible. It’s utterly magical.”
Technically, what we call the Green Flash is numerous distinct phenomena, scientifically referred to as inferior-mirage flashes, mock-mirage flashes, sub-duct flashes, or the rarer green ray.
As the name suggests, the Green Flash is often a fleeting moment – just one or two seconds in Cornwall. Unlike the starry night sky, which reveals itself lazily through the gloaming, this quarry is furtive; a case of blink and you’ll miss it.
“When the Green Flash occurs, the length of time it is visible depends on the rate at which the sun sets or rises, which in turn depends on the observer’s latitude and the time of year,” explains Johannes Courtial in his research paper ‘A Simple Experiment that Demonstrates the Green Flash’. “During the polar summer, a Green Flash has been observed to last for more than 30 minutes. A more typical duration at temperate latitudes is on the order of a second.”
To be in with any chance of catching a rare glimpse, the best strategy is to closely watch the first instants of the sunrise, or last embers of sunset. What you will witness if you are very, very lucky is a sudden, bright colour change as the tiniest sliver of sun crosses the vanishing point. Briefly, the colour green illuminates the horizon in a vivid dot or ray – or the most fortunate might see a pathway of light shoot up into the sky, or stretch back towards the land.
“If you want to see a green ray you have to have a totally clear horizon, no obstructions at all. When the sun sets into a clear line, like a pencil line of the sea, you have a good chance of seeing it. And what you are seeing is the slowest light from the sun. It looks like a tiny pulsating lentil.” – Artist Tacita Dean talking Green Flash spotting in an Independent newspaper interview, 2007.
One common misconception is that the Green Flash only happens in far-flung places. Cornwall is actually an ideal location. A clear sky and far off horizon offer the greatest chance – the reason sailors see it more often than anyone, because of the uninterrupted horizons and their watchful attitude.
Throughout history there has been a degree of mysticism associated with the phenomenon. As Tacita Dean said in her film The Green Ray, “Sailors see them more than the rest of us, and they have come to signify for some the harbinger of great change or fortune in their lives.” This perhaps dates back to Jules Verne’s 1882 novel Le Rayon-Vert, which states that, “He who has been fortunate enough once to behold it is enabled to see closely into his own heart and to read the thoughts of others.”
And while the Green Flash may not bestow physical fortunes on those who witness it, it is a powerful and moving experience. Watergate Bay Hotel co-founder Mary Ashworth has seen it four times (once in the Caribbean and three times at Watergate Bay). She first learned about it from her husband, who had been in the navy and saw it multiple times in the Indian Ocean. It made her feel exhilarated, she says. “And it’s always satisfying when you find something you’re looking for. Especially when many people think that you are pulling their leg.”
Whether you see it or not, there’s joy to be had in searching all the same. As Tacita Dean says, scanning the horizon for the Green Flash is something that becomes, “about the act of looking itself, about faith and belief in what you see”.
Lord Rayleigh, F.R.S, believed the Green Flash could be recreated with knife edges, pinholes and prisms. He also knew that to see it in the wild, you'd need luck. Rayleigh was a physicist – the first to know why the sky is blue. In 1934, he got closer to the science behind the Green Flash. In his paper, ‘Further Experiments in Illustration of the Green Flash at Sunset’, published that year, he states:
“An experimental imitation of the phenomenon of the green flash at sunset is described, an artificial source of light and a prism whose dispersion is equal to the atmospheric dispersion being used. A straight edge parallel to the base of the prism plays the part of the horizon. The observer was 74 metres from the prism. On moving the eye into the shadow of the straight edge, the bluish-green flash was well seen […] This discussion seems to show as clearly as could be wished, that the normal dispersion and resolving power of the atmospheric spectroscope are ample to account for the ‘Green Flash’.”
“Green is quieter still, rest and content, the emerald ripple of wave and flow.” – Winifred Nicholson, St Ives Painter