A private wink from Paradise

For a remarkable 10 days or so last month, the weather decided to show me up for a woeful Cassandra in the matter of climate change. No sooner had I stepped out into the news-features pages to warn of hurricanes, floods and tidal surges than January turned dry and demure as if a zephyr wouldn't melt in its mouth.

The fine spell brought clear horizons and vintage ocean sunsets: on some evenings, the riveting minutes of the star's final plunge halted me in my tracks with the power of a pagan Angelus. Yet a moment of - literally - dazzling revelation came in a casual, quite undeserving, glance.

Where the sun sets, exactly, depends on where you're standing, and from the foot of Mweelrea at this point in the year its descent is intercepted by the low bulk of Tully Mountain, across the bay in Connemara. The peak of Mweelrea on such evenings flushes a deep, rosy pink, with violet shadows in the buttresses, and it was this I was enjoying on a visit to a friend.

Then, swinging round, I found the sun two-thirds obscured by the sharp ridge of Tully, its fiery last segment visibly tilting to a close. A second later, I saw the rim of the sun blaze out in a brilliant, emerald flash of light. Yes - emerald!

I was thrilled at my luck, but not utterly amazed - the "green flash" is a phenomenon science knows about, and has photographed, but which few scientists - comparatively few people, indeed - have seen.

My first thought was of the scientist who alerted me to its existence - this not in a research paper, but a lyrical, poetic meditation on the more elusive wonders of the natural world.

Chet Raymo is Professor of Physics and Astronomy at Stonehill College, Massachusetts, and his books explore the relationship between science, nature and the spiritual. Exceptionally affecting is Honey from Stone, which grew out of his experiences in roaming the Dingle Peninsula, his summer home. In 1997, Brandon published its own edition of it, with some striking illustrations by Bob O Cathail.

In one of the essays, Raymo tells of climbing the mountain behind his house every clear summer evening for a month, hoping to see the green flash. Indeed, he had been watching for it, on and off, for 30 years, ever since reading an article on the subject by the astronomer D.J.K. O'Connell of the Vatican Observatory. The effect this described "was so evanescent, so unexpected, so marvellous, that I have pursued it ever since," wrote Raymo.

Scientific interest in the green flash began with a romantic Jules Verne novel set in Scotland, Le Rayon Vert, published in 1882. Its heroine describes the final sunbeam as "a ray of unparalleled purity, a ray of the most wonderful green which no artist could ever obtain upon his palette, and which neither the varied tints of vegetation nor the shades of the most limpid sea could ever reproduce! If there be green in Paradise, it cannot but be this."

Until O'Connell's research, however, it was widely assumed the phenomenon was in the observer's mind or eye. A well-known effect of retinal fatigue is that staring at one brilliant colour can produce an after-image of its "opposite", or complementary colour.

What I saw, however, was not an afterimage, but a seeming change within the disc itself - an effect, as O'Connell's research showed, that is quite independent of the human retina. In 1965, he and his observatory colleagues succeeded in capturing its fleeting beauty. As Raymo writes: "The green flash is not an artefact of the eye. At the top margin of the sun's disk - in photograph after photograph - is a brilliant strip of emerald green!"

The cause of the flash is, as you might expect, a matter of physics. As the sun sinks, its light reaches the viewer more obliquely and passes through a greater thickness of atmosphere. As this reduces the velocity of the light, its rays bend differentially, depending on their colour, and are spread, as through trough a prism. When the sun sets, its colours should disappear over the horizon one by one, but the atmosphere absorbs some and scatters others, leaving only green to pierce the Earth's molecular miasma.

"Normally," as meterologist Brendan McWilliams explains in his A Weather Eye on Literature, "this upper green rim cannot be seen with the naked eye; it only becomes visible when a very rare, almost freak, thermal structure of the atmosphere makes the air act like a giant magnifying glass. The whole sun is not magnified in these circumstances; only a very small portion of it is, and as the green rim passes through this magnifying region it is momentarily enlarged, producing the beautiful, spectacular but very ephemeral green flash."

It is, as Raymo puts it "a marvel of optical legerdemain", and one whose magic he must continue to take on trust. He has friends who say they "often" saw the flash from the deck of a tanker plying equatorial waters, yet his own vigils, from the Australian outback to the west coast of America, have all been in vain.

There are things I'd dearly love to have seen - a total solar eclipse, for example, or the aurora borealis in an Arctic night. I did live for a summer with the midnight sun, lighting the tundra like a 40-watt bulb, but have rather missed out on shooting stars and rings around the moon. The green flash may only be a matter of timing and physics, but I'll not forget the glory of my private wink from Paradise.