An extract from Philip Marsden’s The Summer Isles: A Voyage of the Imagination – our Book of the Month for October, published by today by Granta.

We were sailing back from Fowey. It was late in the year and the October sun lay sharp on the Dodman point. The boat was bowling along in short seas. I squinted up at the sails, at their cream-coloured arcs against the blue of the autumn sky. At the top of the mast, where the rigging converged, the cups of the anemometer spun in a brisk following wind. With me was my friend Mike, skipper of countless passages in the windier corners of the world. I, on the other hand, had never skippered a boat to anywhere I couldn’t reach by lunchtime. Come spring, I was due to sail up the Irish coast to the top of Scotland, single-handed, aiming for a small group of islands that, more than twenty years ago, I’d vowed to reach in memory of someone I loved.

Hence the overnight trip to Fowey, so late in the season, grabbing all the tips I could. We’d spent the previous evening going through the electrics and the radio, and the navigation equipment, and looking at a few changes to make to the halyards and sheeting – and I had carefully taken in the details, filling the pages of a notebook with technical information, and all the while thinking to myself: What, in God’s name, have I taken on?

Now it was morning, and the sails were well set, and the boat was moving with ease, and we were sitting in the sun. I watched the Dodman grow larger off the starboard bow – golden fern and pale grass and the steep snub of the rock. Around its foot was a ruff of white where the seas rose and fell against the cliff. To the south ran a shifting surface of swells, their backs glittering with jewels, and I watched the waves roll on into the distance, becoming smaller as they headed out to the horizon, and thought to myself: It is for such moments that all the labour of boats is worth it, that everything is worth it.

Then I saw something strange. Some twenty miles to the west stood three tall shapes. I couldn’t make out what they were. Ships’ gantries? Masts? Too grey, too bulky. I took my binoculars and looked again. They resembled rocks or skerries, but they were elevated, like sea-stacks. Also, they shouldn’t be there. What should be there, on that horizon, in that position, was the Lizard peninsula the long, flat strip of land leading down towards Britain’s most southerly point.

What I was seeing was a trick of the light – atmospheric refraction: the sun’s rays are slowed in the earth’s atmosphere, causing anything that appears far off to distort; sometimes the light even bends over the horizon to reveal things that are below it, beyond the normal line of sight. That afternoon the distant skyline of the Lizard had been altered and broken.

It is not an uncommon occurrence around the coast, and has caused some spectacular effects. In his Letters in Natural Magic, addressed to Walter Scott in 1831, David Brewster records all sorts of refractive phenomena – cows suspended in mid-air, ships appearing upside down. It hadn’t been many years since the people of Hastings had spilled out onto the town’s beach in alarm: the coast of France generally safe and hidden below the horizon – was suddenly right there, just a few miles away.

Through binoculars, the rocks were altering. They had steepened and narrowed at the base. They now looked like the inverted pinnacles of Phuket. There were more too; they were multiplying to the south. Some of these ones were larger, their bluish shapes suggesting an entire archipelago. I had the explanation: I knew I was looking at an illusion. But something about the sight of new islands, unfamiliar ground in a familiar setting, persuaded me to suspend disbelief. I found myself picturing gull-flecked cliffs. I could hear the cries and see caves beneath, and breaking waves; I could see along the coast to where there were beaches and dunes and small houses pale in the sun, and people in strange dress moving along grass-centred tracks, driving herds of white cattle.

A tiny split had opened in the fabric of the world, and I found myself eagerly passing through it.

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