Shetland in the simmer dim, when the night forgets to come

Lerwick lies at about sixty degrees north, on much the same latitude as Bergen and St Petersburg, and a good deal closer to Norway than it is to London. In June this has a peculiar effect. The sun goes down late in the evening, travels a short way below the northern horizon, and comes up again in the small hours, and in between the sky never gets properly dark. Shetlanders call this the simmer dim, the summer twilight, and it is the best reason to go there at that time of year: a night that is really only a long, pale evening joined to an early morning.

It does odd things to visitors. You find yourself reading on deck at an hour you would normally be asleep, or walking along the harbour at what feels like dusk and turns out to be half past eleven. Birds sing at times birds should not. The islanders tend to take it in their stride, having paid for it with the winter, when the days are short and the wind does most of the talking.

Norse, until the fifteenth century

Shetland was Norwegian for far longer than it has been Scottish. The Norse settled the islands in the ninth century, and they remained under the Norwegian crown until 1469, when they were pledged to Scotland as security for the dowry of Margaret of Denmark on her marriage to James III. The dowry was never paid and the islands were never redeemed, which is one of history's more consequential unpaid debts.

The Norse inheritance is everywhere once you look for it. Almost every place name is Norse; the old language, Norn, was spoken in some form into the eighteenth century, and the Shetland dialect still carries its words. The islanders' flag is a Nordic cross, white on blue. And every January Lerwick holds Up Helly Aa, a fire festival in which a squad of men in Viking costume march through the town by torchlight and set fire to a replica longship. The festival in its present form dates from the Victorian era rather than the Viking one, but nobody watching the galley burn seems to mind.

The connection with Norway ran the other way in the Second World War, when small Norwegian fishing boats, and later submarine chasers, carried agents, weapons and refugees between occupied Norway and Shetland across the North Sea in winter. The operation was known as the Shetland Bus, and it ran largely from Scalloway, the old capital on the west side of Mainland.

What there is to see

Lerwick is a small grey stone town built along its harbour, with a main street that runs parallel to the water and narrow lanes climbing the hill behind. Along the shore are the lodberries, houses and stores built with their feet in the sea so that boats could unload straight into them, which was convenient for trade and, by long local tradition, for smuggling. Fort Charlotte, begun in the seventeenth century, sits above the harbour.

Beyond the town, most of what people come for is older or wilder. On the island of Mousa stands the best-preserved broch in the world, an Iron Age drystone tower more than twelve metres high, still with its stairs inside the walls. At the southern tip of Mainland, Jarlshof has layer upon layer of settlement, from Bronze Age houses through Norse longhouses to a ruined laird's house. And in summer the cliffs are full of seabirds: puffins at Sumburgh Head from late spring to late summer, gannets in their thousands at Noss and Hermaness. Fair Isle, which gave its name to a style of patterned knitting, lies between Shetland and Orkney and is counted as part of Shetland.

Arriving by sea, in the long days

A cruise call at Lerwick is usually a single day, which is enough for the town and one excursion, to Mousa, say, or to Sumburgh and Jarlshof. What matters most is timing. The simmer dim is at its height around the solstice in late June, but the nights are pale from late May into July, and the seabirds are ashore through most of that time.

Ambassador's Springtime Shores sailing is the closest fit: eight nights from Belfast in June 2027, calling at Tobermory on Mull, Stornoway on Lewis, Scrabster for John O'Groats, Lerwick, Douglas on the Isle of Man and Liverpool before returning to Belfast. It is a tour of the northern and western edges of the British Isles at exactly the right time of year.

From the south of England, Fred. Olsen's Scenic Summertime Scottish Isles leaves Southampton in August 2027 for ten nights, with Rosyth, Lerwick, Stornoway and Portree on Skye, and a run down the Sound of Mull. The simmer dim will be fading by then, but the evenings are still long and the heather is in flower. Ambassador's Celtic Capitals sailing from Tilbury in May 2027 includes Lerwick among a dozen nights round the British Isles, with Brest in Brittany thrown in.

Take warm clothes, whatever the month. Shetland in summer is light rather than hot, and the wind comes a long way across open sea to get there. But a pale midnight over Lerwick harbour, with the boats still and the sky the colour of a pearl, is one of the stranger and more beautiful things the British Isles can offer.