Northern lights from a boat in Tromso: darker water, honest odds

The boat. Complete darkness, an open horizon and, if the lights come, a reflection in the water that no land tour can offer. It is also warm inside between sightings, which matters at minus four in January. The limitation is that it goes where the fjord goes.

The coach. A guided aurora bus can drive inland, or up the coast, or across to Finland if the forecast demands it. In a cloudy week that flexibility is worth more than any other factor. What you get instead of a deck is a lay-by with eighty other people.

Standing still. Tromsø itself is bright, but the edges are not: Telegrafbukta, the road up Fløya, the shore across the bridge. On a strong clear night you can see a good display for the price of a walk. This is the option nobody sells and everybody who lives here uses.

The sensible plan for a three-night trip is one boat, one coach and one walk, in that order of preference — and to book the boat early enough that a cancellation is not the end of the story.

Camera sensors gather light for seconds; eyes do not. Almost every aurora photograph you have seen is brighter and greener than what the photographer was looking at, and knowing that in advance is the difference between delight and disappointment.

A moderate display to the naked eye is a pale grey-green arc low in the north that could be mistaken for thin cloud lit from below. It brightens, it develops structure, and it moves — and movement is the thing photographs cannot convey at all. A strong display fills the sky, turns pink at the top and shifts fast enough to make people shout.

The colour is real: green from oxygen at around a hundred kilometres, red or pink higher up, purple where nitrogen contributes. Your eyes are simply poor at colour in the dark, so the green reads as grey until the display is strong.

What this means practically: give it twenty minutes, keep screens off, and do not judge the night on the first five minutes. Displays come in waves with quiet gaps between them, and the people who go inside at the first lull miss the good part.