Sweden

Read the last two columns together and the market looks upside down. The base with roughly half the clear nights costs more and offers a quarter of the choice. That is what a reputation is worth, and it is the clearest example in this catalogue of a price set by a story rather than by a measurement.

In practice the distinction matters less than it looks, because several Kiruna departures will drive to Abisko when the forecast points that way. Booking from Kiruna and letting the guide decide is the version that needs no argument: you get the better base rate and the option on the valley.

Two things decide how much aurora time a month contains: how often the sky is usable, and how many hours of real darkness there are to use it in. Multiplied together, the Swedish season produces an answer that appears nowhere in the marketing.

November is the best month in Sweden, on 35.7% of Kiruna nights and the full eight hours of astronomical darkness. February and January follow. The month everyone recommends, September, comes last at both bases and by a wide margin: 2.5 hours of dark at Kiruna and 2.3 at Abisko, with only 27% and 23% of nights free of moonlight.

March deserves a note of its own. It has by far the highest cloud figure anywhere in Europe, 54.8% at Kiruna, and 4.3 hours of darkness drops it to sixth of seven. It is a lovely month to be here and it is not the month with the most aurora time in it. That gap between the headline number and the useful one is exactly what this site exists to close.

The other thing the table shows is Kiruna's remarkable evenness: six of its seven months sit within half an hour of each other, and every one of those six beats Abisko's best. You do not have to pick the right month in Kiruna. You cannot pick a good one in Abisko.

One limit, stated because it argues against this ranking: it measures how much dark clear sky a month contains, not the chance the aurora does something inside it. Geomagnetic storms are close to twice as frequent around the equinoxes, favouring September and March. Doubling September brings it level with the Kiruna pack rather than ahead of it, and at Abisko it would move to the top of a very flat and very low row.

Sweden has allemansrätten, the right of public access, written into the constitution since 1994. In practice it means you may walk, stand, camp and look at the sky on almost any land in the country, including private land, provided you do no damage and keep away from houses.

That is not a detail. In much of the world, finding somewhere dark and legal to stand at eleven at night is a real obstacle, and half the value of a guided tour is that somebody else has solved it. In Sweden the problem does not exist. If you are staying in the north and the sky opens at two in the morning, you put a coat on and walk out of the door.

Which is worth weighing against the ticket price. With one night and no car, buy the tour: a Swedish winter road in the dark is not a job for a hire car and a phone map. With three nights in one place, the country is doing most of the work for free.

Kiruna has been an auroral research town since 1957, when a geophysical observatory was set up there during the International Geophysical Year. It is now the Swedish Institute of Space Physics, and it has been measuring the field above this region continuously ever since. Nine years later Esrange opened 40 km east and began firing sounding rockets directly into the auroral zone to sample it from inside, which it still does. Abisko, meanwhile, has had a scientific field station since 1903 holding one of the longest unbroken climate records in the Arctic.

None of that changes what you will see on a given Tuesday. It does explain why claims made in Swedish listings tend to be narrower and better supported than the ones made further south, and it is a little ironic that the one exception is the most repeated claim of all.