Canada

The confusion is worth clearing up because it changes how you read every aurora forecast. There are two northern magnetic poles and they are in different places.

The magnetic dip pole is the point where a compass needle stands vertically. James Clark Ross located it in June 1831 on the Boothia Peninsula in what is now Nunavut, planted a flag, and made it a Canadian possession by accident. It has not stayed put: over the last century it has drifted north and then east, accelerating to something like 50 kilometres a year, and it is now well out across the Arctic Ocean heading for Siberia.

The geomagnetic pole is different. It is the pole of the simplified dipole that best describes the Earth's field as a whole, it lies off northwest Greenland, and it moves far more slowly. That is the one the auroral oval is centred on, and it is why the Canadian north remains the best-placed inhabited region on the planet for this even as the dip pole wanders off.

Compare those magnetic figures with the European ones and the picture reverses. Tromsø at 69°39′N geographic comes out around 67 magnetic; Rovaniemi at 66°33′N comes out around 64. A Canadian town that looks southerly on a map is sitting further inside the ring than either, and the sky agrees: 47.8% of usable nights against Tromsø's 25.4%.

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, both Canadian bases give the same answer, and it is not the one the cloud column alone suggests.

February is the month at both, and it is decisive. Yellowknife's 3.67 expected usable dark hours is the second-highest figure of any month at any of the fourteen destinations we measure, behind only January in Fairbanks.

The instructive row is March. Yellowknife's March cloud figure of 61.6% is the highest in this entire catalogue, and Whitehorse's September figure of 57.7% is the second. Both come near the bottom of their own columns, because the nights at these latitudes are already shortening fast by March and have not properly arrived by September. A number that would headline any other site turns out to describe a month you should not pick.

That gap between the impressive figure and the useful one is the reason we publish both columns rather than one.

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, an effect described by Russell and McPherron in 1973, which favours September and March. At Whitehorse that correction is large enough to lift September to the top; at Yellowknife it narrows the gap without closing it. February survives either way.

Every operator will tell you to check the cloud and the Kp index. Almost none will tell you to check the lunar calendar, and it is the one variable in this entire business that can be predicted with certainty months in advance.

A full moon in a clear winter sky throws a great deal of light. It will not hide a strong display, but a faint arc, which is what most nights offer for the first hour or two, can be washed out entirely. If you are choosing dates and have any freedom at all, aim for the week either side of a new moon. Whitehorse in March has the highest moon-free share of any month at any destination we measure, 65%, which is the one thing that month has going for it.

The counter-argument is real. Moonlight lights the landscape, and a photograph of an aurora over a moonlit frozen lake is often better than the same aurora over a black void. Photographers frequently prefer a quarter moon for exactly that reason. So: new moon if you want to see the most, quarter moon if you want the best picture, full moon only if the dates are fixed.

Yellowknife's country is more water than land, and that splits its season in two. In autumn the lakes are open and still, and a mirror surface doubles whatever the sky is doing; those reflections are the images that made this town's reputation. From November the ice takes over, and by midwinter it carries vehicles, at which point the buses drive out into the middle of a lake for a horizon that is unbroken in every direction.

The same ice supports a winter road running hundreds of kilometres north to the diamond mines, built fresh each year and gone by spring. It is a reminder of what kind of country this is: a place where the infrastructure itself is seasonal, and where the darkness you need for an aurora is not something you drive to but the default state of everything outside town.