Norðurljós — the Northern Lights

This is built now. Go out on a dark, clear night when the sun is awake and you may see them. You may also go a long time without. That is the point. See also Weather and Time, the Day and the Year.

They really happened

The same idea as the weather: we did not invent these.

The Sun throws out streams of particles, and when they strike the air high above the Earth they make it glow. Scientists have been measuring how disturbed the Earth's magnetism is — which is a good measure of how strong that stream is — every three hours since 1932, without a break.

This game reads those measurements. So on a game night, the lights you see are the lights that were really there on that date, in the real year your world happens to be playing.

If you have read the weather chapter, you know the game quietly picks a real year and lives through it. The lights come from the same year and the same day as the weather. A night that was cloudy really was cloudy, and a night the Sun was busy really was busy.

Three things have to line up

This is the part worth understanding, because it explains every night you go out and see nothing.

Miss any one of the three and there is nothing to see. All three at once is not as common as the photographs make it look.

Which is why you will never see them in June

In midsummer the sky here never really gets dark — you know this if you have been outside on a June night in Iceland.

So there are no northern lights in the game in June. Not because we switched them off, but because the first of the three things is missing. It falls out of the same arithmetic that puts the sun where it belongs.

Go looking in September to March. That is when the dark is long enough.

And cloud is the one that will disappoint you most

You can still make out the sun and the moon through cloud in this game — dimmed, but there. You cannot make out the northern lights.

That is honest rather than harsh. They happen about a hundred kilometres up, far above any weather, so a sky full of cloud is simply a lid.

Every Icelander knows the feeling of a beautiful forecast and a grey sky.

What you will actually see

On an ordinary night, a pale green arc low over the northern horizon. Smooth, quiet, not much movement. Easy to mistake for a wisp of cloud until you look properly.

On a good night, the arc climbs higher and breaks into rays — vertical streaks standing up from it, spreading across the northern sky, drifting slowly sideways.

On a rare night, it reaches high overhead, moves visibly while you watch, and a purple-pink fringe appears along the top edge. The purple comes from much higher up and needs a strong night to show at all.

The rays always stand upright. They follow the Earth's magnetic field, which runs almost straight down here — so however far along the arc you look, the streaks point the same way.

Look north. That is where the arc sits.

They do nothing, and that is deliberate

They will not warm you, feed you, hurt you or help you.

We decided this on purpose. The northern lights are worth having because they are beautiful, and a thing that is only beautiful is allowed to exist in a game about surviving a hard place.

Icelandic folklore has plenty to say about them, and one day some of that may find its way in. Nothing about how they look would need to change — which is exactly why the two were kept apart.

⬜ One thing they do not do yet

A real curtain of light is a sheet with depth to it — when it is close you can see the near edge and the far edge at once, and it folds like a cloth in a draught.

Ours is a single sheet, seen flat. It arcs, it snakes and its rays stand up properly, but it does not fold.

It is written down as work to come back to, rather than forgotten.

If you just want to see them

There is a Norðurljós button in the developer tools that poses the best case: a strong night, a clear sky, and no moon to wash them out.

The real sky is far less obliging — which is rather the point of going out to look.

Where all this comes from

• How active the Sun was — the Kp index, published by the GFZ Helmholtz Centre for Geosciences at the Geomagnetic Observatory Niemegk, freely and under a Creative Commons Attribution licence. Continuous since 1932.
• The cloud — Veðurstofa Íslands, the Icelandic Meteorological Office.
• The colours — measured from twenty photographs of real Icelandic aurora rather than invented, then adjusted once by eye, because a camera left open for several seconds sees far more than you do standing there.

We did not make the numbers up, and where we chose something for the look of it rather than for accuracy, that is written down too.