An endless summer day in December

Hello there! Rose again. I write for the blog at Datawrapper, and today I’m back for the third in a streak of Weekly Charts about the changing seasons.

Some seasonal patterns can surprise us in a delightful way. You never know exactly which week a fruit will ripen or a tree will change color. And some seasonal patterns can surprise us in an ominous way — like when the days are way hotter, way more often than ever before.

And some seasonal patterns… don’t surprise us at all. They’re so regular that we can calculate exactly when they happened a hundred years ago and exactly when they’ll happen a hundred years from now. Every year, everywhere, the days get longer for six months and then start to get shorter again. And here in the Northern Hemisphere, they’re starting to feel really short.

This week’s chart has two kinds of symmetry. If you stay put throughout the year, you’ll be traveling along a single colored line, corresponding to your town’s latitude. For example, the bold blue line represents roughly where I am in Berlin — and roughly where you are too, if you live in central Europe, southern Canada, or most of Kazakhstan. Since the winter solstice is basically at the beginning of the year and the summer solstice basically in the middle, our blue line is symmetrical from left to right: the lengthening of the days between December and June perfectly matches the shortening between June and December. 

To make the chart easier to read, I added some shading to represent the seasons: white for the summer months when the days here are longer than the nights, and gray for the winter months when the opposite is true. But if you live in the Southern Hemisphere, then that shading should really be reversed. South of the equator, the long days are in the “gray” season from September to March, and the short days are in the “white” season from March to September. That gives this chart its second, top-to-bottom symmetry. For every blue line representing a northern latitude, there’s an orange counterpart in the south, following the same seasonal pattern in reverse.

Now you might have noticed that this chart only shows latitudes between 60°N and 60°S. That’s because going any farther would take us across the polar circles — and inside the polar circles, the seasons of daylight get a little weird.

The polar circles are the latitudes where, for at least one day a year, the sun never sets. At 66°N, that happens just once, on the summer solstice in June. But the farther north you keep going, the longer that endless day lasts — until, at the North Pole itself, you reach a solstice “day” that lasts for six entire months, with the sun never setting between March and September.

Six months of daylight might sound pretty strange. But the truth is, we all get six months of daylight, wherever we are on earth; north or south, polar or tropical, the sun shines on us half of the time. And when you’ve run through that allotted six months, you won’t be getting any more until next year — which means that after every six-month summer “day" at the poles, there comes a six-month winter “night.” The vertical lines on the equinoxes of this chart, where daylight hours go from zero to 24 in a single day, are the sunrise and sunset of the polar year.^[1]

I started writing about this topic because I was feeling gloomy about the short November days. And honestly, it’s cheered me up quite a bit. Down here below the Arctic circle, no matter how early the sun may set, we can always be sure it will rise in the morning.

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That's all for today! Get excited for next Thursday, when we'll be treated to a Weekly Chart from our support engineer Margaux.

1. In reality, the sun shines so obliquely at the poles that a lot of this summer “day” is very dim, and a lot of the winter “night” is not particularly dark. There are a few other simplifications in this post as well. (For example, we all actually get ever so slightly more than half a year of daylight per year.) Scientists beware! ↩︎