This week we arrived at the spring equinox, the day when, officially, we have the same amount of light here as everywhere else in the world. If you have no interest in this fact (and why should you?), or if you already have a degree in geodesy, you may stop reading now.

For the rest of you we will enlist the help of my able assistant, Rolf. If you are listening to this on the radio, Rolf is holding an apple in his left hand, and an orange in his right. This is the sun (Rolf holds up the apple). This is the earth (Rolf holds up the orange), and you will see it has a flat top and bottom, and is leaning slightly to one side as if someone has just knocked it off balance. Over a year the earth rotates around the sun, but keeps the same angle as it moves. It’s also spinning on its axis every day, which gives us day and night.

Now (pay attention at the back) if the sun (apple) is in the middle, say, and the earth (orange) is at 12 noon, and the lean of the earth is towards the sun, then the sun is shining directly (is ‘in zenith’) at the tropic of cancer (the circle of latitude north of the equator). Then we are at midsummer in the northern hemisphere – it’s light all the time in Kiruna, light more than usual in the UK, and light as it always is (half the day) down at the equator. At the other pole it’s dark all the time.

Rolf moves the orange anti-clockwise to 9, keeping the same angle. Now it is the autumn equinox – because the earth is leaning at the same angle the sun is now ‘in zenith’ over the equator and so is shining equally up and down the whole earth, with equal length night and day all over the globe.

Rolf moves the orange on to 6, and now the sun is shining at the tropic of capricorn in the south. The northern part of the globe is leaning away from the sun. It’s midwinter here in Kiruna and very dark, in the UK it’s darker than usual, it’s light as it always is (half the day) at the equator, and it’s light all the time at the southern pole.

Rolf moves the orange to 3, and it’s the spring equinox, where we are this week – the sun is ‘in zenith’ over the equator and light is shining equally up and down the earth again.

It wasn’t until I moved so far north that I was aware of the movement of the earth – it isn’t something one gets to think about so often. But because light conditions are rather extreme up here, it makes you think about it. The further north you go in the globe the more extreme these effects will be, and because of the flat top of the earth the differences are very marked north of the arctic circle. At the arctic circle, which is only about three hours’ drive south of here, there is only one day a year when the sun never rises, and one day a year when it never sets, and south of there this never happens. In Kiruna there are about four weeks when the sun doesn’t rise at all, and about six weeks when it never sets.

But this week we are all experiencing the same amount of night and day, just like it is at the equator all year round. Before long I will be desperately fixing up the blackout blinds to keep the sun out of my eyes at midnight, but for now the equinox means in Kiruna we have long enough days to enjoy the winter sunshine and long enough nights to have a good night’s sleep.