Why we have seasons
It is not distance. Follow one stubborn tilt of twenty three and a half degrees around a whole orbit and watch summer, winter and both equinoxes fall out of it.
Earth circles the Sun
Earth takes three hundred and sixty five and a quarter days to travel once around the Sun, and that spare quarter is where the leap year comes from. This journey is what gives us the year. What it does not give us, by itself, is the seasons.
Is summer just being closer?
Almost everyone's first guess is distance. It feels obvious that summer is when Earth swings nearer the fire and winter is when it drifts away again. It is a good guess, it is simply wrong, and there is one fact that settles the matter.
No, January is our closest month
Earth's orbit is very nearly a circle. We are actually closest to the Sun in early January, about five million kilometres nearer than we are in July, and January is deep winter across Europe and North America. So distance cannot be the answer.
The Earth leans on its axis
The real cause is a lean. Earth spins on an axis tipped about twenty three and a half degrees away from upright, probably knocked sideways by a colossal impact when the planet was young. Everything about the seasons falls out of that single angle.
The lean always points one way
Here is the part most people miss. The tilt does not swivel to follow the Sun. It stays aimed at the same distant spot all year long, currently close to the star Polaris, so as Earth travels, first one hemisphere leans sunward and later the other does.
Sunlight arrives steeper
When your half of the world leans towards the Sun, the light comes down closer to straight overhead instead of slanting in low. That change of angle is the main engine of summer, and you can feel the same effect between midday and early evening.
The same beam spreads thinner
Picture a torch shone at a wall. Straight on, you get a small fierce circle of light. Tilt the torch and that identical light smears into a long dim oval. Winter sunshine is the oval, the same energy spread thinly over far more ground.
And the days grow longer
The tilt stretches the day as well. Midsummer in London brings about sixteen and a half hours of daylight, midwinter only eight, so summer heats harder and heats for twice as long. North of the Arctic Circle the Sun does not set at all.
June, the north leans in
Around the twenty first of June the north pole is tipped as far towards the Sun as it ever gets. This is the summer solstice, the longest day of the northern year, and the moment the noon Sun stops climbing any higher in the sky.
September, day equals night
Three months on, around the twenty second of September, Earth has covered a quarter of its orbit and the tilt now points sideways to the Sun. Neither pole leans in, so day and night are almost exactly twelve hours each, at the same time, everywhere on the planet.
December, the north leans away
By the twenty first of December that unchanged tilt is leaning the north away. The noon Sun hangs low, the days are short, and each patch of ground now collects roughly a fifth of the energy it soaked up in June. Winter is a shadow, not a distance.
March, day equals night again
Around the twentieth of March the balance returns and day equals night once more. Notice that Earth never altered its lean at any point. All four of these moments are one fixed tilt seen from four corners of the orbit, and today the Sun rises due east for everyone alive.
The south gets the opposite
Every one of those descriptions flips below the equator. When the north leans in, the south leans out, so Australians spend Christmas on the beach and the coldest month in Argentina is July. There is no global summer, only two opposite ones.
Heat lags weeks behind light
One last puzzle. The longest day falls in June, yet the hottest weather usually turns up in late July or August, because land and especially the sea take weeks to soak up heat. The same lag is why the bitterest cold comes after midwinter.
One tilt, four seasons, forever
So the seasons have nothing to do with distance. They are the story of a planet travelling with a slight, stubborn lean, and that one small angle is what hands you blossom, harvest, falling leaves and snow, in the same order, forever.
Watch it explain itself
Every step above is narrated aloud. Play it, or open it in the editor and make it yours — no account needed.