Earth is tilted like a spinning top that never straightens up. For half the year your half of the world leans toward the sun — long, warm days. For the other half it leans away — short, cold ones.
What's actually happening
The most popular wrong answer in science: "summer is when we're closer to the sun." Earth's orbit is nearly circular, and the small difference there is runs backwards — we're closest in early January, the depths of northern winter. Distance is innocent.
The culprit is the tilt. Earth's spin axis leans 23.5° and, like a gyroscope, keeps pointing at the same patch of sky (near Polaris) all year. So as we travel around the sun, there's a month where your hemisphere leans sunward — the sun climbs high, days run long, and sunlight hits the ground steeply, concentrated like a torch aimed straight on. Six months later the same hemisphere leans away: the sun stays low, days shrink, and the same sunlight smears across more ground at a shallow angle. Summer and winter are a lighting-angle story.
The tilt also draws the calendar's strangest lines. Above 66.5° latitude, there are summer days when your latitude line never crosses into the night side at all — the midnight sun — and winter days when it never leaves it. And while the north roasts, the south freezes: the two hemispheres always run opposite seasons off the same single lean.
- 1In a dark room, shine a torch straight down at a sheet of graph paper and trace the bright patch.
- 2Now tilt the torch to hit the paper at a shallow angle from the same distance and trace again — the same light now smears over a far bigger patch, so each square gets less.
- 3That's winter: not less sunlight, but the same sunlight spread thin.