Earth is tilted like a spinning top that never straightens up. For half the year your half of the world leans toward the sun, for long, warm days. For the other half it leans away, for short, cold ones.
Most people think summer happens because Earth is closer to the sun. In fact Earth is closest in early January, and seasons come entirely from the 23.5° tilt swinging each hemisphere's sun angle and day length.
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.
Seasons are a lighting-angle story, not a distance story: the tilt sets how steeply and how long the sun shines on you.
- 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.
Common questions
The two hemispheres always lean opposite ways off the same single tilt, so when the north leans sunward into summer, the south leans away into winter. December is high summer in Australia.
Above about 66.5° latitude, the tilt is steep enough that in summer your latitude line never crosses into the night side at all, so the sun never sets for weeks. In winter it never rises.
In winter the noon sun sits up to 47° lower in the sky, so the same sunlight strikes the ground at a shallow angle and smears over more area. It is spread thin, not reduced.