Space · the reason for the seasons
Ask a room full of people why summer is warm and a lot of them will say: because Earth is closer to the Sun. It sounds sensible. It's also backwards. Earth is at its closest to the Sun around 3 January, in the middle of the northern winter, and at its farthest around 4 July, right when Europe and North America are sweltering.
So distance can't be the story. The real reason is a lean.
Earth's orbit is an ellipse, but only just. At its closest (perihelion) we are about 147.1 million km from the Sun; at its farthest (aphelion), about 152.1 million km. That's a difference of roughly 3%. Drawn to scale, you could not tell it from a circle.
Because light spreads out with the square of distance, that 3% turns into about 7% more sunlight reaching Earth in January than in July. Real, measurable, and completely swamped by what the tilt does. Try it below.
Drag Earth around its orbit (or use the date slider). Pick a latitude, then switch the tilt off and watch the seasons collapse.
Earth spins on an axis tipped about 23.4° from upright, and that axis keeps pointing the same way in space (towards Polaris) all year. In June the northern half leans towards the Sun; six months later, on the far side of the orbit, the same lean points it away.
Leaning towards the Sun changes two things. First, the Sun climbs higher. At 50° north the noon Sun stands about 63° above the horizon at the June solstice but only about 17° in late December. A low Sun spreads the same beam over a much longer patch of ground, like a torch shone at a shallow angle. At 17° each square metre gets roughly a third of what it gets under a 63° Sun.
Same beam, same energy. Spread over more ground, each square metre gets less.
Second, the days get longer. Leaning towards the Sun means your spot on the spinning Earth spends more of each rotation on the lit side. At 50° north that is about 16 hours of daylight in June against about 8 in December (a little more in practice, because the atmosphere bends light over the horizon).
Multiply a higher Sun by a longer day and the effect is enormous. Above the atmosphere, a square metre at 50° north collects roughly 11.6 kWh of sunlight on the June solstice and only about 2.1 kWh on the December solstice: more than five times as much. Set against that, the 7% from distance is a rounding error.
If distance ran the seasons, the whole planet would have summer at the same time. Instead, Sydney's summer comes in December, exactly when London shivers. Drag the latitude slider below the equator in the season machine and the yearly curve flips upside down.
The distance effect doesn't vanish; it just rides along. Because the southern hemisphere leans towards the Sun at almost the same time Earth makes its closest pass, southern summers get a slightly stronger dose of sunlight than northern ones. The south's huge oceans soak much of that up, which is part of why it doesn't show up as scorching southern summers.
Distance has one more, sneakier effect. Earth moves fastest when it's closest to the Sun and slowest when it's farthest (Kepler's second law). The slow stretch falls in the northern summer, so the half of the year from the March equinox to the September equinox is roughly a week longer than the half from September back to March. In 2026, for example, the equinoxes fell on 20 March and 23 September: 187 days one way, 178 the other.
Hit 0° (no tilt) in the season machine. At every latitude the curve flattens almost to a line, with just the gentle 7% ripple from distance, peaking in January everywhere at once. Days would be about 12 hours long all year. No midnight sun, no long winter nights, and no real seasons at all.
So next time someone says summer is when we're closest to the Sun, you can tell them: in the north, summer is when we're farthest. It's not how near you are to the fire. It's which way you're facing.